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The skeletal bonding topology as well as the Re=Re distances and Wiberg bond indices in the experimentally known oblatocloso dirhenaboranes Cp*2Re2Bn-2Hn-2 (Cp*=η5Me5C5, n=8-12) suggest formal Re=Re double bonds through the center of a flattened Re2Bn-2 deltahedron. Removal of a boron vertex from these oblatocloso structures leads to oblatonido structures such as Cp2W2B5H9 and Cp2W2B6H10. Similar removal of two boron vertices from the Cp2Re2Bn-2Hn-2 (n=8-12) structures generates oblatoarachno structures such as Cp2Re2B4H8 and Cp2Re2B7H11. Higher energy Cp2Re2Bn-2Hn-2 (Cp=η5-C5H5, n=8-12) structures exhibit closo deltahedral structures similar to the deltahedral borane dianions BnHn2-. The rhenium atoms in these structures are located at adjacent vertices with ultrashort Re≣Re distances similar to the formal quadruple bond found in Re2Cl82- by X-ray crystallography. Such surface Re≣Re quadruple bonds are found in the lowest energy PnRe2Bn-2Hn-2 structures (Pn=η5,η5-pentalene) in which the pentalene ligand forces the rhenium atoms to occupy adjacent deltahedral vertices. The low-energy structures of the tritungstaboranes Cp3W3(H)Bn-3Hn-3 (n=5-12), related to the experimentally known Cp*3W3(H)B8H8, have central W3Bn-3 deltahedra with imbedded bonded W3 triangles. Similar structures are found for the isoelectronic trirhenaboranes Cp3Re3Bn-3Hn-3. The metal atoms are located at degree 6 and 7 vertices in regions of relatively low surface curvature whereas the boron atoms are located at degree 3-5 vertices in regions of relatively high surface curvature. The five lowest-energy structures for 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 sW-W edge of ~2.7 to ~2.8 Å corresponding to a surface deltahedral edge. [ABSTRACT FROM AUTHOR]

The opposed and parallel structures for the binuclear bis(azulene) 'submarine' sandwiches (C10H8)2M2 (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been optimized using density functional theory. The lowest energy (C10H8)2M2 structures of the early transition metals Ti, V, Cr, and Mn have the azulene units functioning as bis(pentahapto) ligands to each metal atom similar to the azulene ligand in the long-known molybdenum carbonyl complex (η5,η5-C10H8)Mo2(CO)6. The metal-metal bonds in these early transition metal structures have distances and Wiberg bond indices consistent with the formal bond orders required to give each metal atom an 18-electron configuration for the singlet structures and a 17-electron configuration for the triplet structures. For the later transition metals Fe, Co, and Ni, the lowest energy (C10H8)2M2 structures contain pentahapto-trihapto azulene ligands with an uncomplexed CC double bond, similar to that in the long-known iron carbonyl complex (η5,η3-C10H8)Fe2(CO)5. The parallel (η5,η3-C10H8)2M2 (M = Fe, Co, Ni) structures contain metallocene subunits with their metal atoms at long nonbonding distances of 3.5-3.9 Å from the other metal atom, which is located between the azulene C7 rings. Higher energy opposed (C10H8)2Fe2 structures contain an unprecedented distorted η6,η4-azulene ligand using six carbon atoms for bonding to one iron atom as a hexahapto fulvene ligand and the remaining four carbon atoms for bonding to the other iron atom as a tetrahapto diene ligand. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: The following four Cp2M2B n−2H n−2 structure types (M = Mo, W) have been found in the low-energy structures using density functional theory: (1) Flattened oblatocloso deltahedra related to the experimentally known Cp2Re2B n−2H n−2 structures but not as severely flattened with intradeltahedral M–M (M = Mo, W) distances of ∼2.8 to ∼3.2 Å, which are 0.15–0.30 Å less than those in their Tc and Re counterparts; (2) Structures based on closo or isocloso deltahedra with short M M distances (M = Mo, W) of ∼2.4 to ∼2.6 Å along an edge corresponding to deltahedral surface metal–metal triple bonds; (3) Three 11-vertex deltahedral Cp2M2B9H9 structures (M = Mo, W) structures having two non-adjacent degree 6 vertices occupied by the metal atoms; (4) Two 8-vertex Cp2M2B6H6 structures with multiple degree 3 vertices arising from the capping of smaller deltahedra. In many of these structures one or two of the hydrogen atoms bridge M–B or B–B edges of the deltahedron. [Copyright &y& Elsevier]

Abstract: Dicobalt octacarbonyl is known to react with diolefins to give substitution products of the types (diene)Co2(CO)6 and (diene)2Co2(CO)4. The butadiene derivatives (C4H6)Co2(CO) n (n =6, 5, 4, 3, 2) have been investigated by density functional theory using the B3LYP and BP86 methods. The lowest energy (C4H6)Co2(CO) n (n =6, 5, 4) structures have bridging CO groups and terminal butadiene ligands. For the (C4H6)Co2(CO)6 and (C4H6)Co2(CO)5 structures the Coces of ∼2.5Å suggest formal single bonds. However, for the lowest energy (C4H6)Co2(CO)4 structure the significantly shorter Coce of ∼2.3Å suggests the formal triple bond required to give the cobalt atom the favored 18-electron configuration. Bridging butadiene ligands are also found in (C4H6)Co2(CO) n structures including all of the lowest energy (C4H6)Co2(CO)3 and (C4H6)Co2(CO)2 structures. Both the B3LYP and BP86 methods predict butadiene dissociation from (C4H6)2Co2(CO)4 to be energetically favored over CO dissociation by ∼8kcal/mol. For (C4H6)2Co2(CO) n (n =3, 2) the BP86 method predicts CO dissociation to be favored energetically over butadiene dissociation by ∼8kcal/mol. However, the B3LYP method predicts essentially equal CO and butadiene dissociation energies within ∼1kcal/mol from (C4H6)2Co2(CO) n (n =3, 2). [Copyright &y& Elsevier]

Transition metal complexes with bridging PF3 groups are unknown analogous to metal carbonyls with bridging carbonyl groups. The possibility of bridging PF3 groups in binuclear Co2(PF3)n complexes (n = 9, 8, 7, 6, 5) and the tetranuclear Co4(PF3)12 has now been investigated by density functional theory (DFT). The lowest energy Co2(PF3)8, Co2(PF3)7, and Co4(PF3)12 structures are predicted to be unbridged structures with cobalt-cobalt bond distances of ∼2.8 Å, ∼2.5 Å, and ∼2.6 Å, respectively. The lowest energy Co2(PF3)6 structure is an unsymmetrical unbridged structure with a Co-Co distance of ∼2.5 Å. For the more highly unsaturated Co2(PF3)5 both triplet and singlet structures are found, consisting of two Co(PF3)2 units linked by two bridges, namely a PF2 group and a F atom. Such Co2(PF3)5 structures have square planar cobalt coordination and a Co...Co distance too long for direct bonding. In addition, a singlet Co2(PF3)5 structure is found, with two semi-bridging PF3 groups and a very short Co ··· Co distance of ∼2.1 Å indicative of the formal quadruple bond required to give both cobalt atoms the favoured 18-electron configuration. Bridging PF3 groups with no cobalt-cobalt bond are found in a high energy structure of Co2(PF3)7 and in a Co2(PF3)9 structure unstable with respect to PF3 loss to give Co2(PF3)8. The dissociation of Co2(PF3)8 into two Co(PF3)4 radicals is predicted to be only slightly endothermic at -14.3 kcal/mol (B3LYP) or 7.1 kcal/mol (BP86). However, in contrast to its carbonyl analogue Co4(CO)12, the dissociation of Co4(PF3)12 into 2 Co2(PF3)6 is highly exothermic. Optimization of the structure of the experimentally known but not structurally characterized bis(difluorophosphido) derivative Co2(PF3)6(µ-PF2)2 predicts the expected structure with a long Co ··· Co distance of ∼3.5 Å indicating the lack of a direct cobalt-cobalt bond. DFT predicts tetrahedral structures for Co(PF3)4 and HCo(PF3)3 as well as the experimentally known trigonal bipyramidal structure for HCo(PF3)4. [ABSTRACT FROM AUTHOR]

