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The d4 halide complexes [MX(CO)(η-RCCR)Tp′] {R = Me, M = W, X = F; R = Ph, M = Mo or W, X = F or Cl; Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} undergo two-electron oxidation in the presence of a halide source to give the d2 monocations [MX1X2(η-PhCCPh)Tp′]+ (R = Me, M = W, X1 = X2 = F; R = Ph, M = Mo, X1 = X2 = F or Cl; M = W, X1 = X2 = F or Cl; X1 = F, X2 = Cl). Each monocation (R = Ph) shows two reversible one-electron reductions (the second process was not detected for R = Me) corresponding to the stepwise formation of the neutral d3 and monoanionic d4 analogues, [MX1X2(η-PhCCPh)Tp′] and [MX1X2(η-PhCCPh)Tp′]− respectively; the potentials for the two processes can be ‘tuned’ over a range of ca. 1.0 V by varying M and X. Chemical one-electron reduction of [MX2(η-PhCCPh)Tp′]+ gave [MX2(η-PhCCPh)Tp′] (M = Mo or W, X = F or Cl). X-Ray structural studies on the redox pairs [WX2(η-PhCCPh)Tp′]z (X = F and Cl, z = 0 and 1+) show the alkyne to bisect the X–W–X angle in the d2 cations but align more closely with one M–X bond in the neutral d3 molecules, consistent with the anisotropic ESR spectra of the latter; the solution ESR spectrum of [MoF2(η-PhCCPh)Tp′] showed equivalent fluorine atoms, i.e the alkyne oscillates at room temperature. The successful isolation of [MX2(η-PhCCPh)Tp′]+ and [MX2(η-PhCCPh)Tp′] completes a series in which d6 to d2alkyne complexes are linked in a redox family tree by sequential one-electron transfer and substitution reactions. The implications for such trees in the production of new species and the selective synthesis of paramagnetic complexes acting as synthetically useful ‘alkyne radicals’ are discussed.

The d4 halide complexes [MX(CO)(η-RCCR)Tp′] {X = F, Cl, Br or I; R = Me or Ph; M = Mo or W; Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} undergo one-electron oxidation to the d3 monocations [MX(CO)(η-RCCR)Tp′]+, isolable for M = W, R = Me. X-Ray structural studies on the redox pairs [WX(CO)(η-MeCCMe)Tp′]z (X = Cl and Br, z = 0 and 1), the ESR spectra of the cations [WX(CO)(η-RCCR)Tp′]+ (X = F, Cl, Br or I; R = Me or Ph), and DFT calculations on [WX(CO)(η-MeCCMe)Tp′]z (X = F, Cl, Br and I; z = 0 and 1) are consistent with electron removal from a HOMO (of the d4 complexes) which is π-antibonding with respect to the W–X bond, π-bonding with respect to the W–C(O) bond, and δ-bonding with respect to the W–Calkyne bonds. The dependence of both oxidation potential and ν(CO) for [MX(CO)(η-RCCR)Tp′] shows an inverse halide order which is consistent with an ionic component to the M–X bond; the small size of fluorine and its closeness to the metal centre leads to the highest energy HOMO and the lowest oxidation potential. In the cations [MX(CO)(η-RCCR)Tp′]+ electronegativity effects become more important, leading to a conventional order for Cl, Br and I. However, high M–F π-donation is still facilitated by the short M–F distance.

The salts [NEt4][Ru(CN)(CO)2L(o-O2C6Cl4)] {L=PPh3 or P(OPh)3}, which undergo one-electron oxidation at the catecholate ligand to give neutral semiquinone complexes [Ru(CN)(CO)2L(o-O2C6Cl4)], react with the dimers [{Ru(CO)2L(micro-o-O2C6Cl4)}2] {L=PPh3 or P(OPh)3} to give [NEt4][(o-O2C6Cl4)L(OC)2Ru(micro-CN)Ru(CO)2L'(o-O2C6Cl4)] {L or L'=PPh3 or P(OPh)3}. The cyanide-bridged binuclear anions are, in turn, reversibly oxidised to isolable neutral and cationic complexes [(o-O2C6Cl4)L(OC)2Ru(micro-CN)Ru(CO)2L'(o-O2C6Cl4)] and [(o-O2C6Cl4)L(OC)2Ru(micro-CN)Ru(CO)2L'(o-O2C6Cl4)]+ which contain one and two semiquinone ligands respectively. Structural studies on the redox pair [(o-O2C6Cl4)(Ph3P)(OC)2Ru(micro-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)]- and [(o-O2C6Cl4)(Ph3P)(OC)2Ru(micro-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)] confirm that the C-bound Ru(CO)2(o-O2C6Cl4) fragment is oxidised first. Uniquely, [(o-O2C6Cl4){(PhO)3P}(OC)2Ru(micro-CN)Ru(CO)2(PPh3)(o-O2C6Cl4)]- is oxidised first at the N-bound fragment, indicating that it is possible to control the site of electron transfer by tuning the co-ligands. Crystallisation of [(o-O2C6Cl4)(Ph3P)(OC)2Ru(micro-CN)Ru(CO)2{P(OPh)3}(o-O2C6Cl4)] resulted in the formation of an isomer in which the P(OPh)3 ligand is cis to the cyanide bridge, contrasting with the trans arrangement of the X-Ru-L fragment in all other complexes of the type RuX(CO)2L(o-O2C6Cl4).

