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Silver and copper clusters capped by external chelating dithiolate ligands can be classified according to the cavities in their central coinage metal polyhedra. Silver clusters composed exclusively of fused tetrahedra are analogous to simple saturated organic compounds. The only interstitial atom that can be fit into an Ag4 tetrahedron is hydrogen. Silver polyhedra with larger trigonal prismatic or cubic cavities, including highly distorted cubic cavities, can accommodate halide and chalcogenide anions. The still larger 12-vertex icosahedral and cuboctahedral coinage metal cavities can accommodate oxoanions of the types SO32− and SO42− and their heavier congeners or alternatively interstitial coinage or platinum group metals leading to central M′@M12 units. Copper clusters with central cuboctahedra and silver clusters with central icosahedra possessing interstitial metal atoms approximate sphericality and provide examples of electron-rich metal superatoms with an average metal oxidation state of less than +1. Such copper cluster superatoms have two extra electrons corresponding to a filled 1S2 superatomic orbital. The silver cluster superatoms are electron richer with eight extra electrons corresponding to filled 1S2 + 1P6 superatomic orbitals. In these silver clusters seven or eight faces of the central Ag12 icosahedron are capped by additional silver atoms in order to provide these additional electrons. [ABSTRACT FROM AUTHOR]

The currently unknown phosphorus allotrope P8 is of interest since its 40 total valence electrons is a "magic number" corresponding to a filled 1S21P61D101S21F142P6 shell such as found in the relatively stable main group element clusters Al13− and Ge94−. However, P8 still remains as an elusive structure not realized experimentally. The lowest energy P8 structure by a margin of ∼9 kcal mol−1 is shown by density functional theory to be a cuneane analogue with no P＝P double bonds and two each of P5, P4, and P3 rings. Higher energy P8 structures are polycyclic systems having at most a single P＝P double bond. These P8 systems are not "carbon copies" of the corresponding (CH)8 hydrocarbons with exactly one hydrogen atom bonded to each carbon atom. Thus the lowest energy (CH)8 structure is cyclooctatetraene with four C＝C bonds followed by benzocyclobutene with three C＝C bonds. The cuneane (CH)8 structure is a relatively high energy isomer lying ∼36 kcal mol−1 above cyclooctatetraene. The cubane P8 and (CH)8 structures are even higher energy structures, lying ∼37 and ∼74 kcal mol−1 in energy above the corresponding global minima. Our results demonstrate differences in medium sized aggregates of elemental phosphorus and isolobal hydrocarbon species. [ABSTRACT FROM AUTHOR]

The structures and energetics of species with the stoichiometries (Me2CNO)M(CO)nCp (M = Cr, Mo, W; n = 3, 2, 1; Cp = η5-C5H5) have been studied by density functional theory. The experimental structure Mo2-1S with an NO-dihapto Me2CNO ligand found by X-ray crystallography for (Me2CNO)Mo(CO)2Cp is shown to be the lowest energy isomer by a substantial margin. The expected initial Me2CNOMo(CO)3Cp product from the reaction of NaMo(CO)3Cp with Me2C(NO)Br used to synthesize (Me2CNO)Mo(CO)2Cp is predicted to undergo facile CO dissociation based on ΔH and ΔG values. The energetics of the currently unknown analogous chromium and tungsten systems appear to be similar to that of the molybdenum system. The structures Me2NC(CO)OM(CO)2Cp (M = Cr, Mo, W) with a dimethylcarbamate ligand are much lower energy (Me2CNO)M(CO)3Cp isomers but are clearly not accessible from reactions of the anions CpM(CO)3− with Me2C(NO)Br. The lowest energy structures for the monocarbonyl (Me2CNO)M(CO)Cp are of the type (Me2C＝N)M(O)(CO)Cp with separate dimethylimino and oxo ligands formed by oxidative addition of the Me2CNO unit to the central metal atom. [ABSTRACT FROM AUTHOR]

Thorium encapsulated metallofullerenes (Th-EMFs) with external C76, C80, C82, and C86 cages have been synthesized, with the 13C-NMR spectrum recorded for Th@C82. Here, we explore computationally the chemical bonding, NMR and spherical aromaticity of Th@C82 and related thorium-encapsulated metallofullerenes. Our results show that these Th-EMFs are new examples of spherical aromatic structures, representing interesting low-symmetry exceptions to the Hirsch 2(N + 1)2 rule of spherical aromaticity. Their electronic structures are based on π-electron counts of 80, 84, 86, and 90, respectively, with a shell structure ranging from S2P6D10F14G18H22I8 to S2P6D10F14G18H22I18, where the partially filled I-shell remains as a frontier orbital. Their behavior is comparable to that of the spherical aromatic alkali-C606− phases, which in addition to the favorable endohedral Th-fullerene bonding account for their particular abundance exhibiting the ability to sustain a long-range shielding cone as a result of the favorable metal–cage bonding. This rationalization of such species as neutral spherical aromatic EMFs suggests the possibility of an extensive series of aromatic fullerenes with nuclearity larger than C60 buckminsterfullerene as stable building blocks towards nanostructured metal–organic materials. [ABSTRACT FROM AUTHOR]

Structures of the slightly hypoelectronic 14-vertex species Cp2M2B12H12 (M = Rh, Ir) and Cp2M′2C2B10H12 (M′ = Ru, Os) with 28 Wadean skeletal electrons, including the experimentally known Cp*2Ru2C2B10H12, have been optimized by density functional theory. All low-energy such structures have the metal atoms located at degree 6 vertices. The lowest energy structures have central M2B12 and M′2C2B10 deltahedra with three degree 6 vertices, one degree 4 vertex, ten degree 5 vertices, and the metal atoms located at non-adjacent non-antipodal degree 6 vertices similar to meta isomers of icosahedral dimetallaboranes. Slightly higher energy structures are based on the same central deltahedron but with the metal atoms located at adjacent degree 6 vertices similar to ortho isomers of dimetallaboranes. Isomeric structures with central M2B12 and M′2C2B10 central closo deltahedra, namely bicapped hexagonal antiprisms with the metal atoms in antipodal positions at the two degree 6 vertices similar to the para isomers of icosahedral dimetallaboranes, also lie at low energies. [ABSTRACT FROM AUTHOR]

Multiple silicon solar cell technologies have surpassed or are close to surpassing 26% efficiency. Dielectric and amorphous silicon-based passivation layers combined with minimal metal/silicon contact areas were responsible for reducing the surface saturation current density below 3 fA cm−2. At open-circuit, in passivated contact solar cells, the recombination is mainly from fundamental mechanisms (Auger and radiative) representing over 3/4 of the total recombination. At the maximum power point, the fundamental recombination fraction can drop to half, as surface and bulk Shockley–Read–Hall step in. As a result, to further increase the performance at the operating point, it is paramount to reduce the bulk dependence and secure proper surface passivation. Bulk recombination can be mitigated either by reducing bulk defect density or by reducing the wafer thickness. We demonstrate that for commercially-viable solar-grade silicon, thinner wafers and surface saturation current densities below 1 fA cm−2, are required to significantly increase the practical efficiency limit of solar cells up to 0.6% absolute. For a high-quality n-type bulk silicon minority-carrier lifetime of 10 ms, the optimum wafer thickness range is 40–60 μm, a very different value from 110 μm previously calculated assuming undoped substrates and solely Auger and radiative recombination. In this thickness range surface saturation current densities near 0.1 fA cm−2 are required to narrow the gap towards the fundamental efficiency limit. We experimentally demonstrate surface saturation currents below 0.5 fA cm−2 on pi/CZ/in structures across different wafer thicknesses (35–170 μm), with potential to reach open-circuit voltages close to 770 mV and bandgap-voltage offsets near 350 mV. Finally, we use the bandgap-voltage offset as a metric to compare the quality of champion experimental solar cells in the literature, for the most commercially-relevant photovoltaic cell absorbers and architectures. [ABSTRACT FROM AUTHOR]