The heterometallic chromium-manganese carbonyl nitrosyls CrMn(NO)(CO)n (n = 9, 8) have been investigated by density functional theory. The lowest energy CrMn(NO)(CO)9 structures have unbridged staggered conformations with a ~2.99 Å Cr–Mn single bond similar to the experimental and lowest energy structures of the isoelectronic Mn2(CO)10 and Cr2(NO)2(CO)8. A significantly higher energy CrMn(NO)(CO)9 isomer has a nearly symmetrical bridging nitrosyl group and a very weakly semibridging carbonyl group. The two lowest energy structures of the unsaturated CrMn(NO)(CO)8 have a five-electron donor bridging η2-µ-NO nitrosyl group or a four-electron donor bridging η2-µ-CO group, as well as a Cr–Mn single bond of length ~2.94 Å. The next higher energy CrMn(NO)(CO)8 structure has exclusively terminal CO and NO ligands and a shorter Cr–Mn single bond of ~2.85 Å, suggesting an 18-electron configuration for the manganese atom and a 16-electron configuration for the chromium atom indicated by a vacant coordination site nearly perpendicular to the Cr–Mn bond. [ABSTRACT FROM AUTHOR]

The structures and energetics of the tetracarbindanes C4Inn−4Men (n = 6‐14) have been determined by density functional theory. In contrast to their aluminum and gallium analogs, the lowest energy tetracarbindanes typically have all four carbon atoms segregated into a single C4 unit. Thus, linear C4 units resembling butadiene are found in the lowest energy C4Inn−4Men structures. In addition, some higher energy tetracarbindane structures have a structural feature not found in any of the corresponding tetracarbalanes and tetracarbagallanes, namely closed trapezoidal C4 units resembling cyclobutene. Such trapezoidal C4 units bind to the Inn−4 subcluster with the CC edge bonding to a single indium atoms as an olefin‐metal or 3‐center 2‐electron bond. These differences may be attributed to the larger size of indium atoms (1.42 Å covalent radius) relative to gallium atoms (1.22 Å covalent radius). [ABSTRACT FROM AUTHOR]

The structures and energetics of the binuclear pentalene titanium carbonyls PnTi2(CO)n (Pn = η5,η5‐C8H6; n = 8, 7, 6, 5, 4, 3, 2) and their 1,4‐bis(triisopropylsilyl) derivatives related to experimentally studied systems have been examined by density functional theory. Geometric constraints within a η5,η5‐PnTi2 unit having exclusively pentahapto ring‐titanium bonds limit the TiTi distances to a maximum of ~3.1 Å in the PnTi2(CO)n derivatives in contrast to the cyclopentadienyl analogues Cp2Ti2(CO)n where bonding TiTi distances as long as ~3.9 Å are found. Furthermore, at least one 4‐electron donor η2‐μ‐CO group but never more than 2 such groups are found in the lowest energy PnTi2(CO)n structures. The lowest energy PnTi2(CO)n structures (n = 4, 3, 2) by margins of at least 10 kcal/mol have 2 η2‐μ‐CO groups and formal TiTi triple bonds in the singlet spin state. The carbonyl rich species PnTi2(CO)n and Pn†Ti2(CO)n (n = 8, 7) are not viable with respect to CO dissociation to give the corresponding hexacarbonyls. The structures and energetics of the binuclear pentalene titanium carbonyls PnTi2(CO)n (Pn = η5,η5‐C8H6; n = 8, 7, 6, 5, 4, 3, 2) and their 1,4‐bis(triisopropylsilyl) derivatives related to experimentally studied systems have been examined by density functional theory. Geometric constraints limit the TiTi distances to a maximum of ~3.1 Å. At least one 4‐electron donor η2‐μ‐CO group but never more than 2 such groups are found in the lowest energy PnTi2(CO)n structures. [ABSTRACT FROM AUTHOR]

Cyclooctatetraene has been found experimentally to form the perpendicular binuclear sandwich compounds (η 3 ,η 3 -C 8 H 8 ) 2 Ni 2 and [η 5,5 -(Me 3 Si) 2 C 8 H 6 ] 2 Ti 2 . The entire series of such cyclooctatetraene sandwich compounds (C 8 H 8 ) 2 M 2 has now been examined by density functional theory. Bis(pentahapto) folded η 5 ,η 5 -C 8 H 8 rings are found in the lowest energy (C 8 H 8 ) 2 M 2 derivatives of the early transition metals Ti, V, Cr, and Mn. The lowest energy Ti derivative is the singlet spin state structure (η 5 ,η 5 -C 8 H 8 ) 2 Ti 2 containing two such rings with geometry close to the experimental structure for the known silylated derivative [η 5,5 -(Me 3 Si) 2 C 8 H 6 ] 2 Ti 2 . The lowest energy (C 8 H 8 ) 2 M 2 structures for V, Cr, and Mn are (η 5 ,η 5 -C 8 H 8 )M 2 (η 3 ,η 3 -C 8 H 8 ) having metal–metal multiple bonds of various orders. For (η 5 ,η 5 -C 8 H 8 )V 2 (η 3 ,η 3 -C 8 H 8 ) singlet and triplet spin state structures are approximately equal in energy. For (η 5 ,η 5 -C 8 H 8 )M 2 (η 3 ,η 3 -C 8 H 8 ) (M = Cr, Mn) the triplet state structures are the lowest energy structures. The lowest energy (C 8 H 8 ) 2 M 2 structures for the later transition metals Fe, Co, Ni are triplet spin state structures for Fe and Co and a singlet spin state structure for Ni. All three such (η 3 ,η 3 -C 8 H 8 ) 2 M 2 (M = Fe, Co, Ni) structures have two bis(trihapto) η 3 ,η 3 -C 8 H 8 rings containing uncomplexed C C double bonds. [ABSTRACT FROM AUTHOR]