Reduction of [M(CO)2(η-RCCR′)Tp′]X {Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate, M = Mo, X = [PF6]−, R = R′ = Ph, C6H4OMe-4 or Me; R = Ph, R′ = H; M = W, X = [BF4]−, R = R′ = Ph or Me; R = Ph, R′ = H} with [Co(η-C5H5)2] gave paramagnetic [M(CO)2(η-RCCR′)Tp′], characterised by IR and ESR spectroscopy. X-Ray structural studies on the redox pair [Mo(CO)2(η-PhCCPh)Tp′] and [Mo(CO)2(η-PhCCPh)Tp′][PF6] showed that oxidation is accompanied by a lengthening of the CC bond and shortening of the Mo–Calkyne bonds, consistent with removal of an electron from an orbital antibonding with respect to the Mo–alkyne bond, and with conversion of the alkyne from a three- to a four-electron donor. Reduction of [Mo(CO)(NCMe)(η-MeCCMe)Tp′][PF6] with [Co(η-C5H5)2] in CH2Cl2 gives [MoCl(CO)(η-MeCCMe)Tp′], via nitrile substitution in [Mo(CO)(NCMe)(η-MeCCMe)Tp′], whereas a similar reaction with [M(CO){P(OCH2)3CEt}(η-MeCCMe)Tp′]+ (M = Mo or W) gives the phosphite-containing radicals [M(CO){P(OCH2)3CEt}(η-MeCCMe)Tp′]. ESR spectroscopic studies and DFT calculations on [M(CO)L(η-MeCCMe)Tp′] {M = Mo or W, L = CO or P(OCH2)3CEt} show the SOMO of the neutral d5 species (the LUMO of the d4 cations) to be largely dyz in character although much more delocalised in the W complexes. Non-coincidence effects between the g and metal hyperfine matrices in the Mo spectra indicate hybridisation of the metal d-orbitals in the SOMO, consistent with a rotation of the coordinated alkyne about the M–C2 axis.

5 páginas, 2 figuras, 2 tablas, esquemas., A new method for the synthesis of cationic Pd(IV) complexes, based on the chemical oxidation of an anionic Pd(II) metallacycle in the presence of an additional ligand, is presented. Electrochemical analysis of these compounds reveals that the stability of the +4 oxidation state is sensitive to the electron-donor capability of the added co-ligand and to the steric features of the Tp‘ ligand., Financial support from the DGI (Project PPQ2003-00975), the Ministerio de Educación, Cultura y Deporte (research studentship, D.d.R.), and the Junta de Andalucía is gratefully acknowledged.

Reaction of the hydride complexes [M 2 Cp 2 (μ-H)(μ-PPh 2 )(CO) 4 ] (M = Mo, W; Cp = η 5 -C 5 H 5 ) with [FeCp 2 ]BF 4 proceeds in a 1:2 ratio to give the unsaturated tetracarbonyl salts [M 2 Cp 2 -(μ-PPh 2 )(CO) 4 ]BF 4 , which spontaneously (M = Mo) or in the presence of CO (M = W) transform into the corresponding electron-precise pentacarbonyls [M 2 Cp 2 (μ-PPh 2 )(μ-CO)-(CO) 4 ]BF 4 . In contrast, reaction of the above hydride complexes with [FeCp 2 ](BAr' 4 ) (Ar' = 3,5-C 6 H 3 (CF 3 ) 2 ) proceeds in a 1:1 ratio to give the paramagnetic salts [M 2 Cp 2 (μ-H)(μ-PPh 2 )-(CO) 4 ](BAr' 4 ). No further reaction takes place with [FeCp 2 ](BAr' 4 ) unless a proton acceptor such as water or tetrahydrofuran is present, in which case the corresponding salts [M 2 Cp 2 -(μ-PPh 2 )(CO) 4 ](BAr' 4 ) or [M 2 Cp 2 (μ-PPh 2 )(μ-CO)(CO) 4 ](BAr' 4 ) are rapidly formed. Reactions of the anions [M 2 Cp 2 (μ-PPh 2 )(CO) 4 ] - (as (H-DBU) + salts, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) with [FeCp 2 ]BF 4 proceed stepwise to give first the 33-electron radicals [M 2 Cp 2 (μ-PPh 2 )(CO) 4 ], which can be isolated as very air-sensitive green solids, and then the unsaturated salts [M 2 Cp 2 (μ-PPh 2 )(CO) 4 ]BF 4 , which are identical to those formed from the neutral hydrides. The structure and dynamic behavior of the new compounds are analyzed on the basis of solution IR, 1 H, 3 1 P, and 1 3 C NMR or ESR spectroscopic data. In addition, the redox and other chemical transformations connecting all the new compounds are??

The complex trans-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2] is obtained in 91% yield by refluxing toluene solutions of [Fe2Cp2(CO)4] (Cp = η5-C5H5) and the secondary phosphine PPh2H. This compound isomerizes upon irradiation with visible−UV light under a CO atmosphere to yield cis-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2]. The above hydride complexes react under photochemical conditions with 1 equiv of secondary phosphines PR2H (R = Et, Ph) to give the corresponding monocarbonyl compounds [Fe2Cp2(μ-PPh2)(μ-PR2)(μ-CO)] via the hydride intermediates [Fe2Cp2(μ-H)(μ-PPh2)(CO)(PR2H)] (detected and isolated for R = Et). Deprotonation of trans-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2] with LiBu gives the binuclear anion [Fe2Cp2(μ-PPh2)(CO)2]-. This highly nucleophilic carbonylate reacts rapidly with [AuCl(PiPr3)] or MeI to give the corresponding gold diiron cluster [AuFe2Cp2(μ-PPh2)(CO)2(PiPr3)] or methyl derivative [Fe2Cp2(Me)(μ-PPh2)(μ-CO)(CO)2], respectively. Both hydrides cis- and trans-[Fe2Cp2(μ-H)(μ-PPh2)(CO)2] can be reversibly oxidized at low tempe...

The reaction of the complex [Mo(OTf)(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (1) (OTf = trifluoromethylsulfonate; phen = 1,10-phenanthroline) with tetrabutylammonium fluoride trihydrate afforded the fluoride complex [MoF(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (2). The IR spectrum and the oxidation potential of 2 reflect the fact that its metal center is more electron-rich than that of the chloro analogue [MoCl(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)]. Compound 2 reacted with 1 affording the homobinuclear complex [[Mo(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)](2)(mu-F)][OTf] (3), with a fluoride bridge. Compound 2 also reacts with the species generated in situ by triflate abstraction from [M(OTf)(CO)(3)('N-N')] (M = Mn, Re; 'N-N' = 2,2'-bipyridine (bipy), phen) using NaBAr'(4) (Ar' = 3,5-bis(trifluoromethyl)phenyl), affording the heterobinuclear complexes [[Mo(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)](mu-F)[M(CO)(3)('N--N')]][BAr'(4)] (M = Mn, 'N-N' = bipy (4); M = Re, 'N-N' = phen (5)). All new compounds have been characterized by spectroscopic methods (IR and NMR) and, in the case of 1, 2, 3, and 4, also by means of X-ray diffraction analysis.