The thiophosphoryl complexes (PS)M(CO)n (M from V to Co) and their corresponding dimers with P2S2 ligands have been studied at the DLPNO-CCSD(T)/cc-pVTZ//M06L/cc-pVTZ level of theory. For the mononuclear complexes containing the highly bent 1-e donor PS group with ∠SPM angles of ∼115°, the (SP)V(CO)6 and (SP)Co(CO)4 complexes are energetically disfavored and (SP)Mn(CO)5 is only slightly favored towards CO loss. The (SP)Cr(CO)5 and (SP)Fe(CO)4 complexes with a bent 2-e donor PS group with ∠SPM angles of ∼133° are both thermodynamically and kinetically disfavored towards dimerization. Similarly all of the (SP)M(CO)n (M from V to Co) complexes with linear 3-e donor PS ligands are energetically disfavored towards dimerization, excepting for Mn(CO)4(PS). The trans-influence of PS follows the sequence 3-e NO ≥ PS (linear) > CS > PS (2-e bent) ≈ CO. [ABSTRACT FROM AUTHOR]

Binuclear manganese complexes with covalent Mn–Mn bonds include dimanganese carbonyl complexes and dimanganese non-carbonyl complexes with bidentate anionic ligands. Density functional theory (DFT) using the M06-L functional is used to study some known dimanganese complexes and three series of model paddlewheel-type dimanganese complexes M2Lx and M2Lx·2H2O (L = HNCHNH, HNC(NH2)NH and OCHO respectively; x = 2, 3, 4). The DFT results for model structures for known dimanganese complexes agree well with the experimental structures and lead to predictions of ground state spin multiplicities. For the model paddlewheel-type complexes, predictions are made with regard to ground state spin multiplicities, and Mn–Mn bond lengths. Formal bond orders (fBO) from 0.5 to 4.5 for all the Mn–Mn bonds are assigned by electron counting. Molecular orbital analysis yields Mn–Mn bond orders (BO) matching the fBO values. Four "super-short" Mn–Mn bonds (lengths 1.618 to 1.639 Å) are found in higher energy isomers, along with other short Mn–Mn quadruple bonds. The effects of axial ligation by two H2O ligands are also noted. The Mn–Mn bond lengths are categorized into distinct ranges as per the Mn–Mn fBO values from 0.5 to 4.5, ranges which are compared with other experimental and computational estimates. [ABSTRACT FROM AUTHOR]

Endohedral metallofullerenes are key species for expanding the range of viable fullerenes, their versatility, and applications. Here we report our computational evaluation on the formation of spherical aromatic counterparts of the C60 fullerene from relativistic DFT calculations, based on the inclusion of Cr, Mo and W endohedral atoms. The resulting M@C60 endohedral fullerenes are 66-π electron neutral species exhibiting bonding properties and electronic structure mimicking the aromaticity and diamagnetic insulator behavior of alkali-C606− phases. The resulting structures are interesting candidates for further experimental realization as novel neutral building blocks for more flexible nanostructured organic materials, highlighting truly spherical aromatic neutral species retaining the truncated icosahedral structure of the seminal Buckminster fullerene. [ABSTRACT FROM AUTHOR]

The 1-norbornyl (nor) ligand is known experimentally to form stable transition metal alkyl derivatives through direct metal–carbon bond formation. This appears to be related to its steric bulk and inaccessibility towards β-hydrogen elimination, as exemplified by the tetraalkyls (nor)4M, some of which are very stable. In this connection we have used density functional theory and the DLPNO-CCSD(T) method to investigate the 1-norbornylcobalt carbonyl derivatives (nor)Co(CO)n (n = 4, 3, 2, 1) and (nor)2Co2(CO)n (n = 7, 6, 5). Low-energy structures of the unsaturated systems (nor)Co(CO)n (n = 3, 2) and (nor)2Co2(CO)n (n = 6, 5) are found to have agostic hydrogen atoms from a CH2 group adjacent to the Co–C bond. Such agostic hydrogen atoms form a C–H–Co bridge with a bonding Co–H distance less than ∼2 Å. In such structures unsaturation is relieved by donation of an additional two electrons from the C–H bond of this norbornyl CH2 group. In addition, structures in which carbonyl migration from cobalt to carbon has occurred to form acyl norCO ligands are among the lowest energy structures. The resulting acyl carbonyl groups of the norCO ligands serve as spacers between the bulky 1-norbornyl ligand and the cobalt carbonyl moiety. Furthermore, such neutral norCO acyl ligands can either be one-electron donors to a cobalt atom, bonding solely through the carbonyl carbon, or three-electron donor η2-μ-norCO groups bridging a central Co2 unit through both the acyl carbon and oxygen atoms. The strengths of the agostic C–H–Co interactions have been characterized by their reduced density gradient (RDG) values. [ABSTRACT FROM AUTHOR]

The geometries and energetics of the bis(bicyclo[3.2.1]octa-2,6-dien-4-yl) complexes of the first row transition metals, (bcod)2M (M = Ti to Cu), including the experimentally known chromium and iron derivatives, have been examined by density functional theory. Sandwich-type structures in which each bcod ligand is bonded to the central metal atom through both the trihapto allylic unit and the double bond are energetically preferred for the metals from titanium to iron. However, higher energy (bcod)2M (M = V, Cr, Mn) isomers are found in which one of the bcod ligands is bonded to the central metal atom through the trihapto allylic unit and an agostic hydrogen from the CH2 bridge leaving an uncomplexed C＝C double bond. The experimentally known iron complex (bcod)2Fe, an analogue of ferrocene, has the favored 18-electron configuration and a singlet spin state. For the metals to the left of iron from manganese to vanadium the energetically preferred spin states range from doublet to quartet, respectively, corresponding to holes in the closed shell 18-electron configuration. However, a singlet structure is preferred for (bcod)2Ti despite the 14-electron configuration of the central titanium atom. The (bcod)2M (M = Co, Ni, Cu) derivatives of the electron richer later transition metals are characterized by several structures of different types at closely spaced energies in which at least one of the bcod ligands is only partially bonded to the central metal atom. [ABSTRACT FROM AUTHOR]