The cyclopentadienyliridium hydrides Cp 2 Ir 2 H n (Cp = η 5 -C 5 H 5 ; n = 6, 4, 2, 0), and CpIrH n ( n = 4, 2) related to the experimentally known pentamethyl cyclopentadienyliridium hydrides Cp ∗ 2 Ir 2 H n (Cp ∗ = η 5 -Me 5 C 5 ; n = 6, 2) and Cp ∗ IrH 4 have been investigated by density functional theory. The lowest energy Cp 2 Ir 2 H n ( n = 6, 4, 2) structures are predicted to have terminal Cp rings with the central Ir 2 unit bridged by two hydrogen atoms. For the hexahydride Cp 2 Ir 2 H 6 , such doubly bridged Cp 2 Ir 2 (μ-H) 2 H 4 structures are bent, leading to trans and cis structures of similar energies with an unbridged Cp 2 Ir 2 H 6 isomer lying only ∼4 kcal/mol above the bridged structures. This suggests fluxional behavior consistent with experimental data on the temperature dependence of the proton NMR spectrum of the closely related Cp ∗ 2 Ir 2 H 6 . The tetrahydride Cp 2 Ir 2 H 4 is predicted to undergo slightly exothermic disproportionation into Cp 2 Ir 2 H 6 + Cp 2 Ir 2 H 2 and thus not be a viable species. This is consistent with the failure to find any Cp ∗ 2 Ir 2 H 4 in the Cp ∗ 2 Ir 2 H n systems. The doubly bridged dihydride Cp 2 Ir 2 (μ-H) 2 is a particularly favorable species since it lies more than 18 kcal/mol in energy below any other isomer. Higher energy Cp 2 Ir 2 H n ( n = 4, 2) structures have one or two bridging Cp rings and exclusively terminal hydrogen atoms. Related structures are the lowest energy structures for the hydride-free Cp 2 Ir 2 . A higher energy Cp 2 Ir 2 structure consists of two CpIr units linked solely by an Ir–Ir bond. Analysis of the frontier molecular orbitals indicates this Ir–Ir bond to be the quadruple bond required to give each iridium atom the favored 18-electron configuration. However, this quadruple bond is a 2 σ + 2π bond with no δ components and thus differs from the σ + 2π + δ quadruple bond found in the long-known Re 2 Cl 8 2− . [ABSTRACT FROM AUTHOR]

The Cp 2 Fe 2 (vegiMe)(CO) n (n = 3, 2, 1) complexes of the chelating biscarbene vegiMe ligand have been investigated by density functional theory. Triplet and quintet Cp 2 Fe 2 (vegiMe)(CO) n (n = 2, 1) structures have similar energies to their singlet isomers relating to the relatively weak field biscarbene ligand. [Display omitted] • The Cp 2 Fe 2 (vegiMe)(CO) n (n = 3, 2, 1) complexes of the weak field rigid dicarbene vegiMe have been investigated by DFT. • The tricarbonyls Cp 2 Fe 2 (vegiMe)(CO) 3 are disfavored relative to CO dissociation. • Dicarbonyl structures have the vegiMe ligand bridging an Fe-Fe σ bond. • Triplet and quintet Cp 2 Fe 2 (vegiMe)(CO) n (n = 2, 1) structures have comparable energies to singlet structures. Binuclear cyclopentadienyliron carbonyl derivatives of the weak-field rigid chelating bidentate dicarbene vegiMe ligand with a tricyclic backbone are shown by theoretical studies on the Cp 2 Fe 2 (vegiMe)(CO) n (n = 3, 2, 1) systems to have higher spin triplet and quintet structures of comparable energies to isomeric singlet structures satisfying the 18-electron rule. Thus triplet structures of the tricarbonyl Cp 2 Fe 2 (vegiMe)(CO) 3 with the vegiMe ligand as a bidentate chelate to a single iron atom are of comparable energies to singlet isomers with no iron-iron bond and the vegiMe ligand bridging the two iron atoms. Similarly, triplet and quintet structures for the dicarbonyl Cp 2 Fe 2 (vegiMe)(CO) 2 are of comparable energies to singlet structures with Fe–Fe single bonds that are substitution products of the well-known Cp 2 Fe 2 (CO) 2 (µ-CO) 2. The only low-energy singlet structure for the monocarbonyl Cp 2 Fe 2 (vegiMe)(CO) has an unusual six-electron donor bridging vegiMe ligand forming not only the usual two C(carbene)→Fe dative bonds but also a N→Fe dative bond. The other low-energy Cp 2 Fe 2 (vegiMe)(CO) structures are various types of triplet and quintet spin state structures. [ABSTRACT FROM AUTHOR]

Abstract: The recent synthesis by Vidović and Aldridge of the complex (η 5-C5H5)2Ru2(CO)4(μ-BF) containing the fluoroborylene ligand (BF), isoelectronic with CO, suggests the possibility of introducing the BF ligand into binuclear cobalt carbonyl derivatives of the type Co2(BF)2(CO) n (n =7, 6, 5, 4, 3). This has now been explored using density functional theory. Structures in which both BF groups bridge a cobalt–cobalt bond are energetically preferred for the Co2(BF)2(CO) n (n =6, 5, 4, 3) derivatives over other types of structures. Thus the Co2(μ-BF)2(CO)6 structure with two bridging BF groups lies more than 18kcal/mol below the next lowest Co2(BF)2(CO)6 structure and appears to be a reasonable synthetic objective. This differs from the well-known isoelectronic Co2(CO)8 for which the structures with two bridging CO groups and with all terminal CO groups are so close in energy that they are found experimentally to exist in equilibrium. Examples of Co2(B2F2)(CO) n (n =7, 4, 3) derivatives are found in which the two BF groups have coupled to form a bridging difluorodiborene (B2F2) ligand with a B–B distance of ∼1.9Å. However, the Co2(B2F2)(CO)7 structures do not appear to be viable since loss of a CO group to give Co2(BF)2(CO)6 is predicted to be an exothermic process. [Copyright &y& Elsevier]

Abstract: The qualitative aspects of the chemistry of the fluoroborylene iron and ruthenium carbonyls M(BF)(CO) m (m =4, 3) and M2(BF)2(CO) n (n =7, 6) (M=Fe, Ru) are predicted to be very similar. For Ru(BF)(CO)4 the trigonal bipyramidal structures with the BF group in an equatorial position and in an axial position are both found with the equatorially substituted isomer lying ∼2kcal/mol below the axially substituted isomer. For the coordinately unsaturated Ru(BF)(CO)3 both singlet and triplet structures are found derived from the equatorially substituted Ru(BF)(CO)4 structure by removal of an equatorial CO group. The structures for the binuclear derivatives Ru2(BF)2(CO) n (n =7, 6) provide examples of lower energy structures with bridging BF groups relative to similar structures with bridging CO groups. The lowest energy Ru2(BF)2(CO)7 structure has two bridging BF groups and one CO group and is qualitatively similar to the well-known Fe2(CO)9 structure with three bridging CO groups. All of the structures of the unsaturated Ru2(BF)2(CO)6 within 30kcal/mol of the global minimum have two bridging groups. The structure with one bridging CO group and one bridging BF group lies ∼16kcal/mol above the global minimum with two bridging BF groups again showing the preference for bridging BF groups over bridging CO groups. A triplet Ru2(BF)2(CO)6 structure with two bridging BF groups was also found but at the high energy of ∼26kcal/mol above that of the corresponding singlet global minimum. This singlet–triplet splitting is much larger than the singlet–triplet splitting of the corresponding Fe2(BF)2(CO)6. [Copyright &y& Elsevier]

• Low-energy (Me 2 C 2 N 2 P) 2 Ru 2 (CO) n (n = 4, 3) structures typically have dihapto bridging diazaphospholyl ligands. • Terminal η5-Me 2 C 2 N 2 P ligands or bridging η5,η1-Me 2 C 2 N 2 P ligands are found in some higher energy structures. • The (Me 2 C 2 N 2 P) 2 Fe 2 (CO) n (n = 4, 3) systems have complicated potential energy surfaces with higher spin state structures. The structures and energies of the (Me 2 C 2 N 2 P) 2 Ru 2 (CO) n (n = 4, 3) complexes of the four isomeric diazaphospholyl ligands have been studied by density functional theory. Most of the low-energy structures in these systems have one or both diazaphospholyl rings bridging a central Ru Ru bond through Ru N and/or Ru P bonds without involvement of the other three ring atoms. A few higher energy structures are found in these systems having either terminal pentahapto η5-Me 2 C 2 N 2 P ligands or bridging η5,η1-Me 2 C 2 N 2 P ligands bonded to one ruthenium atom as a pentahapto ligand and to the other ruthenium atom through a P → Ru or N → Ru dative bond. The potential surfaces of the analogous iron systems (Me 2 C 2 N 2 P) 2 Fe 2 (CO) n (n = 4, 3) are complicated considerably by other triplet and even quintet structures of comparable energies to structures analogous to the low-energy ruthenium structures. This relates to the lower ligand field strengths of iron complexes as compared with their ruthenium analogues. [Display omitted] [ABSTRACT FROM AUTHOR]