Cationic nitrile complexes and neutral halide and cyanide complexes, with the general formula [MnL1L2(NO)(eta-C5H4Me)]z, undergo one-electron oxidation at a Pt electrode in CH2Cl2. Linear plots of oxidation potential, Eo', vs. nu(NO) or the Lever parameters, EL, for L1 and L2, allow Eo' to be estimated for unknown analogues. In the presence of TlPF6, [MnIL'(NO)(eta-C5H4Me)] reacts with [Mn(CN)L(NO)(eta-C5H4Me)] to give [(eta5-C5H4Me)(ON)LMn(mu-CN)MnL'(NO)(eta5-C5H4Me)][PF6] which undergoes two reversible one-electron oxidations; DeltaE, the difference between the potentials for the two processes, differs significantly for stable cyanide-bridged linkage isomers. Novel pentametallic complexes such as [Mn[(mu-NC)Mn(CNBut)(NO)(eta5-C5H4Me)]4(OEt2)][PF6]2 and [Mn[(mu-NC)Mn(CNXyl)(NO)(eta5-C5H4Me)]4(NO3-O,O')][PF6], containing a trigonal bipyramidal and a distorted octahedral Mn(II) centre, respectively, result either from slow decomposition of the binuclear cyanide-bridged species or from the reaction of anhydrous MnI2 with four equivalents of [Mn(CN)L(NO)(eta5-C5H4Me)] in the presence of TlPF6.

The nominally square-planar coordination of the d8 complexes [MClL1L2(p-XC6H4NNNHC6H4X-p)] (M = Rh, L1 = L2 = CO, X = H, Me, Et or F; M = Ir, L1 = L2 = CO, X = Me; M = Pd or Pt, L1 = Cl, L2 = PPh3, X = Me; M = Pd, L1L2 = η3-C3H5, X = Me), with the triazene N-bonded via the imine group, is supplemented by an axial M⋯H–N interaction involving the terminal amino group.

Fast scan rate cyclic voltammetry has been used to elucidate the heterogeneous and homogeneous aspects of the redox chemistry of cis- and trans-[Mn(CN)(CO)2{P(OPh)3}(Ph2PCH2PPh2)]. The cis species, on oxidation, isomerizes to the trans conformation, and the various processes can be understood as components of a square reaction scheme. In solution phase, the homogeneous rate constant for isomeric conversion is found to be independent of solvent and concentration, with a value of 38 s-1 obtained by comparison with digitally simulated responses. The heterogeneous rate constants for the cis0/cis+ and trans0/trans+ couples in acetonitrile have values of 0.06 and 0.065 cm s-1 respectively. The cis or trans complex was readily immobilized on an electrode surface, either by mechanical attachment or by precipitation from solution. However, electrolysis in the solid state was strongly dependent on the identity of the supporting electrolyte anion. This dependence is similar to the Hofmeister series of anions with la...

The complex [Rh(CO)2{HC(pz′)3}][PF6], 1+[PF6]− {HC(pz′)3 = tris(3,5-dimethylpyrazolyl)methane}, prepared by reacting [{Rh(CO)2(μ-Cl)}2] with HC(pz′)3 in the presence of Tl[PF6], has a distorted square pyramidal structure with a κ3-HC(pz′)3 ligand. Carbonyl substitution with Lewis bases gives [Rh(CO)L{HC(pz′)3}][PF6] {L = PPh3, 2+[PF6]−; L = AsPh3, 3+[PF6]−; L = P(o-tolyl)3, 4+[PF6]−}, which have square planar κ2 structures, confirmed by X-ray crystallography for 2+[PF6]−. The cations 2+ and 3+ have the third pyrazolyl ring orientated pseudo-parallel to the square planar metal whereas 4+ more likely has the third ring orientated exo to that plane. One-electron oxidation of 2+ and 3+ gives the Rh(II) dications [Rh(CO)(PPh3){HC(pz′)3}]2+, 22+, and [Rh(CO)(AsPh3){HC(pz′)3}]2+, 32+, characterised by ESR spectroscopy. Complex 1+[PF6]−reacts with PhCCPh to give [Rh(CO)(η2-PhCCPh){HC(pz′)3}][PF6], 5+[PF6]−, in which the two-electron donor alkyne occupies an equatorial position in a trigonal bipyramidal κ3 structure. With MeCCR (R = Me or Et), 1+[PF6]− gives the κ2 square planar complexes [Rh{η4-C4Me2R2C(O)}{HC(pz′)3}][PF6] (R = Me, 6+[PF6]−; R = Et, 7+[PF6]−) in which the cyclopentadienone ligands are coordinated via two Rh–monoalkene bonds; the structurally characterised form of 7+ has the two alkyne units linked head-to-head with the CEt termini bound to the ketonic CO group. With HCCPh or HCCH, 1+ gives the octahedral, κ3 rhodium(III) metallacyclopentadienes [Rh(CO)(η1:η1′-CHCRCHCR){HC(pz′)3}][PF6] (R = Ph, 8+[PF6]−; R = H, 9+[PF6]−) with the two alkynes linked head-to-tail in 8+. The reaction of 1+ with HCCH also gives the cycloheptatrienone (tropone) derivative [Rh{η4-C6H6C(O)}{HC(pz′)3}][PF6], 10+[PF6]−, with a κ3 ligand and the cycloheptatrienone ligand bound to the metal via two Rh–C σ-bonds and one Rh–monoalkene interaction.

Frozen solution electron paramagnetic resonance (EPR) spectra are reported for [(η 4 -cod)Rh(μ-RNNNR) 2 -Ir(CO) 2 ] + (cod = 1,5-cyclooctadiene), [(PPh 3 )(CO)Rh(μ-RNNNR) 2 Ir(CO)(PPh 3 )] + (R = p-tolyl) and [Tp'Ir-(CO)(PPh 3 ] + (Tp' = hydrotris(3,5-dimethylpyrazolyl)borate). In the first spectrum, the Rh hyperfine coupling dominates and there are no significant quadrupolar effects. In the second spectrum, the low-field (g x and g y ) features are 1:2:1 triplets rather than 1:1:1:1 quartets (I = 3/2 for 1 9 1 Ir and 1 9 3 Ir), and in the third, the x and y features appear to be doublets. These anomalies result from the very large quadrupole moment for the iridium isotopes. This can be partially understood from a perturbation theory treatment, but quantitative simulations require direct matrix diagonalization and careful treatment of the problem, with respect to the orientation of the electron spin and nuclear spin quantization axes. The theoretical aspects of the calculations are presented, together with an interpretation of the results in terms of the electronic structures of the complexes.