Fluorocarbons have been shown experimentally by Baker and coworkers to combine with the cyclopentadienylcobalt (CpCo) moiety to form fluoroolefin and fluorocarbene complexes as well as fluorinated cobaltacyclic rings. In this connection density functional theory (DFT) studies on the cyclopentadienylcobalt fluorocarbon complexes CpCo(L)(CnF2n) (L = CO, PMe3; n = 3 and 4) indicate structures with perfluoroolefin ligands to be the lowest energy structures followed by perfluorometallacycle structures and finally by structures with perfluorocarbene ligands. Thus, for the CpCo(L)(C3F6) (L = CO, PMe3) complexes, the perfluoropropene structure has the lowest energy, followed by the perfluorocobaltacyclobutane structure and the perfluoroisopropylidene structure less stable by 8 to 11 kcal mol−1, and the highest energy perfluoropropylidene structure less stable by more than 12 kcal mol−1. For the two metal carbene structures Cp(L)Co＝C(CF3)2 and Cp(L)Co＝CF(C2F5), the former is more stable than the latter, even though the latter has Fischer carbene character. For the CpCo(L)(C4F8) (L = CO, PMe3) complexes, the perfluoroolefin complex structures have the lowest energies, followed by the perfluorometallacycle structures at 10 to 20 kcal mol−1, and the structures with perfluorocarbene ligands at yet higher energies more than 20 kcal mol−1 above the lowest energy structure. This is consistent with the experimentally observed isomerization of the perfluorinated cobaltacyclobutane complexes CpCo(PPh2Me)(–CFR–CF2–CF2–) (R = F, CF3) to the perfluoroolefin complexes CpCo(PPh2Me)(RCF＝CF2) in the presence of catalytic quantities of HN(SO2CF3)2. Further refinement of the relative energies by the state-of-the-art DLPNO-CCSD(T) method gives results essentially consistent with the DFT results summarized above. [ABSTRACT FROM AUTHOR]

The singlet–triplet energy splittings (STES) of dication carbon chains capped by three 16-electron and two 14-electron configuration transition metal termini have been investigated using density functional theory. All five transition metal-capped carbon chains (MCC) exhibit an odd–even STES alternation, suggesting that it is a general feature of the MCCs. Analysis of the frontier molecular orbitals indicates that the frontier and neighboring molecular orbitals (MOs) are π orbitals delocalized over the entire carbon chain, the transition metal termini, and the ancillary ligands. In even and odd metal–carbon chains the HOMOs and LUMOs are nearly degenerate and non-degenerate, respectively, resulting in the even–odd STES alternation. Further analysis of the MOs in the MCCs and the uncapped carbon chains indicate that the STES of the MCCs are determined substantially by the uncapped carbon chain. Other ancillary ligands also play important roles in tuning the energy splitting through their π donor and acceptor abilities. These observations are helpful for the design of cumulene materials exhibiting tunable electronic and optical properties. [ABSTRACT FROM AUTHOR]

The diferratetracarbaboranes Cp2Fe2C4Bn−6Hn−2 (n = 10 to 14; Cp = η5-C5H5) as well as the experimentally known C-tetramethyl derivatives Cp2Fe2C4Me4B8H8 have been studied by density functional theory methods. For the Cp2Fe2C4Me4B8H8 system, the three structurally characterized isomers produced under relatively mild conditions having an "open" tetragonal or pentagonal face correspond to the lowest energy structures not based on the 14-vertex closo deltahedron, namely the bicapped hexagonal antiprism. These structures provide examples of kinetically favored but thermodynamically disfavored high-energy metallacarborane structures. Thus the lowest energy such structure lies ∼22 kcal mol−1 above the global minimum, namely a C2vcloso structure with no C–C deltahedral edges. This latter global minimum 14-vertex closo structure is found experimentally to be the ultimate pyrolysis product in the Cp2Fe2C4Me4B8H8 system at 300 °C. The lowest energy structures for the smaller 11 to 13 vertex Cp2Fe2C4Bn−6Hn−2 systems are the corresponding most spherical closo deltahedra as expected by the Wade–Mingos rules for these 2n + 2 skeletal electron systems. However, for the 11- and 12-vertex systems, less spherical deltahedral structures providing additional degree 6 vertices for the iron atoms and degree 4 vertices for the carbon atoms become energetically competitive. For the 10-vertex Cp2Fe2C4B4H8 system a relatively non-spherical deltahedral structure with four degree 4 vertices for the carbon atoms and two degree 6 vertices for the iron atoms is energetically preferred by a substantial margin. Thus such a structure lies ∼23 kcal mol−1 in energy below the isomeric 10-vertex closo bicapped tetragonal antiprism structure expected from the Wade–Mingos rules for this 2n + 2 skeletal electron system. [ABSTRACT FROM AUTHOR]

Density functional theory studies on the experimentally known Cp*3Rh3B12H12Rh(B4H9RhCp*) as well as the model compounds Cp4Rh4B12H12 and Cp3Rh3B12H12Rh(η3-C3H5) indicate low energy structures with central Rh4B12 tetracapped tetratruncated tetrahedra (TTT) for these 32 Wadean skeletal electron systems. This skeletal electron count corresponds to 2k2 (k = 4) skeletal electrons suggesting a spherical aromatic system with filled 1s + 1p + 1d + 1f molecular orbitals as well as an isocloso 2n (= 32 for n = 16) skeletal electron count. Similar TTT structures are found for the valence isoelectronic 32 skeletal electron systems [Cp4M′′4B12H12]4+ (M′′ = Ni, Pd, Pt) and [Cp4M′4B12H12]4− (M′ = Fe, Ru, Os). The preferred structures of the 34 skeletal electron systems [Cp4M4B12H12]2− (M = Co, Rh, Ir), [Cp4M′′4B12H12]2+ (M′′ = Ni, Pd, Pt) are not the most spherical TTT despite their 2n + 2 skeletal electron count (= 34 for n = 16) for a closo structure by the Wade–Mingos rules. Instead they are prolate (elongated) polyhedra with two degree 6 and two degree 5 metal vertices with a central M4 macrobutterfly having one long M⋯M distance of ∼5.0 Å between the wingtips. The preferred structures for the still electron richer 36 skeletal electron systems Cp4M′′4B12H12 (M′′ = Pd, Pt) are derived from triple square antiprisms with two open M′′2B2 square faces. A distorted version of this polyhedron is the deltahedral structure with four degree 5 metal vertices and four degree 6 boron vertices found in the valence isoelectronic 36 skeletal electron first row transition metal derivatives Cp4Ni4B12H12 and [Cp4Co4B12H12]4−. However, this polyhedron is not found in the 36 skeletal electron [Cp4M4B12H12]4− (M = Rh, Ir), that instead have symmetrical central M4B12 TTTs. For some 16-vertex [Cp4M4B12H12]z systems deviating from the favored 32 skeletal electron count, low-energy structures are found in which hydrogen atoms migrate to bridge B–B edges or bend over to bridge M–B edges. In addition, the hypoelectronic hexacations [Cp4M4B12H12]6+ (M = Co, Rh, Ir; Ni, Pd, Pt) are found to have low-energy structures in which three of the four Cp rings are hydrogenated to give tetrahapto cyclopentadiene η4-C5H6 rings. [ABSTRACT FROM AUTHOR]