Graphical abstract The lowest energy structures for the tetracarbonyl (C 4 H 4 BH) 2 Co 2 (CO) 4 are doubly bridged structures similar to (C 4 H 4 BCH 3) 2 Co 2 (CO) 4. The lowest energy structures for the unsaturated derivatives (C 4 H 4 BH) 2 Co 2 (CO) n (n = 3, 2, 1) have bridging borole ligands with agostic hydrogen atoms from their B–H groups to a cobalt atom. Highlights • The (C 4 H 4 BH) 2 Co 2 (CO) n (n = 4, 3, 2, 1) systems have been examined by density functional theory. • Low-energy doubly bridged (C 4 H 4 BH) 2 Co 2 (CO) 2 (µ-CO) 2 structures are found similar to (C 4 H 4 BCH 3)Co 2 (CO) 2 (µ-CO) 2. • The lowest energy structures for (C 4 H 4 BH) 2 Co 2 (CO) n (n = 3, 2, 1) have agostic B–H hydrogens from bridging borole ligands. Abstract The (C 4 H 4 BH) 2 Co 2 (CO) n (n = 4, 3, 2, 1) systems have been examined by density functional theory for comparison with the analogous (C 4 H 4 BCH 3) 2 Co 2 (CO) n systems. For the tetracarbonyl (C 4 H 4 BH) 2 Co 2 (CO) 4 a higher energy unbridged structure is found at ∼7 kcal/mol above the lowest energy doubly bridged structure similar to (C 4 H 4 BCH 3) 2 Co 2 (CO) 4. The lowest energy structures for the unsaturated derivatives (C 4 H 4 BH) 2 Co 2 (CO) n (n = 3, 2, 1) have bridging borole ligands with agostic hydrogen atoms from their B–H groups to a cobalt atom. Structures without agostic hydrogens analogous to the low-energy structures for the corresponding methylborole derivatives (C 4 H 4 BCH 3) 2 Co 2 (CO) n are higher energy (C 4 H 4 BH) 2 Co 2 (CO) n structures. [ABSTRACT FROM AUTHOR]

Metal complexes of the 2,4-di- t- butyl-1,3-diphosphacyclobutadiene ligand have been synthesized by head-to-tail dimerization of t- BuC P: on transition metal sites. In this connection the geometries and energetics of the complete series of binuclear cobalt carbonyl complexes (Me 2 C 2 P 2 ) 2 Co 2 (CO) n ( n = 5, 4, 3, 2, 1) of the simpler 2,4-dimethyl-1,3-diphosphacyclobutadiene ligand have been investigated by density functional theory. The lowest energy structures of these complexes have bridging η 4 ,η 1 -Me 2 C 2 P 2 ligands donating six electrons to the central Co 2 unit through a tetrahapto ring-metal linkage to one cobalt atom and a phosphorus lone pair to the other cobalt atom. For the tetracarbonyl (Me 2 C 2 P 2 ) 2 Co 2 (CO) 4 doubly CO-bridged structures with terminal η 4 -Me 2 C 2 P 2 ligands similar to known (η 5 C 5 H 5 ) 2 Fe 2 (μ-CO) 2 (CO) 2 and (η 4 -Me 4 C 4 ) 2 Co 2 (μ-CO) 2 (CO) 2 structures lie only ∼5 kcal/mol in energy above the η 4 ,η 1 -Me 2 C 2 P 2 bridged structures. For the unsaturated (Me 2 C 2 P 2 ) 2 Co 2 (CO) 3 and (Me 2 C 2 P 2 ) 2 Co 2 (CO) 2 systems, structures with one or two bridging η 4 ,η 1 -Me 2 C 2 P 2 ligands, respectively, are preferred energetically over isomeric structures with formal Co Co double bonds and Co Co triple bonds, respectively. The lowest energy structure for the monocarbonyl (Me 2 C 2 P 2 ) 2 Co 2 (CO) is a triplet structure in which a (Me 2 C 2 P 2 ) 2 Co sandwich unit functions as a tridentate ligand to a CoCO unit through a phosphorus atom on each ring as well as the central cobalt atom. [ABSTRACT FROM AUTHOR]

The experimentally observed reductive CO coupling in organouranium systems observed by Cloke and co-workers makes of interest the exploration of the structures and thermochemistry of simple Cp 2 U 2 (CO) n ( n = 5, 4, 3, 2) derivatives. In this connection, the low-energy Cp 2 U 2 (CO) n ( n = 2, 3, 4) structures contain two to four bridging η 2 -μ-CO groups supplemented, in some cases, by one or two terminal CO groups. No examples of CO coupling to form C 2 O 2 ligands are found in any of these Cp 2 U 2 (CO) n structures in contrast to the previously studied Cp 2 Th 2 (CO) 4 system in which CO coupling to form C 2 O 2 ligands is observed. The retention of uranium valence electrons in low-energy Cp 2 U 2 (CO) n ( n = 3, 4) structures, even after partial donation of uranium electrons to reduce Cp to Cp – and CO to CO 2- , leads to a rich and complicated variety of U U bonding modes as well as spin states from singlet to quintet. Of particular interest are uranium-uranium multiple bonds containing multiple single electron half-bond components in triplet and quintet spin state Cp 2 U 2 (CO) n structures. The formal uranium-uranium multiple bonds in such Cp 2 U 2 (CO) n structures exhibit some of the features of the quintuple bond of diuranium, U 2 , in which a typical σ + 2π triple bond similar to the C C triple bond of acetylene is supplemented by no less than four separate one-electron half-bonds. Thus the U U bonds in the lowest energy Cp 2 U 2 (CO) n ( n = 2, 3) structures can be interpreted as formal triple bonds of various types with unusually short bond distances of ∼2.5 Å and Wiberg bond indices of ∼3. [ABSTRACT FROM AUTHOR]

The binuclear 1,3-diphosphacyclobutadiene manganese carbonyls (Me 2 C 2 P 2 ) 2 Mn 2 (CO) n ( n = 6, 5, 4, 3) have been investigated by density functional theory. The lowest energy (Me 2 C 2 P 2 ) 2 Mn 2 (CO) n ( n = 6, 5) structures are found to have two mononuclear (Me 2 C 2 P 2 ) 2 Mn 2 (CO) m fragments linked by P→M dative bonds. The lowest energy isomers with terminal η 4 -Me 2 C 2 P 2 rings and Mn–Mn bonds lie ∼15 and ∼22 kcal/mol above these global minima for (Me 2 C 2 P 2 ) 2 Mn 2 (CO) 6 and (Me 2 C 2 P 2 )Mn 2 (CO) 5 , respectively. For the (Me 2 C 2 P 2 ) 2 Mn 2 (CO) 4 system, singlet structures with terminal η 4 -Me 2 C 2 P 2 rings and formal Mn Mn triple bonds are of comparable energies to triplet structures consisting of (Me 2 C 2 P 2 ) 2 Mn 2 (CO) m fragments linked by two P→M dative bonds. All of the low-energy (Me 2 C 2 P 2 ) 2 Mn 2 (CO) 3 structures have terminal η 4 -Me 2 C 2 P 2 rings and three bridging CO groups with short Mn–Mn distances suggesting formal multiple bonds. [ABSTRACT FROM AUTHOR]