The complex [Mo(CO)(PhC≡CPh) 2 (η 5 -C 5 Me 5 )][BF 4 ] reacts with 3 equiv of CNBu t to yield the mixed alkyne-isocyanide complex [Mo(CNBu t ) 3 (PhC≡CPh)(η 5 -C 5 Me 5 )][BF 4 ], 1. Reaction of 1 with a fourth equivalent of CNBu t generates [Mo{=C(NHBu t )C(Ph)=C(Ph)CN}(CNBu t ) 2 -(η 5 -C 5 Me 5 )][BF 4 ], 2, containing an η 3 -vinylcarbene ligand formed from the coupling of two of the coordinated isocyanide ligands with the diphenylacetylene, with concomitant protonation of one of these isocyanide fragments and dealkylation of the other. Complex 2 may be deprotonated to give [Mo{C(=NBu t )C(Ph)=C(Ph)CN}(CNBut) 2 (η 5 -C 5 Me 5 )], 3. Protonation of 1 with HBF 4 .Et 2 O generates [Mo{=C(Ph)C(Ph)C=NHBu t }(CNBu t ) 2 (η 5 -C 5 Me 5 )][BF 4 ] 2 , 4, by inducing the coupling of a protonated isocyanide ligand with diphenylacetylene, and 4 reacts with CNBu t to give 2. A similar reaction of 4 with P(OMe) 3 generates [Mo{=C(NHBu t )C-(Ph)=C(Ph)CN}(CNBu t ){P(OMe) 3 }(η 5 -C 5 Me 5 )][BF 4 ], 5, involving the same coupling and elimination pattern. The diphenylacetylene and CNBu t ligands of 1 may also be reductively coupled. Thus, treatment of 1 with sodium-mercury amalgam and subsequent protonation with HBF 4 .Et 2 O gives the metallacyclopentatriene-like [Mo{=C(NHBut)C(Ph)=C(Ph)C-(NHBu t )}(CNBu t )(η 5 -C 5 Me 5 ][BF 4 ], 6, or [Mo{=C(NHBu t )C(Ph)C(Ph)CH-NHBu t }(CNBu t )(η 5 -C 5 Me 5 )][BF 4 ], 7, depending upon the solvent. Complex 7 contains an N-protonated η 4 -monoazadiene ligand with a pendant carbene functionality that is also coordinated to the metal. The crystal structures of 1, 2, 3, 6, and 7 have been determined.

X-Ray structural studies on the redox pair [Cr(CO)2(η-PhCCPh)(η-C6Me5H)]z (z = 0 and 1) show that one-electron oxidation of the neutral complex results in a shortening of the Cr–Calkyne bonds and a lengthening of the Cr–C(O) bonds, consistent with depopulation of a HOMO antibonding with respect to the metal–alkyne interaction. Oxidation leads to an increase in the substitutional lability of the Cr–CO bonds so that [Cr(CO)2(η-RCCR)(η-C6Me6)]+ (R = Ph or C6H4OMe-p) reacts with Lewis bases to give [Cr(CO)L(η-RCCR)(η-C6Me6)]+ {L = CNXyl, P(OMe)3 and P(OCH2)3CEt}, X-ray studies on which show a rotation of the alkyne to align with the remaining Cr–CO bond. ESR spectroscopic studies on [Cr(CO)L(η-RCCR)(η-C6Me6)]+ show delocalisation of the unpaired electron onto the alkyne ligand, consistent with its description as a three-electron donor. The cations [Cr(CO)L(η-RCCR)(η-C6Me6)]+ undergo both one-electron reduction and oxidation, and chemical oxidation of [Cr(CO){P(OCH2)3CEt}(η-p-MeOC6H4CCC6H4OMe-p)(η-C6Me6)]+ with AgPF6 gives the dication [Cr(CO){P(OCH2)3CEt}(η-p-MeOC6H4CCC6H4OMe-p)(η-C6Me6)]2+. Thus the two-electron alkyne of [Cr(CO)2(η-RCCR)(η-C6Me6)] is converted into the four-electron alkyne of [Cr(CO)L(η-RCCR)(η-C6Me6)]2+ by an ECE (E = electrochemical, C = chemical) process in which all of the intermediates have been fully characterised.

The synthesis, electrochemistry, and structural characterization of the mononuclear complex [Pd{C6H4N(H)NC(CH3)C5H4N}(p-tolN−NNp-tol)] (1) containing the monodentate 1,3-di-p-tolyltriazenido ligand is described. Compound 1 is an example of a stable species containing a Pd−N amido bond cis to a Pd−C aryl bond. Kinetic parameters for the dynamic intramolecular N(1)−N(3) exchange of the monodentate ligand in complex 1 have been calculated. The cis and trans isomers of the orthometalated face-to-face complex [{Pd(C6H4NNC6H5)(μ-p-tolNNNp-tol)}2] (2) have also been prepared, and the crystal structure of the trans isomer is reported. There are noticeable differences in the electrochemical behavior of the mononuclear and binuclear species. From the electrochemical experiments on both isomers of 2 it is possible to recognize different redox sites, to calculate the electronic coupling between them, and to suggest where the reversible electron transfers occur. Each isomers of 2 undergoes two one-electron oxidations ...