The experimentally known cobalt ethylenedithiolate complex CpCoS2C2H2 as well as its rhodium and iridium analogues are aromatic systems that can function as pentahapto six-electron donor metallodithiolene ligands by binding through all five atoms of its CoS2C2 ring. The lowest energy CpMS2C2H2·Cr(CO)n (n = 3, 2) and CpMS2C2H2·Fe(CO)2 structures (M = Co, Rh, Ir) by substantial margins are predicted by density functional theory to have structures with such pentahapto η5-(CpMS2C2H2) (M = Co, Rh, Ir) ligands. The CpMS2C2H2·Cr(CO)3 structures appear to be favorable as indicated by high carbonyl dissociation free energies (ΔG) exceeding 15 kcal mol−1. The lowest energy structures for the carbonyl-richer CpMS2C2H2·Cr(CO)4 and CpMS2C2H2·Fe(CO)3 systems have trihapto CpMS2C2H2 ligands involving a heterometallic metal–metal bond and two metal–sulfur bonds retaining an uncomplexed C＝C double bond in the ethylenedithiolate ligands. The CpMS2C2H2·Fe(CO)3 structures can be derived from the structures of the long-known (RS)2Fe2(CO)6 derivatives by replacing one of the Fe(CO)3 units by an isoelectronic/isolobal CpM unit. Such CpMS2C2H2·Fe(CO)3 structures appear to be favorable as indicated by high carbonyl dissociation free energies (ΔG) exceeding 15 kcal mol−1. Higher energy CpMS2C2H2·Cr(CO)4 and CpMS2C2H2·Fe(CO)3 structures involve coordination of the C＝C double bond and at least one sulfur atom of the CpMS2C2H2 ligand to the chromium or iron atom but have long metal–metal distances suggesting lack of a direct heterometallic metal–metal bond. In the lowest energy structures of the still carbonyl richer CpMS2C2H2·Cr(CO)5 and CpMS2C2H2·Fe(CO)4 the CpMS2C2H2 ligand is a dihapto ligand that is bonded to the chromium or iron atom through a heterometallic M–M′ (M′ = Cr, Fe) bond and a metal–sulfur bond. Other types of dihapto ligand–metal bonding found in higher energy CpMS2C2H2·Cr(CO)5 and CpMS2C2H2·Fe(CO)4 structures include bonding only the C＝C double bond of the ethylenedithiolate ligand to the chromium or iron atom as in a simple (olefin)M(CO)n complex or bonding of adjacent carbon and sulfur atoms in the CpMS2C2H2 ligand to the chromium or iron atom. The latter mode of dihapto C,S bonding of the CpMS2C2H2 ligand can be interpreted as a π-bond from a C＝S double bond in the dithioglyoxal structure for CpMS2C2H2. [ABSTRACT FROM AUTHOR]

The energetically preferred structures of the gold clusters Au9+, Au113+, and Au124+ with eight skeletal electrons have been studied by density functional theory for comparison with the 8-electron Au102+ cluster shown previously to have a highly favored Td tetracapped octahedral structure. The low-energy structures for the Au9+ and Au113+ clusters are found to be similar relatively spherical polyhedra. Such systems can be considered to exhibit spherical aromaticity in accord with their filled 1S21P6 shells, their diatropic NICS(0) values ranging from −21.4 to −44.3 ppm, and their shielding cone surfaces. However, the preferred spherical polyhedra for Au9+ and Au113+ are not the same as the closo deltahedra found in the BnHn2− borane dianions. Instead they have smaller internal cavities formed by capping faces of smaller deltahedra or by formation of internal Au–Au bonds. The lowest energy Au124+ structures are not similar nearly spherical polyhedral structures. Instead they are derived from planar gold subclusters by adding more gold atoms to form tetrahedral Au4 bubbles. The planar origin of the low-energy Au124+ structures relates to the energetic preference for neutral Au<14 clusters for planar structures or nearly planar structures containing small polyhedral bubbles. The presence of σ-holes has been identified on the surfaces of the complete series of the Aun(n−8)+ (n = 9 to 12) clusters. The strength of their electrostatic interactions is predicted to increase upon increasing cluster size. [ABSTRACT FROM AUTHOR]

The geometries and energetics of the binuclear fluoroborylene manganese carbonyls Mn2(BF)2(CO)n (n = 8, 7, 6) have been investigated using density functional theory. Coupling of two fluoroborylene ligands to give a bridging difluorodiborene μ-B2F2 ligand with a relatively long ∼1.9 Å B–B bond has been observed in two of the three lowest energy Mn2(BF)2(CO)7 structures as well as the lowest energy Mn2(BF)2(CO)6 structure. Higher energy Mn2(BF)2(CO)6 structures have two separate bridging BF groups and short Mn≡Mn distances of ∼2.3 Å suggesting the formal triple bond required by the 18-electron rule. [ABSTRACT FROM AUTHOR]

Density functional theory studies show that the lowest energy C4F8Fe(CO)4 structure is not the very stable experimentally known ferracyclopentane isomer (CF2CF2CF2CF2)Fe(CO)4 obtained from Fe(CO)12 and tetrafluoroethylene. Instead isomeric (perfluoroolefin)Fe(CO)4 structures derived from perfluoro-2-butene, perfluoro-1-butene, and perfluoro-2-methylpropene are significantly lower energy structures by up to ∼17 kcal mol−1. However, the activation energies for the required fluorine shifts from one carbon to an adjacent carbon atom to form these (perfluoroolefin)Fe(CO)4 complexes from tetrafluoroethylene are very high (e.g., ∼70 kcal mol−1). Therefore the ferracyclopentane isomer (CF2CF2CF2CF2)Fe(CO)4, which does not require a fluorine shift to form from Fe3(CO)12 and tetrafluoroethylene, is the kinetically favored product. The lowest energy structures of the binuclear (C4F8)2Fe2(CO)n (n = 7, 6) derivatives have bridging perfluorocarbene ligands and terminal perfluoroolefin ligands. [ABSTRACT FROM AUTHOR]

The known sandwich compound [η5-(CH2)3N2(BPh)2CMe]2Fe in which adjacent C2 units are replaced by isoelectronic BN units can be considered as a boraza analogues of ferrocene similar to borazine, B3N3H6, considered as a boraza analogue of benzene. In this connection, the related bis(1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl) derivatives (Me4B2N2CH)2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) for all of the first row transition metals have been optimized using density functional theory for comparison with the isoelectronic tetramethylcyclopentadienyl derivatives (Me4C5H)2M. Low-energy sandwich structures having parallel B2N2C rings in a trans orientation are found for all seven metals. The 1,2-diaza-3,5-diborolyl ligand appears to be a weaker field ligand than the isoelectronic cyclopentadienyl ligand as indicated by higher spin ground states for some (η5-Me4B2N2CH)2M sandwich compounds relative to the corresponding metallocenes (η5-Me4C5H)2M. Thus (η5-Me4B2N2CH)2Cr has a quintet ground state in contrast to the triplet ground state of (η5-Me4C5H)2Cr. Similarly, the sextet ground state of (η5-Me4B2N2CH)2Mn lies ∼18 kcal mol−1 below the quartet state in contrast to the doublet ground state of the isoelectronic (Me4C5H)2Mn. These sandwich compounds are potentially accessible by reaction of 1,2-diaza-3,5-diborolide anions with metal halides analogous to the synthesis of [η5-(CH2)3N2(BPh)2CMe]2Fe. [ABSTRACT FROM AUTHOR]