The structures and thermodynamics of the binuclear phospholyl iron carbonyls (C 4 H 4 P) 2 Fe 2 (CO) n ( n = 6, 5, 4, 3, 2) have been investigated using density functional theory. The low-energy (C 4 H 4 P) 2 Fe 2 (CO) n ( n = 4, 3, 2) structures are found to have direct iron–iron bonds and terminal five-electron donor pentahapto η 5 -C 4 H 4 P rings with the phosphorus lone pairs not involved in the bonding to the iron atoms. They are thus analogous to the corresponding cyclopentadienyliron carbonyl derivatives. However, they differ from the binuclear phospholyl manganese carbonyls (C 4 H 4 P) 2 Mn 2 (CO) n ( n = 5, 4) for which structures with bridging seven-electron donor η 5 , η 1 -C 4 H 4 P phospholyl rings are the lowest energy structures by substantial margins. Partially bonded phospholyl rings, as well as direct Fe–Fe bonds, are found in the carbonyl-rich (C 4 H 4 P) 2 Fe 2 (CO) n ( n = 6, 5) species. The pentacarbonyl (C 4 H 4 P) 2 Fe 2 (CO) 5 does not appear to be a viable species since it is disfavored relative to CO loss and to disproportionation into (C 4 H 4 P) 2 Fe 2 (CO) 6 + (C 4 H 4 P) 2 Fe 2 (CO) 4 . [ABSTRACT FROM AUTHOR]

The structures and energetics of the binuclear ruthenium carbonyl nitrosyls Ru 2 (NO) 2 (CO) n ( n = 7, 6, 5, 4) have been studied by density functional theory for comparison with the isovalent Fe 2 (NO) 2 (CO) n and Rh 2 (CO) n +2 systems. The lowest energy Ru 2 (NO) 2 (CO) 6 structure has two bridging NO groups analogous to the lowest energy Fe 2 (NO) 2 (CO) 6 and Rh 2 (CO) 8 structures, which are also doubly bridged structures. The lowest energy Ru 2 (NO) 2 (CO) 5 structures are two nearly degenerate structures. One of these structures has a single bridging NO group. The other low energy Ru 2 (NO) 2 (CO) 5 structure and the lowest energy Ru 2 (NO) 2 (CO) 4 structure have semibridging NO and CO groups. The energetics of the Ru 2 (NO) 2 (CO) n ( n = 6, 5, 4) systems indicate the preference for bridging NO groups relative to bridging CO groups in otherwise equivalent structures. In addition, Ru 2 (NO) 2 (CO) 5 is disfavored relative to disproportionation into Ru 2 (NO) 2 (CO) 6 + Ru 2 (NO) 2 (CO) 4 . The lowest energy structure of the carbonyl-rich Ru 2 (NO) 2 (CO) 7 , like that of Ru 2 (NO) 2 (CO) 6 , has two bridging NO groups but a long Ru⋯Ru distance greater than 3.3 Å indicating the lack of a direct Ru–Ru bond. However, this Ru 2 (NO) 2 (CO) 7 structure is disfavored relative to both CO loss and fragmentation into Ru(NO) 2 (CO) 2 + Ru(CO) 5 . [ABSTRACT FROM AUTHOR]

The pentamethylcyclopentadienylosmium (Cp*Os) system is of interest in forming a stable binuclear hydride Cp* 2 Os 2 (μ-H) 4 with four bridging hydrogen atoms as well as a stable mononuclear hydride Cp*OsH 5 . Density functional theory (DFT) studies on the corresponding unsubstituted systems Cp 2 Os 2 H n ( n = 8, 6, 4, 2, 0) and CpOsH n ( n = 5, 3, 1) are reported. The quadruply bridged structure Cp 2 Os 2 (μ-H) 4 is shown to be the lowest energy Cp 2 Os 2 H 4 structure. For the other Cp 2 Os 2 H n systems ( n = 8, 6, 2) the lowest energy structures contain doubly bridged Cp 2 Os 2 (μ-H) 2 units with decreasing Os Os distances as the numbers of hydride ligands is decreased. A related doubly bridged Cp 2 Os 2 (μ-H) 2 H 2 structure is predicted to lie only 2.5 kcal/mol above the quadruply bridged Cp 2 Os 2 (μ-H) 4 isomer. The hydrogen-richer structures Cp 2 Os 2 H n ( n = 8 and 6) are predicted to lose H 2 easily to give Cp 2 Os 2 H 4 in reactions within ∼8 kcal/mol of being thermoneutral. However, the H 2 dissociation energy of Cp 2 Os 2 H 4 to give Cp 2 Os 2 H 2 + H 2 is relatively high at ∼42 kcal/mol consistent with its stability. Bridging cyclopentadienyl rings are found in low-energy Cp 2 Os 2 H 2 and Cp 2 Os 2 structures. For Cp 2 Os 2 , two structures, each with a bridging μ-Cp ring, are shown to lie more than 14 kcal/mol in energy below the singlet and triplet unbridged isomers, which were previously shown by frontier molecular orbital analyses to have formal Os Os quintuple bonds. [ABSTRACT FROM AUTHOR]

Cobalt carbonyls such as Co 2 (CO) 8 are among the most frequently used catalyst precursors for the cyclotrimerization of alkynes to form benzene derivatives. In order to gain some insight regarding possible intermediates in such reactions, the species (C 6 R 6 )Co 2 (CO) n ( n = 6, 5, 4, 3; R = H, F, CF 3 ) have been investigated by density functional theory. The thermochemistry of these systems is found to be very dependent on the substituent R. The lowest energy (C 6 H 6 )Co 2 (CO) n structures ( n = 6, 5, 4, 3) by at least 8 kcal/mol all have cis geometries with intact benzene rings and Co–Co bonds. The benzene rings are bent for the more highly unsaturated (C 6 H 6 )Co 2 (CO) n ( n = 4, 3) structures. The tetracarbonyl (C 6 H 6 )Co 2 (CO) 4 is disfavored with respect to disproportionation into (C 6 H 6 )Co 2 (CO) 5 and (C 6 H 6 )Co 2 (CO) 3 . In addition, (C 6 H 6 )Co 2 (CO) 6 is marginally viable with respect to benzene dissociation to give Co 2 (CO) 6 , which has long been recognized as an intermediate in the conversion of Co 2 (CO) 8 to Co 4 (CO) 12 . The perfluoro and trifluoromethyl systems (C 6 R 6 )Co 2 (CO) n (R = F, CF 3 ; n = 6, 5, 4, 3) differ from (C 6 H 6 )Co 2 (CO) n in having “flyover” derivatives rather than cyclized benzene derivatives as the lowest energy structures for the tetracarbonyls (C 6 R 6 )Co 2 (CO) 4 . This may relate to difficulties in the cyclization of the C 6 chain with the more hindered F and CF 3 substituents relative to hydrogen on the end carbon atoms. These “flyover” tetracarbonyl appear to be thermodynamic “sinks” consistent with the experimental isolation of [C 6 (CF 3 ) 6 ]Co 2 (CO) 4 as a stable product from Co 2 (CO) 8 /CF 3 C CCF 3 reactions. [ABSTRACT FROM AUTHOR]