The redox-active cyanomanganese carbonyl ligands trans- and cis-[Mn(CN)(CO)2(PR3)(dppm)] (R = OEt or OPh, dppm = Ph2PCH2PPh2) and trans-[Mn(CN)(CO)(dppm)2] react with SnCl2 to give [Cl2SnII(μ-NC)MnILx] [Lx = trans- and cis-(CO)2(PR3)(dppm), R = OEt or OPh, 1–4, and trans-(CO)(dppm)25]; complex 5 is oxidised by [Fe(η-C5H5)2][PF6] to give [Cl2SnII(μ-NC)MnII(CO)(dppm)2][PF6] 5+[PF6]−. X-Ray structural studies on the redox related pair 5 and 5+ are consistent with oxidation localised mainly at Mn although the SnII–NC distance increases from 2.198(3) A in 5 to 2.303(9) A in 5+. The dicarbonyl cyanomanganese ligands react with SnCl4 to give trinuclear trans-[Cl4Sn{(μ-NC)MnLx}2] [Lx = trans- and cis-(CO)2(PR3)(dppm), R = OEt or OPh, 6–9], X-ray studies on 7 showing octahedral geometry at Sn(IV). The reaction of cis-[Mn(CN)(CO)2{P(OEt)3}(dppm)] with one equivalent each of SnCl4 and PPh3 and of trans-[Mn(CN)(CO)(dppm)2] with one equivalent each of SnCl4 and [N(PPh3)2]Cl gives heterobinuclear [Cl4(Ph3P)SnIV(μ-NC)MnI(CO)2{P(OEt)3}(dppm)-cis] 10 and [N(PPh3)2][Cl5SnIV(μ-NC)MnI(CO)(dppm)2-trans], [N(PPh3)2]+(11−), respectively; the latter is oxidised with [Fe(η-C5H5)2][PF6] to give [Cl5SnIV(μ-NC)MnII(CO)(dppm)2-trans] 11, completing a series of cyanide-bridged complexes with core oxidation states Sn(II)Mn(I), Sn(II)Mn(II), Sn(IV)Mn(I) and Sn(IV)Mn(II).

The complexes [Rh(CO)LTp′] {L = CO or PPh3, Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate} reacted with the ortho-quinone o-C6Cl4O2 (o-chloranil; 3,4,5,6-tetrachloro-1,2-benzoquinone) to give [Rh{C(O)OC6Cl4O}LTp′] (L = CO, 1; L = PPh3, 2). X-Ray structural studies on 2 reveal CO insertion into one Rh–O bond of a rhodium–catecholate ring. Loss of the inserted CO on UV irradiation (of 1) or thermolysis (of 2) gives [Rh(o-O2C6Cl4)LTp′] (L = CO 3 or PPh34); thermal substitution of the CO ligand of [Rh(o-O2C6Cl4)(CO)Tp′] with L provides a second route to [Rh(o-O2C6Cl4)(PPh3)Tp′] as well as [Rh(o-O2C6Cl4)LTp′] {L = AsPh35, P(OPh)36 or py 7} which are oxidised by [NO]+ to the monocations [Rh(o-O2C6Cl4)LTp′]+4++–7++. X-Ray structural studies on the redox pair [Rh(o-O2C6Cl4)(PPh3)Tp′]z (z = 0, 4 or 1, 4++) are consistent with catecholate ligand-based oxidation; the ESR spectra of the paramagnetic cations 4++–7++ suggest little delocalisation of unpaired electron density from the semiquinone ligand to the RhIIILTp′ unit.

The complex [Mo(CO)(PhCCPh)2(η-C5Me5)][BF4] reacted with three equivalents of 2,6-dimethylphenyl isocyanide, CNxyl, to give the tris(isocyanide) complex [Mo(PhCCPh)(CNxyl)3(η-C5Me5)][BF4] 1. With four equivalents of CNxyl, alkyne–isocyanide coupling leads to the formation of the diiminometallacyclopentene [Mo{C(Nxyl)C(Ph)C(Ph)CNxyl}(CNxyl)2(η-C5Me5)][BF4] 2 which decomposes in thf at room temperature to give the tetrakis(isocyanide) complex [Mo(CNxyl)4(η-C5Me5)][BF4] 3. In dichloromethane, 2 gives the diiminocyclobutene xylNCC(Ph)C(Ph)CNxyl 4, the η2-iminoacyl complex [MoCl(CNxyl)2(η2-xylNCCH2Cl)(η-C5Me5)][BF4] 5, formed by an addition reaction with dichloromethane, and the metallacyclobutene complex [Mo{C(Ph)C(Ph)CN(H)xyl}(CNxyl)3(η-C5Me5)][BF4]26. Complex 5 is more efficiently synthesized by photolysis of 3 in dichloromethane, but the same reaction in chloroform produces the dichloromethyl complex [MoCl(CHCl2)(CNxyl)3(η-C5Me5)][BF4] 7 which thermally decomposes to [MoCl2(CNxyl)3(η-C5Me5)][BF4] 8. Reaction of 2 with HCl in diethyl ether results in protonation of the metallacyclic ring and formation of the iminium metallacyclopentene [MoCl{C(Nxyl)C(Ph)C(Ph)CN(H)xyl}(CNxyl)2(η-C5Me5)][BF4] 9. Compound 3 is oxidised by AgBF4 to give the molybdenum(IV) complex [MoF(CNxyl)4(η-C5Me5)][BF4]210. The molecular structures of 3–6, 7/8 and 10 have been determined by X-ray crystallography.

The complexes [Rh(CO)LTp′] {Tp′ = HBR3, R = 3,5-dimethylpyrazolyl; L = PPh32, PCy33, L = P(NMe2)34, P(C6H4Me-p)35 or P(C6H4Me-m)36}, prepared from [Rh(CO)2Tp′] 1 and L, and [Rh(PPh3)2L′] [L′ = Tp′ 8, Tp 9 or B(pz)410 {Tp = HB(pz)3, pz = pyrazolyl}] and [Rh(dppe)Tp′] 11, prepared from [{Rh(μ-Cl)(PPh3)2}2] or [{Rh(μ-Cl)(dppe)}2] and KL′, adopt four-co-ordinate κ2 structures, confirmed in the cases of 2–4, 6 and 8 by X-ray structural studies. By contrast, complex [Rh(CO){P(OPh)3}Tp′] 7 has a distorted five-co-ordinate square pyramidal structure with a long Rh ⋯ N contact [2.764(2) A] in the apical site and an essentially planar Rh(CO)PN2 basal plane. Each complex undergoes fluxional processes on the NMR timescale. One-electron oxidation of 1–11 gives the κ3 rhodium(II) cations 1+++–11++; the crystal structures of salts of 2+ and 8+ confirm stabilisation of the unusual rhodium(II) oxidation state by axial co-ordination of the third pyrazolyl ring as a result of oxidatively induced κ2–κ3 isomerisation. These structures and ESR spectroscopy are consistent with a five-co-ordinate square pyramidal geometry with the unpaired electron in a σ* Rh–Naxial orbital.