The reaction of Mn(CO)5Br with sodium ethylenedithiolate was reported in 1968 to give a dark red binuclear H2C2S2Mn2(CO)6 complex possessing the unusual property of complexing reversibly with ammonia to give a yellow H2C2S2Mn2(CO)6·NH3 adduct. In order to provide some insight into the nature of this adduct, density functional studies were performed on the H2C2S2Mn2(CO)n (n = 4 to 8) systems as well as their relevant ammonia and trimethylphosphine adducts. These theoretical studies support the structure of H2C2S2Mn2(CO)6 originally suggested 50 years ago involving the binding of the ethylenedithiolate C=C double bond as well as the sulfur atoms to the Mn2 unit with a bonding Mn–Mn distance of ∼2.8 Å. Complexation of H2C2S2Mn2(CO)6 with NH3 or Me3P preserves the complexed C=C double bond of the ethylenedithiolate ligand but lengthens the Mn…Mn distance to a non-bonding ∼3.6 Å. Thus H2C2S2Mn2(CO)6 represents a novel type of Lewis acid where reversible complexation with Lewis bases involves the rupture of a metal–metal bond. Carbonyl dissociation energies in the H2C2S2Mn2(CO)n series account for the formation of the hexacarbonyl H2C2S2Mn2(CO)6 as the stable product from the Mn(CO)5Br/ethylenedithiolate reaction. [ABSTRACT FROM AUTHOR]

The two tetranuclear iron carbonyl systems EFe4(CO)n (E = Sn, C) containing central group 14 interstitial atoms differ in that spiropentane-like SnFe4(CO)16 has been synthesized in the tin system whereas the butterfly CFe4(CO)13, with three fewer carbonyl groups is the carbonyl-richest tetranuclear iron carbonyl carbide that has been synthesized. In order to clarify this point, the complete SnFe4(CO)n (n = 16, 15, 14, 13, 12) series has been studied by density functional theory for comparison with earlier similar studies on their CFe4(CO)n analogues. The experimentally observed spiropentane-like Sn[Fe2(CO)8]2 structure is found to be the lowest energy structure for the SnFe4(CO)16 system as it is for the experimentally unknown CFe4(CO)16 system. Loss of a CO group from Sn[Fe2(CO)8]2 joins the two Fe2(CO)8 units by a third Fe–Fe bond to give an SnFe4(CO)15 structure with a bonded four-atom Fe–Fe–Fe–Fe chain. Further CO loss from SnFe4(CO)15 adds a fourth Fe–Fe bond in the lowest energy SnFe4(CO)14 structure. The lowest energy SnFe4(CO)13 structure is analogous to that of the experimentally known iron carbonyl carbide CFe4(CO)13 with a central Fe4 butterfly having five Fe–Fe bonds. The energetics of CO dissociation from the EFe4(CO)n (E = C, Sn; n = 16, 15, 14, 13) species account for the experimentally observed differences between the systems with central tin and central carbon atoms. Thus for the tin systems the CO dissociation energy from SnFe4(CO)16 is relatively high at ∼50 kcal mol−1 consistent with its experimental observation as a stable species. However, for the tetranuclear iron carbonyl carbides CFe4(CO)n, the CO dissociation energies of the species with more than 13 CO groups are all very small or even negative suggesting CFe4(CO)13 to be the carbonyl-richest viable iron tetracarbonyl carbide consistent with experiment. [ABSTRACT FROM AUTHOR]

Iron-capped, linear-carbon-chain dications have been investigated at the M06L/DZP level of quantum chemistry in order to determine their structural and electronic properties, focusing on differences between chains containing odd and even numbers of carbon atoms. Such differences result from changes in the electronic states and the acetylenic or cumulenic nature of the carbon chain. Interestingly, the short even-carbon chains exhibit distinct properties, but upon chain lengthening undergo a transition to structures similar to those of odd-carbon chains, with a turning point around [FeC10Fe]2+. On the other hand, the less extensively investigated short odd-carbon chains, such as [FeC5Fe]2+, [FeC7Fe]2+ and [FeC9Fe]2+, due to synthetic difficulty, are predicted to exhibit more exceptional properties than the short even-carbon chains in every aspect, such as excellent back bonding as well as more cumulenic and more nearly linear structures. This theoretical study suggests that more experimental work should be considered on metal-capped, short linear odd-carbon chains as potential building blocks for novel electronic and optical materials. [ABSTRACT FROM AUTHOR]

Theoretical methods show that the lowest energy bis(butadiene)metal structures (C4H6)2M (M = Ti to Ni) have a perpendicular relative orientation of the two butadiene ligands corresponding to a tetrahedral coordination of the central metal atom to the four C=C double bonds of the butadiene ligands. Distribution of the metal d electrons in the resulting tetrahedral ligand field rationalizes the predicted spin states increasing monotonically from singlet to quartet from nickel to manganese and back from quartet to singlet from manganese to titanium. [ABSTRACT FROM AUTHOR]

The geometries and energetics of tritungstaboranes Cp3W3(H)Bn−3Hn−3 (Cp = η5-C5H5; n = 5 to 12), related to the experimentally known Cp*3W3(H)B8H8 (Cp* = η5-Me5C5), have been investigated using density functional theory and coupled cluster calculations. Such low-energy structures have central W3Bn−3 deltahedra with superimposed bonded W3 triangles. The “extra” hydrogen atom either bridges a deltahedral edge or caps a deltahedral face containing at least one tungsten atom. The tungsten atoms are located at degree 5 to 7 vertices in regions of a relatively low surface curvature whereas the boron atoms are located at degree 3 to 5 vertices in regions of a relatively high surface curvature. The five lowest-energy structures of the 11-vertex tritungstaborane Cp3W3(H)B8H8 all have the same central W3B8 deltahedron and differ only by the location of the “extra” hydrogen atom. The isosceles W3 triangles in these structures have two long ∼3.0 Å W–W edges through the inside of the deltahedron with the third shorter W–W edge of ∼2.7 to ∼2.8 Å corresponding to a surface deltahedral edge. The five Cp3W3(H)B8H8 structures differ only by the location of the “extra” hydrogen atom. The lowest energy such structure has the “extra” hydrogen atom bridging the surface W–W bond and corresponds to the experimental Cp*3W3(H)B8H8 structure, as determined using X-ray crystallography. [ABSTRACT FROM AUTHOR]

The unexpected 2004 discovery of decamethyldizincocene suggested some new possibilities for organometallic compounds in which an M2 unit is sandwiched between two planar carbocyclic rings. Density functional theory shows that the low-energy structures for the dizincocenes Zn2(C5X5)2 (X = H, F) are singlet coaxial structures having two (η5-C5X5)Zn units linked by a Zn–Zn single bond of length ∼2.3 Å. However, the low-energy M2(C5H5)2 (M = Cu, Ni, Co, Fe) structures have perpendicular configurations with bridging C5H5 ligands. They exhibit increasingly higher spin states from singlet (Cu, Ni) to triplet (Ni,Co), quintet (Co,Fe), and septet (Fe). The low-energy structures of the perfluorinated systems M2(C5F5)2 (M = Co, Fe) have irregular geometries with one bent bridging C5F5 ring and one planar terminal pentahapto η5-C5F5 ring, with the metal atom bearing the terminal η5-C5F5 ligand has the favored 18-electron configuration while the other metal atom has only a lower electron configuration and vacant coordination sites. [ABSTRACT FROM AUTHOR]