The structures and thermochemistry of the formally unsaturated binuclear iron carbonyl thioboronyls Fe 2 (BS) 2 (CO) n ( n = 7, 6) have been examined using density functional theory. The CO dissociation from the previously studied octacarbonyl Fe 2 (BS) 2 (CO) 8 is shown to be essentially thermoneutral so that the heptacarbonyl Fe 2 (BS) 2 (CO) 7 is predicted to be the binuclear iron carbonyl thioboronyl most likely to be synthesized. However, the chemistry of Fe 2 (BS) 2 (CO) 7 is predicted to be very rich with several different types of unusual ligands arising from coupling of BS and CO groups bridging the central Fe 2 unit. Unusual bridging ligands found in the lowest energy Fe 2 (BS) 2 (CO) 7 structures include the η 3 ,η 3 -μ-SBB(S)CO ligand, the trans -η 3 ,η 1 -μ-SBBS ligand, and the η 2 ,η 2 -μ-SBCO ligand donating four, six, and four electrons, respectively, to the central Fe 2 unit. The bridging ligands found in the lowest energy structures of the hexacarbonyl Fe 2 (BS) 2 (CO) 6 include cis and trans μ-SBBS stereoisomers as well as the related CO adduct cis -η 4 ,η 2 -SB(CO)BS. [ABSTRACT FROM AUTHOR]

The recently synthesized 1,2-diaza-3,5-diborolyl ligand is a BN analog of the cyclopentadienyl ligand that has been recently used for the synthesis of a BN analog of ferrocene. The binuclear 1,2-diaza-3,5-diborolyl iron carbonyl derivatives (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) n ( n = 5, 4, 3, 2) have now been investigated by density functional theory for comparison with the corresponding cyclopentadienyliron carbonyl systems, (C 5 H 5 ) 2 Fe 2 (CO) n . The lowest energy (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) n ( n = 4, 3, 2) structures have two bridging CO groups. For (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) 3 a triplet triply bridged (Me 4 B 2 N 2 CH) 2 Fe 2 (μ-CO) 3 structure is analogous to the experimentally known stable compounds (η 5 -C 5 R 5 ) 2 Fe 2 (μ-CO) 3 (R = H, CH 3 ). From the thermochemistry results, the doubly bridged singlet (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) 2 (μ-CO) 2 structure appears to a promising synthetic objective. The energies of the triplet (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) 3 and (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) 2 structures are lower than those of the corresponding singlet structures. The Wiberg bond indices of the iron–iron bonds determined by the NBO method correlate well with the formal bond orders suggested by the Fe Fe distances and electron counting. The (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) 5 structures lack an iron–iron bond and require only ∼2 kcal/mol in energy for decarbonylation to (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) 4 . In all of the (Me 4 B 2 N 2 CH) 2 Fe 2 (CO) n ( n = 4, 3, 2) structures, the predicted Fe N and Fe B distances are close to those in the ferrocene analog [η 5 -(CH 2 ) 3 N 2 (BPh) 2 CMe] 2 Fe. [ABSTRACT FROM AUTHOR]

Highlights: [•] The C7H7 ring can be either a heptahapto or pentahapto ligand in low-energy (C7H7)2V2(CO) n (n =5, 4, 3) structures. [•] (C7H7)2V2(CO) n (n =5, 4, 3) structures with V V triple bonds and 18-electron vanadium configurations are preferred. [•] A low-energy linear triplet (C7H7)2V2 structure has rare D 7h symmetry and a V V triple bond. [•] The lowest energy singlet (C7H7)2V2 structure has an ultrashort V–V distance of 1.84Å and a formal sextuple bond. [ABSTRACT FROM AUTHOR]

Reactions of acetylene, thiophene, or even tellurophene with iron carbonyls have been found to give the tricarbonylferrole iron tricarbonyl, [η4-C4H4Fe(CO)3]Fe(CO)3, which has been shown by X-ray crystallography to contain a metallacyclic FeC4 ring with an Fe–Fe bonding distance of ∼2.5Å to the exocyclic iron atom. This structure has been shown by density functional theory to be the lowest energy C4H4Fe2(CO)6 structure by more than 30kcal/mol relative to the lowest energy isomeric triplet structure. Dissociation of carbonyl groups from this structure requires ∼40kcal/mol per carbonyl group. The central C4H4Fe2 unit in the resulting unsaturated C4H4Fe2(CO) n (n =5, 4, 3) structures is relatively rigid upon carbonyl loss with the Fe–Fe distance decreasing only to ∼2.4Å in even the highly unsaturated C4H4Fe2(CO)3. Thus 16-electron metal configurations with iron–iron single or double bonds rather than the normally favored 18-electron metal configurations are preferred over structures with higher order iron–iron multiple bonds. For the carbonyl-rich species the lowest energy C4H4Fe2(CO)8 structure has one C C double bond of a tetracarbonylferrole ring bonded to an exocyclic Fe(CO)4 moiety with a long Fe⋯Fe non-bonding distance of ∼3.8Å. The low-energy structures for C4H4Fe2(CO)7 either have a dihapto ferrole ligand with an iron–iron bond or a tetrahapto ferrole ligand without an iron–iron bond. However, C4H4Fe2(CO)7 does not appear to be viable since carbonyl dissociation from the lowest energy C4H4Fe2(CO)7 structure to give C4H4Fe2(CO)6 is predicted to be an exothermic process. [ABSTRACT FROM AUTHOR]

Highlights: [•] The structures of (C5F6)Co2(CO) n (n =8, 7, 6, 5, 4) have been investigated using density functional theory. [•] Trihapto/monohapto bonding of the C5F6 ring to the Co2 unit is preferred in (C5F6)Co2(CO) n (n =7, 6, 5, 4). [•] Viable bis(monohapto) structures are found for C5F6Co2(CO)8. [•] Cobalt–cobalt bonds are found in the lowest energy (C5F6)Co2(CO) n (n =6, 5, 4) structures. [ABSTRACT FROM AUTHOR]

The structures and energetics of the cyclobutadiene manganese carbonyls (C4H4)2Mn2(CO) n (n =6, 5, 4, 3) have been investigated using density functional theory. In this connection the lowest energy (C4H4)2Mn2(CO)6 structure consists of two C4H4Mn(CO)3 units coupled through an Mn–Mn bond of length ∼3.0Å. This rather fragile dimer is predicted to dissociate readily into C4H4Mn(CO)3 radicals. A higher energy (C4H4)2Mn2(CO)6 structure has an agostic hydrogen atom and an Mn⋯Mn distance of ∼4.3Å, too long for a direct bond. The unsaturated pentacarbonyl (C4H4)2Mn2(CO)5 system resembles its (C5H5)2Cr2(CO)5 counterpart by having low energy triplet spin state structures and being disfavored relative to (C4H4)2Mn2(CO)6 +(C4H4)2Mn2(CO)4. A singlet tetracarbonyl (C4H4)2Mn2(CO)4 structure is found with a short Mn Mn distance of ∼2.2Å suggesting a formal triple bond analogous to the known (C5H5)2Cr2(CO)4 structure. However, the lowest energy (C4H4)2Mn2(CO)4 state is a novel triplet spin state octahedral Mn2C4 cluster with Mn(CO)3 and (η4-C4H4)Mn(CO) vertices. The lowest energy (C4H4)2Mn2(CO)3 structure by ∼13kcal/mol is an unsymmetrical triplet spin state structure with a C4H4 ligand bridging a dative formal Mn Mn triple bond connecting an Mn(CO)3 group to a (η4-C4H4)Mn group. [ABSTRACT FROM AUTHOR]