The complexes [Rh2(CO)4(μ-p-XC6H4NNNC6H4X′-p)2] (X = X′ = H, Me, Et, OMe, CN, F, Cl or Br; X = H, X′ = OMe or NO2) were prepared in a two-step reaction involving the cleavage of [{Rh(μ-Cl)(CO)2}2] with the diaryltriazene p-XC6H4NNNHC6H4X′-p followed by the deprotonation of the resulting mononuclear triazene complex [RhCl(CO)2{N(C6H4X-p)NNHC6H4X′-p}] with NEt3. Yields of the dimeric products were maximised by carefully controlling the reaction time for each step. Reaction of the tetracarbonyls with PPh3 gave the mono- and di-substituted species [Rh2(CO)4−n(PPh3)n(μ-p-XC6H4NNNC6H4X′-p)2] (n = 1 or 2), the reaction times again depending on the substituents X and X′. Each binuclear complex undergoes at least one reversible one-electron oxidation reaction at a platinum electrode in CH2Cl2. In some cases, e.g. X = X′ = OMe, as many as three oxidation waves are observed; for X = H, X′ = NO2, n = 1 or 2, well-defined reduction waves are apparent. The oxidation potential depends on the extent of carbonyl substitution (for each incremental increase in n the potential is decreased by ca. 300 mV) and on the triazenide ligand substituent such that E°′ for the first oxidation wave can be varied systematically over a range of 800 mV. There is a linear relationship between E°′ for the first oxidation step and the Hammett parameter σp but a poorer correlation for the second oxidation process.

The complex salts [Fe4(μ3-S)4(η-C5H5)4]m[M(mnt)2]n {M = Ni, n = 2, m = 1 1; n = 1, m = 1 2 or 2 3: M = Pt, n = 2, m = 1 4; n = 1, m = 1 5 or 2 6; mnt = S2C2(CN)2} have been synthesized by stoichiometric reactions between [NEt4]n[M(mnt)2] (n = 1 or 2, M = Ni or Pt) and [Fe4(μ3-S)4(η-C5H5)4][PF6]n (n = 1 or 2); the salts 2–6 have been structurally characterised. The molecular structures of the [Fe4(μ3-S)4(η-C5H5)4]z+ (z = 1 or 2) cations and the [M(mnt)2]z− (M = Ni or Pt, z = 1 or 2) anions show mean Fe⋯Fe and M–S distances characteristic of z. The isostructural salts [Fe4(μ3-S)4(η-C5H5)4]2[M(mnt)2] (M = Ni 3 or Pt 6) comprise two monocations and one dianion; the planar anions, which form a ruffled ribbon arrangement, are interleaved by the near-spherical cations. The structure of [Fe4(μ3-S)4(η-C5H5)4][Pt(mnt)2]2, 4, with two monoanions and one dication, has face-to-face metal–metal pairs of [Pt(mnt)2]− ions separated by individual [Pt(mnt)2]− units, leading to anion layers interleaved by layers of cations. Salts 2 and 5 have very different structures. The nickel complex [Fe4(μ3-S)4(η-C5H5)4][Ni(mnt)2] 2 comprises dications and dianions whereas the platinum complex [Fe4(μ3-S)4(η-C5H5)4][Pt(mnt)2] 5 has monocations and monoanions. In salt 2 the dianions are well separated while in 5 the monoanions are weakly dimerised.

The sterically hindered redox-active cyanomanganese carbonyl complex trans-[Mn(CN)(CO)(dppm)2] acts as an N-donor ligand towards MCl2 (M = Mn, Co or Ni) to give the cyanide-bridged tetrahedral (at M) complexes [Cl2(thf)M(μ-NC)MnLx] {Lx = (CO)(dppm)2}. Displacement of the labile thf ligand by chloride ion affords anionic [Cl3M(μ-NC)MnLx]− which is oxidised to [Cl3M(μ-NC)MnLx], containing two paramagnetic centres, tetrahedral M(II) and low spin octahedral Mn(II). The cobalt(II) complex [Cl2(thf)Co(μ-NC)MnLx] also undergoes thf substitution with neutral N-donor ligands to give [Cl2L′Co(μ-NC)MnLx] {L′ = 4,4′-bipy or (NC)Mn(PPh3)(NO)(η-C5H4Me)}; the latter is oxidised to the mixed valence trimetallic cation [Cl2CoII{(μ-NC)MnIILx}{(μ-NC)MnI(PPh3)(NO)(η-C5H4Me)}]+. X-Ray structural studies on [Cl2(thf)M(μ-NC)MnLx] (M = Co, Mn or Ni) and [Cl3Mn(μ-NC)MnLx] show distortion of the M–N–C angle to accommodate non-bonded interactions between the phenyl groups of the bulky Mn(dppm)2 moiety and the ligands at the tetrahedral metal; distortions of the tetrahedral valence angles at the MII centres of [Cl2(thf)M(μ-NC)MnLx] may in part be related to the weak binding of thf.

The complexes [M(NCMe)(RC2R)2(η-C5R‘5)]+ (M = Mo or W, R = Me or Ph; R‘ = H or Me, C5R‘5 = C5HPh4) undergo one-electron reduction to [M(NCMe)(RC2R)2(η-C5R‘5)], which rapidly lose acetonitrile to give the 17-electron complexes [M(RC2R)2(η-C5R‘5)]. The stability of [M(RC2R)2(η-C5R‘5)] depends on R and R‘; for R = Ph, the radical is sufficiently stable so that its reduction to the 18-electron anion [M(RC2R)2(η-C5R‘5)]- is detected in the cyclic voltammogram. Chemical reduction of [Mo(NCMe)(PhC2Ph)2(η-C5HPh4)]+ with [Co(η-C5H5)2] gives the air-sensitive solid [Mo(PhC2Ph)2(η-C5HPh4)], characterized as an alkyne-based radical by ESR spectroscopy, which gives [Mo(NCMe)(PhC2Ph)2(η-C5HPh4)]+ when treated with [Fe(η-C5H5)2]+ in acetonitrile. The carbonyl cations [M(CO)(RC2R)2(η-C5R‘5)]+ (M = Mo, W) undergo two sequential one-electron reductions, the first of which is reversible and gives the 19-electron species [M(CO)(RC2R)2(η-C5R‘5)]. The 19-electron radical [Mo(CO)(PhC2Ph)2(η-C5Me5)] has been characterized in sol...