The long-range characteristics of the induced magnetic field in the bare icosahedral [Al@Al12]− and [Si12]2− clusters reveal inherent characteristics for spherical aromatic and antiaromatic systems. Here, we extend the shielding cone property to these highly symmetrical inorganic examples to achieve a suitable indicator for aromaticity as a reliable method for evaluating the aromaticity of clusters containing interstitial atoms. [ABSTRACT FROM AUTHOR]

Binuclear chromium carbonyl complexes of the general type [MeN(PF2)2]mCr2(CO)n, including the experimentally known [MeN(PF2)2]3Cr2(CO)n (n = 6, 5) species, have been studied by density functional theory (DFT) methods. The lowest energy structures for the three series of [MeN(PF2)2]mCr2(CO)n (m = 1, 2, 3) structures can be grouped into three triads, namely [MeN(PF2)2]Cr2(CO)n (n = 10, 9, 8), [MeN(PF2)2]2Cr2(CO)n (n = 8, 7, 6), and [MeN(PF2)2]3Cr2(CO)n (n = 6, 5, 4). The carbonyl richest structures of each triad, namely [MeN(PF2)2]Cr2(CO)10, [MeN(PF2)2]2Cr2(CO)8, and [MeN(PF2)2]3Cr2(CO)6 have all terminal carbonyl groups, no chromium–chromium bond, and the MeN(PF2)2 ligands bridging the pair of chromium atoms. However, for [MeN(PF2)2]3Cr2(CO)6 a structure with two of the three MeN(PF2)2 ligands chelating to single chromium atoms are energetically competitive. Low-energy singlet spin state structures for the intermediate and carbonyl poorest members of each triad can incorporate a variety of features such as chromium–chromium single and double bonds, MeN(PF2)2 ligands split into bridging MeNPF2 + PF2 groups, and four-electron donor bridging η2-μ-CO groups as required to give each chromium atom the favored 18-electron configuration. Such four-electron donor bridging η2-μ-CO groups are not found in low-energy structures of related binuclear carbonyl complexes [MeN(PF2)2]mM2(CO)n (M = Fe, Ni; m = 1, 2) of the later transition metals iron and nickel. [ABSTRACT FROM AUTHOR]

The structures and energetics of the tetracarbaboranes C4Bn−4Hn (n = 6 to 13) have been investigated by density functional theory and coupled cluster calculations. In general, the lowest energy structures of the tetracarbaboranes C4Bn−4Hn minimize the number of C–C polyhedral edges as well as the degrees of the carbon vertices. For the C4B2H6 and C4B3H7 systems the lowest energy structures are pyramidal structures having all four carbon atoms located on the base of the pyramid. The lowest energy structure for the 9-vertex C4B5H9 system is a capped square antiprism. The frameworks of the lowest energy C4B4H8 and C4B6H10 structures resemble those of the isoelectronic experimentally known B8H12 and B10H14 structures. However, an experimentally known S4 adamantane-like 10-vertex structure found in Me4C4B6Et6 based on a tetracapped octahedron lies only ∼7 kcal mol−1 in energy above the lowest energy structure. The lowest energy structures for the 11- to 13-vertex C4Bn−4Hn (n = 11, 12, 13) systems can be derived from an (n + 1)-vertex closo deltahedron by removing a high-degree vertex. At least three of the four carbon atoms are located on edges of the resulting pentagonal or hexagonal open face in the low-energy structures. However, the structures of the experimentally known R4C4B8H8 (R = Me, Et) obtained from the dimerization of R2C2B4H42− differ from these low-energy structures. The Me4C4B8H8 polyhedron has a C4 chain and two tetragonal faces whereas the Et4C4B8H8 polyhedron has a hexagonal face with two C2 units. These structures lie within 2 kcal mol−1 of each other thereby accounting for the fluxional properties of these systems observed by NMR spectroscopy. [ABSTRACT FROM AUTHOR]

A density functional theory study of experimentally reported [M](C3E2) complexes ([M] = Ir, Ni, Pt surrounded by ligands such as phosphines and halides; E = O, S, Se) suggests that the behaviors of C3O2 and C3E2 (E = S, Se) as ligands in transition metal complexes differ significantly. The lowest energy structures for the [M](C3O2) complexes have the C3O2 ligand bonded to the transition metal through a C=C double bond. However, the lowest energy structures for the [M](C3E2) (E = S, Se) complexes have the C3E2 ligand bonded to the transition metal through a C=E double bond. The latter structures are confirmed by comparison of the experimental infrared frequencies arising from the C3E2 ligand with our theoretical results. [ABSTRACT FROM AUTHOR]

The structures and energetics of the acenaphthylene iron carbonyl complexes (C12H8)Fe(CO)n (n = 4, 3, 2), (C12H8)Fe2(CO)n (n = 8, 7, 6, 5, 4), and (C12H8)Fe3(CO)n (n = 9, 8), including the known (η2-C12H8)Fe(CO)4 and (η5,η3-C12H8)Fe2(CO)5, have been examined using density functional theory. The two experimentally known (η5,η3-C12H8)Fe2(CO)5 structures, related by a thermally reversible photochemical haptotropic rearrangement, lie within ∼1 kcal mol−1 from each other in energy and more than 20 kcal mol−1 below any other (C12H8)Fe2(CO)5 isomers. Decarbonylation of the pentacarbonyl to the tetracarbonyl (C12H8)Fe2(CO)4 is predicted to retain the formal Fe–Fe single bond but to convert one of the CO groups to a four-electron donor bridging η2-μ-CO group. The lowest-energy carbonyl-rich (C12H8)Fe2(CO)n (n = 8, 7, 6) structures lack direct iron–iron bonds and have the ligand hapticities expected from the 18-electron rule. The low CO dissociation energy of ∼5 kcal mol−1 for (C12H8)Fe2(CO)6 suggests that these carbonyl-rich species are not viable. The addition of an Fe(CO)3 moiety to the binuclear (C12H8)Fe2(CO)n (n = 6, 5) complexes to give the trinuclear (C12H8)Fe3(CO)n (n = 9, 8) complexes, respectively, is predicted to be exothermic by ∼50 kcal mol−1. These trinuclear complexes have pentahapto–bis(trihapto) acenaphthylene ligands with an Fe–Fe bond in the octacarbonyl structures. The experimental mononuclear (η2-C12H8)Fe(CO)4 structure with a dihapto acenaphthylene ligand is the lowest energy isomer. The hapticity of the acenaphthylene ligand increases from dihapto to pentahapto or hexahapto upon CO loss to give (C12H8)Fe(CO)n (n = 3, 2). [ABSTRACT FROM AUTHOR]