Highlights: [•] A cis-(η5,η5-C8H6)Co2(CO)4 structure related to the experimental structure is predicted. [•] The Co–Co interaction in cis-(η5,η5-C8H6)Co2(CO)4 has a Wiberg bond index of only 0.06. [•] The low-energy (C8H6)Co2(CO) n (n =3, 2) structures also have pentahapto pentalene ligands. [•] The carbonyl-rich (C8H6)Co2(CO) n (n =6, 5) structures illustrate considerable diversity of pentalene bonding modes. [Copyright &y& Elsevier]

Highlights: [•] The W2(NO)2(CO) n (n =9, 8, 7, 6, 5) systems are studied by density functional theory. [•] An unbridged W2(NO)2(CO)8 structure is predicted with a formal W–W single bond. [•] An unbridged W2(NO)2(CO)7 structure is predicted with a formal W W double bond. [•] A doubly CO bridged W2(NO)2(CO)6 structure is predicted with a formal W W triple bond. [Copyright &y& Elsevier]

Abstract: The structures and thermochemistry of the binuclear cyclopentadienylcobalt carbonyl boronyls Cp2Co2(BO)2(CO) n (Cp = η5-C5H5; n = 2, 1, 0) were investigated by density functional theory. Low energy structures were found of the following types: (1) Structures in which the BO groups are all one-electron donors analogous to the isoelectronic Cp2Fe2(CO) n+2 structures; (2) Structures in which the two BO groups have coupled to form bridging B2O2 ligands, which may be either four- or six-electron donors; (3) Structures with three-electron donor bridging BO groups, bonded to one metal through the boron atom and to the other metal through the oxygen atom. The lowest energy Cp2Co2(BO)2(CO)2 structures are singlet trans and cis isomers of doubly bridged structures analogous to the well-known isoelectronic Cp2Fe2(CO)2(μ-CO)2. The lowest energy Cp2Co2(BO)2(CO) structure is a triplet triply bridged structure analogous to the isoelectronic stable triplet state molecule Cp2Fe2(μ-CO)3. Higher energy Cp2Co2(BO)2(CO) structures have trans or cis bridging B2O2 ligands. The lowest energy Cp2Co2(BO)2 structure is a triplet state structure with a six-electron donor bridging trans-η4-μ-B2O2 ligand. The lowest energy singlet Cp2Co2(BO)2 structure has two one-electron donor bridging μ-BO groups and a formal Co Co triple bond similar to the predicted lowest energy Cp2Fe2(CO)2 structure. [Copyright &y& Elsevier]

Highlights: [•] The species (CF3NC)2Co2(CO) n (n =7, 6, 5, 4) have been studied by density functional theory. [•] Bridging CF3NC groups are energetically preferred over bridging CO groups. [•] Four-electron donor bridging η2-μ-CF3NC groups are found in higher energy (CF3NC)2Co2(CO) n (n =5, 4) structures. [•] Coupling of two CF3NC groups to form a CF3N C C NCF3 ligand occurs in the (CF3NC)2Co2(CO)7 system. [Copyright &y& Elsevier]

Abstract: The diphosphacyclobutadiene sandwich compounds (η4-But 2C2P2)2M z (M=Ni, z =0; M=Co, z =−1, 0; M=Fe, z =−1, 0) as well as the iron carbonyl derivative (η4-But 2C2P2)Fe(CO)3 have recently (2008) been synthesized by Lammertsma and co-workers using the dimerization of the phosphaalkyne ButC P: on suitable reactive transition metal sites. The structures and energetics of the closely related dimethyl derivatives (Me2C2P2)Fe(CO) n (n =3, 2, 1) and (Me2C2P2)2Fe2(CO) n (n =5, 4, 3, 2) have now been investigated by density functional theory. For (Me2C2P2)2Fe2(CO)5 a structure with a bridging η1,η4-Me2C2P2 ring and no iron–iron bond is energetically preferred by more than 24kcal/mol over an alternative structure with only terminal η4-Me2C2P2 rings and an iron–iron single bond. The lowest energy singlet and triplet (Me2C2P2)2Fe2(CO)4 structures have at least one bridging η1,η4 ligand, which in one case is in the form of an (η4-Me2C2P2)2Fe(CO) sandwich bidentate ligand chelating to an Fe(CO)3 unit through two ring phosphorus atoms. In contrast to (Me2C2P2)2Fe2(CO) n (n =5, 4), the lowest energy structures for (Me2C2P2)2Fe2(CO)3 have only terminal η4-Me2C2P2 ligands, three carbonyl groups, and an Fe Fe triple bond. Interesting structures for the dicarbonyl (Me2C2P2)2Fe2(CO)2 include a structure with a (η4-Me2C2P2)2Fe sandwich ligand chelating to an Fe(CO)2 group and a structure with exclusively terminal η4-Me2C2P2 ligands and a short Fe–Fe distance of ∼2.16Å, suggesting a formal quadruple bond. [Copyright &y& Elsevier]

Highlights: [•] Theoretical studies on (C5F5)2M2(CO) n (M=Fe, n =4, 3, 2; M=Co, n =3, 2, 1; M=Ni, n =2, 1) are reported. [•] The structures of the η5-C5F5 derivatives are generally similar to their η5-C5H5 analogs. [•] (C5F5)2M2(CO) n (M=Fe, n =4; M=Co, n =3; M=Ni, n =2) are predicted to have M M single bonds of lengths 2.4–2.6Å. [•] The cis and trans isomers of (C5F5)2Fe2(μ-CO)2(CO)2 are predicted to have similar energies. [•] (C5F5)2M2(CO) n (M=Fe, n =3, 2; M=Co, n =2, 1; M=Ni, n =1) are predicted to have M M double or M M triple bonds. [ABSTRACT FROM AUTHOR]

Highlights: [•] The structures and energetics of (C4H6)2Cr2(CO) n (n =7, 6, 5) are investigated by density functional theory. [•] (C4H6)2Cr2(CO)7 is disfavored with respect to dissociation into mononuclear fragments. [•] (C4H6)2Cr2(CO)6 is disfavored with respect to disproportionation into (C4H6)2Cr2(CO)7 +(C4H6)2Cr2(CO)5. [•] (C4H6)2Cr2(CO)5 is predicted to have all terminal CO groups and a Cr Cr triple bond. [•] The (C4H6)2Cr2(CO) n (n =7, 6, 5) structures are closely related to the isomeric [(CH2)3C]2Cr2(CO) n structures. [ABSTRACT FROM AUTHOR]

Highlights: [•] The potential energy surface of Cp2Pd2(BO)2 has 11 low energy structures. [•] These Cp2Pd2(BO)2 structures includes both perpendicular and coaxial structures. [•] Several Cp2Pd2(BO)2 structures have the BO group directly bonded to a Cp carbon atom. [•] A highly favored coaxial Cp2Pd2(μ-BF)2 structure is found. [Copyright &y& Elsevier]

Highlights: [•] Doubly bridged and unbridged structures are predicted for Mn2(NS)2(CO)7. [•] Triply bridged structures analogous to the isoelectronic Fe2(CO)9 are not found for Mn2(NS)2(CO)7. [•] The lowest energy Mn2(NS)2(CO) n (n =6, 5) structures all have a five-electron donor bridging η2-NS group. [•] The lowest energy Mn2(NS)2(CO)6 structure is a particularly attractive synthetic objective. [Copyright &y& Elsevier]