The reaction of [Mn(CN)(PR3)(NO)(η-C5H4Me)] (R = Ph or OPh) with [AuCl(tht)] (tht = tetrahydrothiophene) in the presence of Na[BPh4] gave [Mn(CNBPh3)(PR3)(NO)(η-C5H4Me)], X-ray studies on which (R = OPh) confirm addition of triphenylboron to the nitrogen atom of the cyanomanganese centre. Comparison of the geometry of the Ph3B–NC interaction, as measured by the mean C–B–C angle and mean B–C length, with those of L·BPh3 species in the Cambridge Structural Database showed the manganese ligand to be a relatively weak donor comparable with a range of other N and O ligands. The complexes [Mn(CNBPh3)(PR3)(NO)(η-C5H4Me)], as well as cis-[Mn(CNBPh3)(CO)2{P(OEt)3}(dppm)] and trans-[Mn(CNBPh3)(CO)(dppm)2], can be prepared directly from BPh3 and the appropriate cyanomanganese ligand. Each of the complexes undergoes one-electron oxidation at a platinum disc electrode in CH2Cl2 and the reaction of trans-[Mn(CNBPh3)(CO)(dppm)2] with [Fe(η-C5H5)2][PF6] gives trans-[Mn(CNBPh3)(CO)(dppm)2][PF6].

A new range of potentially tetradentate proligands, H2L, derived from aromatic aldehydes and ketones and aliphatic diamines has been prepared. Their vanadyl(IV) and vanadyl(V) complexes [VO(L)] and [VO(L)]+, and also some adducts [VO(L)→VO(L)]+, have been synthesized. The structures of four selected complexes have been determined and it is shown that these must be a result of both steric and electronic factors that make prediction of conformation and stacking difficult. The adducts [VO(L)→VO(L)]+ have structures that persist in solution in dichloromethane, where they can undergo redox chemistry, but they apparently dissociate into their component complexes in the donor solvent acetonitrile.

UV irradiation of [M(CO)3(η-arene)] and Me3SiCCSiMe3 gives [M(CO)2{CC(SiMe3)2}(η-arene)] (M = Cr, arene = C6H2Me4-1,2,3,5 2V, C6H3Me3-1,2,3 3V or C6H6 4V; M = Mo, arene = C6Me6 5V or C6H3Me3-1,3,5 6V). The crystal structure of [Cr(CO)2{CC(SiMe3)2}(η-C6H6)] 4V confirms the presence of the vinylidene ligand; the complex has approximate Cs symmetry with the C(SiMe3)2 plane orthogonal to the arenecentroid–Cr–Cα–Cβ plane. Voltammetry and IR and NMR spectroscopy show that in solution [Mo(CO)2{CC(SiMe3)2}(η-C6H3Me3-1,3,5)] 6V thermally equilibrates with the alkyne isomer [Mo(CO)2(η-Me3SiCCSiMe3)(η-C6H3Me3-1,3,5)] 6A. The vinylidene complexes [M(CO)2{CC(SiMe3)2}(η-arene)] 2V–6V undergo one-electron oxidation to the alkyne cations [M(CO)2(η-Me3SiCCSiMe3)(η-arene)]+ 2A+–6A+via fast, redox-induced vinylidene-to-alkyne isomerisation. These cations are reduced to the neutral alkyne complexes [M(CO)2(η-Me3SiCCSiMe3)(η-arene)] 2A–6A which slowly isomerise thermally to the neutral vinylidene complexes 2V–6V. Paramagnetic vinylidene and alkyne complex cations have been characterised by ESR spectroscopy; unpaired electron density is extensively delocalised from the metal centre to the C2 ligand, in agreement with the results of EHMO calculations.

Treatment of cis-[Mn(CN)(CO)2{P(OEt)3}(dppm)] with SbCl5 gave paramagnetic trans-[Mn(CNSbCl5)(CO)2{P(OEt)3}(dppm)][SbCl6] the crystal structure of which shows the geometry of the [Mn(CN)(CO)2{P(OEt)3}(dppm)] fragment to be characteristic of a low-spin manganese(II) centre. The Mn–P(OEt)3 and Mn–Pdppm bond lengths are increased by 0.10 and 0.07 A compared with those in analogous manganese(I) species while the Mn–C distances are little changed. A Cambridge Structural Database study of octahedral L·SbCl5 species indicated that the Sb–Cl distances (cis and trans to the ligand L) and the average L–Sb–Clcis angle are strongly correlated and appear to reflect the donor ability of the ligand L. A comparison of the geometry of the Mn(CNSbCl5) moiety with those of other L·SbCl5 species showed that the manganese ligand is a donor comparable with a range of other nitrogen and oxygen ligands but much weaker than ligands such as the phenyl anion. The geometry variations were analysed using a qualitative molecular orbital model for the bonding in L·SbCl5 species.

The reaction of [NEt4]2[Pt(mnt)2] {mnt = [S2C2(CN)2]2–} with [Pt(CNMe)4][PF6]2 gave the diamagnetic ‘stacked’ salt [Pt(CNMe)4][Pt(mnt)2]. In acetonitrile, nitromethane and nitropropane solution respectively, treatment of [NEt4][Pt(mnt)2] with [Pt(CNMe)4][PF6]2 yielded three different crystal forms of the salt of 1∶2 stoichiometry: [Pt(CNMe)4][Pt(mnt)2]2·2MeCN, [Pt(CNMe)4][Pt(mnt)2]2·MeNO2 and [Pt(CNMe)4][Pt(mnt)2]2. These show structure-dependent antiferromagnetism, reflecting differing arrangements of the formally platinate(III) anions [Pt(mnt)2]– within the crystal lattice. The palladium salts [Pt(CNMe)4][Pd(mnt)2] and [Pt(CNMe)4][Pd(mnt)2]2· 2MeCN are isomorphous with their platinum analogues.

The cyanomanganese carbonyls trans-[Mn(CN)(CO)(dppm)2], and cis- and trans-[Mn(CN)(CO)2{P(OR)3}(dppm)] (R = Ph or Et, dppm = Ph2PCH2PPh2) react with TlPF6 to give [(thf)2Tl(µ-NC)Mn(CO)(dppm)2][PF6] 1, [{Tl(µ-NC)MnLx}2][PF6]2 [Lx = cis-(CO)2{P(OR)3}(dppm), R = Ph 2 or Et 3] and [Tl{(µ-NC)MnLx}2][PF6] [Lx = trans-(CO)2{P(OR)3}(dppm), R = Ph 4 or Et 5] respectively. Complexes 1 and 3 have been structurally characterised by single-crystal X-ray diffraction methods; the former is pyramidal at thallium while the latter provides a rare example of a µ3-κC∶κN∶κN cyanide ligand N-bonded to two thallium atoms and C-bonded to manganese.