Open metallocenes containing a metal sandwiched between two U-shaped pentadienyl or substituted pentadienyl ligands have been synthesized by Ernst and co-workers using reactions of pentadienyl or substituted pentadienyl anion reagents with metal halides. The complete series of such open metallocenes (2,4-Me2C5H5)2M of the first row transition metals (M = Ti to Ni) has now been examined using density functional theory. The experimentally known lowest energy structures of the early transition metals are open metallocene structures with two pentahapto U-shaped η5-2,4-Me2C5H5 ligands. Such structures can exist as syn-eclipsed, gauche-eclipsed, and/or anti-eclipsed conformations of similar energies. The theoretical predictions of the lowest energy conformations of the open metallocenes are in excellent agreement with available experimental data. Singlet and triplet spin state structures are preferred energetically for the Ti and Cr derivatives, respectively. For (η5-2,4-Me2C5H5)2V, doublet and quartet spin state structures are spaced in energy by 7.6 kcal mol−1. The (2,4-Me2C5H5)2Mn energy surface is complicated with doublet, quartet, and sextet spin state structures closely spaced in energy. In the sextet spin state of (2,4-Me2C5H5)2Mn only the end carbon atoms of the 2,4-Me2C5H5 ligand are within bonding distance of the manganese atom. This corresponds to tetrahedral C4Mn coordination similar to that found experimentally in the sextet ground state t-butyl derivative (2,4-tBu2C5H5)2Mn as well as the related Mn(CN)42−. The low energy structures for the “open ferrocene” (η5-2,4-Me2C5H5)2Fe are singlet spin state structures with two U-shaped pentahapto η5-2,4-Me2C5H5 ligands but otherwise analogous to ferrocene. The low energy (2,4-Me2C5H5)2M structures of the late transition metals Co and Ni have at least one trihapto η3-2,4-Me2C5H5 ligand. The lowest energy (2,4-Me2C5H5)2Ni structure is a singlet spin state structure with two trihapto η3-2,4-Me2C5H5 ligands and a 16-electron nickel environment analogous to the well-known bisallylnickel, (η3-C3H5)2Ni. [ABSTRACT FROM AUTHOR]

The geometries and energetics of the trirhenaboranes Cp3Re3Bn−3Hn−3 (Cp = η5-C5H5; n = 5 to 12) have been investigated using density functional theory for comparison with the experimentally known oblatocloso dirhenaboranes Cp*2Re2Bn−2Hn−2 (Cp* = η5-Me5C5; n = 8 to 12). The low-energy Cp3Re3Bn−3Hn−3 (7 ≤n≤ 12) structures are found to be Re3Bn−3 deltahedra with internally bonded Re3 triangles. The rhenium atoms are generally located at degree 6 to 8 vertices representing sites of low local curvature and the boron atoms at degree 3 to 5 vertices representing sites of high local curvature. The Re–Re bonds in the Re3 triangles of such clusters typically range from 2.6 to 2.7 Å if they are located on or near the deltahedral surface and from 2.8 to 3.0 Å if they go through the interior of the deltahedron away from the surface. Such highly non-spherical structures, typically having little symmetry, are related to the oblatocloso structures of the dirhenaboranes. A low-energy, more nearly spherical 12-vertex Cp3Re3B9H9 structure, essentially degenerate with the global minimum, has an Re–Re–Re chain embedded in a deltahedron having degree 5 and 6 rhenium vertices and degree 4 and 5 boron vertices. Low-energy structures for the 5-vertex Cp3Re3B2H2 system are derived from a trigonal bipyramid. Similarly, low-energy 6-vertex Cp3Re3B3H3 structures have central Re3B3 bicapped tetrahedra. [ABSTRACT FROM AUTHOR]

Theoretical studies are reported on methylphosphepine, methylbenzophosphepine, their norcaradiene isomers, and their metal complexes with Fe(CO)3 and CpCo (Cp = η5-C5H5) units. Both methylphosphepine and methylphosphanorcaradiene are C6H6PCH3 species existing as anti/syn stereoisomer pairs with the methylphosphepine structures at slightly higher energies. The transition states for the interconversion of these isomers and their benzo derivatives lie ∼20 kcal mol−1 in energy above the methylphosphanorcaradiene isomers. Complexation of either C6H6PCH3 ligand with the transition metal units Fe(CO)3 and CoCp leads to energetically closely spaced η4-tetrahapto and η3-trihapto isomers of the methylphosphepine complexes and η4-tetrahapto isomers of the methylphosphanorcaradiene complexes. However, the bis(dihapto) (η2,2-C6H6PCH3)Fe(CO)3 and (η2,2-C6H6PCH3)CoCp complexes involving coordination of non-adjacent C=C double bonds lie at significantly higher energies. Fusion of a benzene ring to the C6H6PCH3 rings in methylphosphepine and methylphosphanorcaradiene leads to significantly different structures of their lowest energy metal complexes. Thus the lowest energy (C10H8PCH3)Fe(CO)3 and (C10H8PCH3)CoCp structures are η2,2 and η4 methylbenzophosphepine complexes, which avoid using any carbon atoms of the benzene ring in the ligand for metal complexation. Higher energy (C10H8PCH3)Fe(CO)3 and (C10H8PCH3)CoCp structures have tetrahapto ligands with one or both C=C double bonds of the benzene ring complexed with the metal atom. [ABSTRACT FROM AUTHOR]

The experimentally known CpFeMn(CO)7 system as well as the related unsaturated CpFeMn(CO)n systems (n = 6, 5) have been investigated by density functional theory. For CpFeMn(CO)7 unbridged and doubly bridged structures with a heteronuclear Fe–Mn single bond lie within ∼5 kcal mol−1 of each other with the experimentally known unbridged structure being the lower energy structure. The three lowest-energy unsaturated CpFeMn(CO)6 structures, lying within ∼1 kcal mol−1 of each other, have very diverse structural features. The lowest-energy CpFeMn(CO)6 structure contains an unusual three-center two-electron C–H–Mn bond involving an agostic hydrogen of the Cp ring. Another such structure is a triply bridged triplet CpFe(μ-CO)3Mn(CO)3 closely related to the experimentally known Cp2Fe2(μ-CO)3 having a central UGRAPHIC DISPLAY="INLINE" ID="UGT2" SRC="UGT2"/ Fe=Mn double bond containing the two unpaired electrons of the triplet spin state. The third low-energy CpFeMn(CO)6 structure is a CpFe(CO)Mn(CO)4(η2-μ-CO) structure with a four-electron donor bridging η2-μ-CO group similar to that found in the lowest energy Mn2(CO)9 structure. The lowest-energy structure of the even more unsaturated CpFeMn(CO)5 has a short Fe≡Mn distance of only ∼2.2 Å suggesting a formal triple bond. Higher energy CpFeMn(CO)5 structures have a four-electron donor η2-μ-CO group in addition to a formal Fe=Mn double bond. The CpFeMn(CO)5 structure in which the oxygen atom of the η2-μ-CO group is bonded to manganese lies ∼9 kcal mol−1 in energy below the isomeric structure in which the oxygen of the η2-μ-CO group is bonded to iron. [ABSTRACT FROM AUTHOR]

Uhl and coworkers have synthesized and characterized structurally the 11-vertex tetracarbalanes (AlMe)7(CEt)4(μ-H)2 and (AlEt)7(CCH2Ph)4(C≡CPh)(μ–H). In order to understand the nature of the unusual C4Al7 polyhedra in such systems, the complete series of permethylated tetracarbalanes C4Aln−4Men (n = 6 to 14) as model compounds have been investigated using density functional theory. An overriding factor in determining the polyhedra for the lowest energy structures of a wide range of tetracarbalanes C4Aln−4Men (n = 6 to 14) is the availability of vertices of degrees 3 and 4 for all four carbon atoms. For the 11-vertex C4Al7Me11 system the experimentally observed C4Al7 deltahedron with carbon atoms at the four degree 4 vertices is found in the lowest energy structure but severely distorted from ideal C2h to Cs symmetry. This distortion is removed by adding two bridging hydrogens to the C4Al7Me11 structure in the same locations as in the experimental structures. The lowest energy structures of the tetracarbalanes C4Aln−4Men (n = 12, 13, 14) with more than 11 vertices are deltahedra having the carbon atoms at degree 4 vertices similar to the lowest energy C4Al7Me11 structures. The lowest energy structures for the smaller tetracarbalanes C4Aln−4Men (n = 6, 7, 8) are nido structures with pentagonal or hexagonal open faces related to nido borane structures. The intermediate 9- and 10-vertex tetracarbalanes C4Aln−4Men (n = 9, 10) have relatively complicated energy surfaces. [ABSTRACT FROM AUTHOR]