Prolate Ti 2 C 4 octahedra with titanium atoms in polar positions are found in the lowest energy (C 4 H 4) 2 Ti 2 (CO) n (n = 8, 7) structures. Low-energy (C 4 H 4) 2 Ti 2 (CO) n (n = 6, 5, 4, 3) structures have various combinations of four-electron donor bridging η2-µ-CO groups and six-electron donor bridging η4,η1-C 4 H 4 cyclobutadiene ligands. [Display omitted] • The structures and energetics of (C 4 H 4) 2 Ti 2 (CO) n (n = 8, 7, 6, 5, 4, 3) have been examined by density functional theory. • Low-energy (C 4 H 4) 2 Ti 2 (CO) n (n = 8, 7) structures have central prolate octahedral Ti 2 C 4 clusters. • A low energy (η4-C 4 H 4)Ti(CO) 4 (CO) → Ti(CO) 3 (η4-C 4 H 4) isomer is also found for (C 8 H 8) 2 Ti 2 (CO) 8. • Low-energy (C 4 H 4) 2 Ti 2 (CO) n (n = 6, 5, 4, 3) structures have various combinations of bridging η2-µ-CO groups and bridging η4,η1-C 4 H 4 rings. The structures and energetics of the binuclear cyclobutadiene titanium carbonyl derivatives (C 4 H 4) 2 Ti 2 (CO) n (n = 8, 7, 6, 5, 4, 3) have been investigated by density functional theory. The lowest energy (C 4 H 4) 2 Ti 2 (CO) 8 structure is found to have a central prolate Ti 2 C 4 octahedron with Ti(CO) 5 and (η4-C 4 H 4)Ti(CO) 3 moieties in polar positions. Similar central Ti 2 C 4 prolate octahedra are found in an only slightly higher energy triplet (C 4 H 4) 2 Ti 2 (CO) 8 structure as well as in the lowest energy triplet (C 4 H 4) 2 Ti 2 (CO) 7 structure. An only slightly higher energy (C 4 H 4) 2 Ti 2 (CO) 8 isomer has a totally different type of structure which can be dissected into the complex (η4-C 4 H 4)Ti(CO) 5 with an 18-electron titanium configuration functioning as a Lewis base to a coordinatively unsaturated (η4-C 4 H 4)Ti(CO) 3 fragment through an O → Ti bond from one of its carbonyl groups. The low-energy structures of the more highly unsaturated (C 4 H 4) 2 Ti 2 (CO) n (n = 6, 5, 4, 3) species have various combinations of four-electron donor bridging η2-µ-CO groups and six-electron donor bridging η4,η1-C 4 H 4 cyclobutadiene ligands with agostic C H Ti interactions bridging a central formal Ti Ti double bond. [ABSTRACT FROM AUTHOR]

Highlights: [•] The lowest energy Fe2(CF2)(CO) n (n =7, 6, 5) structures have a bridging rather than terminal CF2 group. [•] Iron–iron triple and quadruple bonds are avoided in the highly unsaturated Fe2(CF2)(CO) n (n =6, 5) structures. [•] The lowest energy singlet Fe2(CF2)(CO)5 structure has an unprecedented kind of four-electron donor η2-μ-CF2 group. [Copyright &y& Elsevier]

Abstract: Density functional theory predicts the chemistry of the binuclear cyclopentadienylrhodium boronyls Cp2Rh2(BO) n (n =4, 3, 2) derivatives to be considerably more complicated than that of the related cyclopentadienylrhodium carbonyls Cp2Rh2(CO) n owing to the greater variety of BO coordination modes in energetically competitive structures. In this connection, the Cp2Rh2(BO) n structures resemble to a certain extent the previously studied isoelectronic cyanide complexes Cp2Rh2(CN) n . All of the energetically low-lying Cp2Rh2(BO) n structures prefer coaxial geometries with terminal Cp rings and bridging BO groups. By considering the bridging BO groups as neutral ligands, both one-electron donor bridging μ-BO groups bonded to the Rh atoms exclusively through their B atoms and three-electron donor η2–μ-BO groups bonded to the Rh atoms through both their B and O atoms are found in low energy structures. The three-electron donor bridging η2-μ-BO groups exhibit longer B–O distances and lower ν(BO) frequencies than their one-electron donor bridging μ-BO counterparts. The thermodynamic stabilities predicted for the binuclear Cp2Rh2(BO) n derivatives suggest them as possible synthetic objectives. [Copyright &y& Elsevier]

Abstract: Fluorophosphinidene (PF) is a versatile ligand found experimentally in the transient species M(CO)5(PF) (M=Cr, Mo) as well as the stable cluster Ru5(CO)15(μ4-PF). Theoretical studies on the cyclopentadienyliron carbonyl derivatives Cp2Fe2(PF)(CO) n (n =4, 3, 2, 1, 0) predict the PF groups in the low-energy structures always to be bridging rather than terminal groups and almost always to be two-electron rather than four-electron donors. Viable structures are found for Cp2Fe2(PF)(CO)4 in which a PF group bridges two CpFe(CO)2 moieties without an Fe–Fe bond. Such structures are potentially accessible from NaFe(CO)2Cp and PF3. The low energy Cp2Fe2(PF)(CO)3 structures have one PF bridge and one CO bridge and are thus analogous to the well-known Cp2Fe2(CO)2(μ-CO)2. The lowest energy Cp2Fe2(PF)(CO)2 structure is a triplet spin state triply bridged structure analogous to the stable Cp2Fe2(μ-CO)3. Such structures have the two unpaired electrons of the triplet spin state in two orthogonal π “half-bond” components in the formal Fe Fe double bond. Higher spin states, even quintets and septets, are energetically preferred over Fe–Fe triple and quadruple bonds for the highly unsaturated Cp2Fe2(PF)(CO) n (n =1, 0). In addition Cp2Fe2(PF)(CO) n (n =1, 0) structures are found in which the P–F bond of the PF ligand has split to form separate P and F ligands. [Copyright &y& Elsevier]

Abstract: The effect of pairwise replacement of terminal CO groups with the umbrella-shaped trimethylenemethane ligand on the structures and stabilities of binuclear chromium carbonyls has been examined using density functional theory. Such species are likely photolysis products of the known (CH2)3CCr(CO)4. The lowest energy [(CH2)3C]2Cr2(CO)7 structure has two semibridging CO groups similar to the lowest energy Cr2(CO)11 structure. This [(CH2)3C]2Cr2(CO)7 structure is disfavored by ∼14 kcal/mol with respect to dissociation into (CH2)3CCr(CO)4 + (CH2)3CCr(CO)3. The lowest energy [(CH2)3C]2Cr2(CO)6 structure is a singly bridged structure in contrast to the doubly semibridged lowest energy structure of the analogous Cr2(CO)10. However, a doubly semibridged [(CH2)3C]2Cr2(CO)6 structure similar to the Cr2(CO)10 structure is predicted to lie ∼16 kcal/mol above the singly bridged global minimum. The lowest energy [(CH2)3C]2Cr2(CO)5 structure is an unbridged structure in contrast to the triply semibridged lowest energy Cr2(CO)9 structure. However, a related triply semibridged [(CH2)3C]2Cr2(CO)5 structure is predicted to lie only slightly in energy above this global minimum. The [(CH2)3C]2Cr2(CO)5 structures, like the corresponding Cr2(CO)9 structure, have formal Cr bonds of lengths ∼2.3 Å and are predicted to be very stable with respect to dissociation into mononuclear fragments with dissociation energies greater than 35 kcal/mol. [Copyright &y& Elsevier]