The structures and energetics of the complete series of hydrogen-rich dimetallaboranes Cp2M2B6H10 and Cp*2M2B6H10 (Cp = η5-C5H5; Cp* = η5-Me5C5; M = Pd, Pt; Rh, Ir; Ru, Os; Re; Mo, W; Ta), including the experimentally known Cp*2Rh2B6H10 and Cp*2W2B6H10 (Cp* = η5-Me5C5), have been investigated by density functional theory. The lowest energy structures of the hyperelectronic Cp2M2B6H10 (M = Pd, Pt; Rh, Ir) systems have central M2B6 frameworks with a hexagonal open face similar to the B8 networks in arachno-B8H14 and nido-B8H12. The two lowest energy structures for Cp2Rh2B6H10 and Cp*2Rh2B6H10, lying within 1 kcal mol−1 of energy, differ only in the locations of the bridging hydrogen atoms around the hexagonal hole consistent with the experimentally observed fluxionality of the hydrogen atoms in Cp*2Rh2B6H10. Most of the lowest energy Cp2M2B6H10 (M = Ru, Os) structures also have a central M2B6 framework similar to B8H12, typically with such additional features as an additional metal–metal bond or a formal metal–metal double bond. A common motif for the low-energy structures of the hypoelectronic Cp2M2B6H10 (M = Re; Mo, W; Ta) systems, including the experimentally known Cp*2W2B6H10, is a central M2B4 octahedron with its two M2B faces capped by the remaining boron atoms and with four M–B edges bridged by hydrogen atoms. Such structures can also be considered as oblatonido structures derived from the experimentally known 9-vertex oblatocloso Cp*2Re2B7H7 structure by removal of the unique degree 4 vertex atom. [ABSTRACT FROM AUTHOR]

Consideration of the well-known very stable icosahedral B12H122− as a three-dimensional analogue of benzene was extended by the recent synthesis of the biicosahedral B21H18− as a three-dimensional analogue of naphthalene. The preferred structures of metallaboranes derived from B21H18− have now been examined by density functional theory. The isoelectronic species CpNiB20H17 and CpCoCB19H17 have the 46 skeletal electrons expected by the Wade-Mingos and Jemmis rules for a structure consisting of two face-sharing fused icosahedra. The CpM units in these structures energetically prefer to be located at a meta vertex of the biicosahedron. The analogous ferraboranes CpFeB20H17 with only 44 skeletal electrons also have related biicosahedral structures. The presence of an agostic hydrogen atom bridging an Fe–B edge compensates for the two-electron deficiency in CpFeB20H17 relative to CpNiB20H17. The nucleus-independent chemical shift (NICS) values of these systems indicate them to be strongly aromatic. [ABSTRACT FROM AUTHOR]

Reactions of cyclooctatetraene with iron carbonyls under various conditions give not only the monomeric (η4-C8H8)Fe(CO)3 but three C8H8Fe2(CO)6 isomers and one C8H8Fe2(CO)5 isomer. Density functional theory on the C8H8Fe2(CO)6 system shows the trans-(η4,η4-C8H8)Fe2(CO)6 isomer to be the lowest energy isomer. The cis-(η3,η3-C8H8)Fe2(CO)6 isomer with an Fe–Fe bond and an uncomplexed C=C double bond in the C8H8 ring lying ∼11 kcal mol−1 in energy above trans-(η4,η4-C8H8)Fe2(CO)6 appears to correspond to one of the metastable C8H8Fe2(CO)6 isomers obtained under relatively mild conditions. However, the cis-(η4,η4-C8H8)Fe2(CO)6 structure without an Fe–Fe bond suggested for the other metastable isomer appears to be a high-energy structure with a large imaginary vibrational frequency. Following the corresponding normal mode leads to cis-(η3,η3-C8H8)Fe2(CO)6. For C8H8Fe2(CO)5 the two lowest energy structures are singly bridged cis-C8H8Fe2(CO)4(μ-CO) structures differing only by a 22.5° rotation of the C8H8 ring around the central Fe2 unit. One of these structures is the experimental C8H8Fe2(CO)5 structure. The closeness in energy of these two C8H8Fe2(CO)5 structures is consistent with the experimentally observed fluxionality of this molecule in the NMR spectrum at low temperatures. The unsaturated C8H8Fe2(CO)n (n = 4, 3) structures obtained by further decarbonylation of C8H8Fe2(CO)5 retain the bridging bis(tetrahapto) or bis(pentahapto) C8H8 rings of C8H8Fe2(CO)5 and provide examples of structures with formal Fe=Fe double bonds with the lowest energy such structures having triplet rather than singlet spin states. Viable carbonyl-rich (η2,η2-C8H8)Fe2(CO)8 and (η4,η2-C8H8)Fe2(CO)7 structures represent possible intermediates in the formation of the various C8H8Fe2(CO)n (n = 6, 5) species from cyclooctatetraene and iron carbonyls. [ABSTRACT FROM AUTHOR]

Thiophene is known experimentally to be desulfurized by Fe3(CO)12 under mild conditions to give the tricarbonyl ferrole (η4,η2-C4H4)Fe2(CO)6. A similar reaction of benzothiophene (thianaphthene) with Fe3(CO)12 gives a (C8H6S)Fe2(CO)6 complex in which an iron carbonyl moiety has inserted into the thiophene ring to give a thiaferranaphthalene ligand. Density functional theory shows this experimental structure to be the lowest energy structure. Furthermore, the lowest energy structures of the diiron pentacarbonyl (C8H6S)Fe2(CO)5 are simply derived from this (C8H6S)Fe2(CO)6 by loss of a CO group retaining the thiaferranaphthalene ligand. However, a higher energy isomeric (η6,η2-C8H6S)Fe2(CO)5 structure retains the original benzothiophene ligand with the C6 ring bonded to an Fe(CO)2 moiety as a hexahapto ligand and the C=C double bond of the C4S ring bonded to an Fe(CO)3 moiety as a dihapto ligand with an Fe→Fe dative bond between the iron atoms. Similar insertion of an iron atom into a thiophene ring to give a thiaferrabenzene ring is predicted to occur in the lowest energy (C6H4S2)Fe2(CO)6 structure derived from either the anti or syn isomers of thienothiophene. However, the bonding of the exocyclic iron atom to the resulting thiaferrabenzothiophene ligand involves atoms in both rings in contrast to the (C8H6S)Fe2(CO)6 complex where the benzene ring is not involved in the ligand–iron bonding. [ABSTRACT FROM AUTHOR]