The redox-active cyanomanganese carbonyl ligands cis- and trans-[Mn(CN)(CO)2{P(OR)3}(dppm)] (R = Ph or Et, dppm = Ph2PCH2PPh2) reacted with ZnBr2, CdI2 and Hg(NO3)2 to give the tetrahedral (at M) complexes [X2M{(µ-NC)MnLx}2] [MX2 = ZnBr2, CdI2 or Hg(NO3)2; Lx = cis- or trans-(CO)2{P(OR)3}(dppm); R = Ph or Et]; similarly [Mn(CN)(NO)(PPh3)(η-C5H4Me)] gives [X2M{(µ-NC)MnLx}2] {MX2 = ZnBr2 or CdI2, Lx = (NO)(PPh3)(η-C5H4Me)}. Treatment of [Br2Zn{(µ-NC)MnLx}2] [Lx = trans-(CO)2{P(OEt)3}(dppm)] 4 with TlPF6 in the presence of 1 or 2 equivalents of trans-[Mn(CN)(CO)2{P(OEt)3}(dppm)] gave the tetra- and penta-metallic complexes [BrZn{(µ-NC)MnLx}3][PF6] 13 and [Zn{(µ-NC)MnLx}4][PF6]2 14 [Lx = trans-(CO)2{P(OEt)3}(dppm)] respectively. Differential pulse volammetry showed that 4, 13 and 14 are oxidised to weakly interacting mixed-valence complexes. The reaction of trans-[Mn(CN)(CO)(dppm)2] with ZnBr2 or CdX2 (X = Br or I) in thf gave the bimetallic species [X2(thf)M(µ-NC)Mn(CO)(dppm)2] (MX2 = ZnBr2 or CdI2) and [Br2Cd(µ-NC)Mn(CO)(dppm)2] which are oxidised by [Fe(η-C5H5)2][PF6] to the MnII complexes [X2(thf)M(µ-NC)Mn(CO)(dppm)2][PF6] and [Br2Cd(µ-NC)Mn(CO)(dppm)2][PF6]. The crystal structures of the tetrahedral polynuclear complexes [I2Cd{(µ-NC)MnLx}2] [Lx = trans-(CO)2{P(OEt)3}(dppm)] 9 and [Br2(thf)Zn(µ-NC)Mn(CO)(dppm)2] 15 are reported, and the importance of steric effects (as quantified by cone angles) in the behaviour of cyanomanganese carbonyl ligands is noted.

The preparation and characterization of [Cr(CO)2L(η-C6Me6)] (L = PEt3, PPh3, P(OEt)3 and P(OPh)3, are reported. One-electron oxidation affords unstable Cr(I) cations, [Cr(CO)2L(η-C6Me6)]+, EPR spectra of which are reported. Detailed analysis of the anisotropic 31P hyperfine interaction indicates that, in frozen CH2Cl2/THF, the phosphine and phosphite complexes have 2A‘ and 2A‘‘ ground states, respectively. The hyperfine anisotropy can be accounted for by dipolar interaction of the 31P nucleus with spin density on Cr and, in the case of the phosphite complexes, with ∼0.01 P 3py spin density resulting from π-backbonding. Line width anomalies observed in EPR spectra of these and other Cr(I) and Mn(II) “piano stool” complexes can be understood in terms of molecular distortions resulting from solvation in frozen solutions.

The reaction of [Mo2(μ-C8Me8)(η-C5H5)2] (1) with 1 equiv of [Fe(η-C5H5)2][PF6] in CH2Cl2 gives [Mo2(μ-C8Me8)(η-C5H5)2][PF6] (1+[PF6]-), which reacts with a second 1 equiv of [Fe(η-C5H5)2][PF6] or 1 equiv of the trityl radical, •CPh3, to give [Mo2(μ-C8Me7CH2)(η-C5H5)2]+ (2+); depending on the oxidant the activation of one C−H bond of 1, with the formation of 2+, can occur by an EC or EEC mechanism. Complexes 1 and 1+ constitute the first isolable redox pair to show the structural effects of one-electron oxidation on a metal−alkene bond. An X-ray crystal structure analysis shows that the geometry of 1+ is similar to that of 1 and has approximate Cs symmetry. The Mo−Mo distance in 1+ is consistent with the presence of a MoMo double bond. The C8 chain acts as a double μ-allylidene ligand while binding to Mo(2) as an η-alkene through C(4) and C(5). The Mo−C distances for atoms C(1), C(2), C(3), C(6), C(7), and C(8) of the C8 chain are remarkably similar in 1 and 1+ while the Mo(2)−C(4) and Mo(2)−C(5) distances...

The complex [FeL2(NO)2](L = PEt31a, L = PPh31b or L2= dppe 1c) prepared from [{Fe(µ-I)(NO)2}2] and PPh3 or Ph2PCH2CH2PPh2(dppe){in the presence and absence of [Co(cp)2](cp =η5-C5H5) respectively} undergo one-electron oxidation at a platinum electrode in CH2Cl2. The complex [{Fe(µ-dppm)(NO)2}2]2, prepared from [{Fe(µ-I)(NO)2}2] and Ph2PCH2PPh2(dppm) in the presence of [Co(cp)2], undergoes two sequential one-electron oxidations. Complex 1b with [Fe(cp)2]+ gave 1b+, X-ray studies of which show a distorted tetrahedral geometry with near C2v symmetry. Oxidation of 1b leads to substantial lengthening of the Fe–P bonds and changes in the P–Fe–P and N–Fe–N angles. These changes are consistent with significant Fe–P π-bonding character in the singly occupied molecular orbital of 1b+. Cation 1b+ reacts with halide ions, giving [FeX(PPh3)(NO)2](X = Cl or I) and then [FeX2(NO)2]–, and with OPPh3 to give [Fe(OPPh3)(PPh3)(NO)2]+3. X-Ray studies on the last, as its [PF6]– salt, show a distorted tetrahedral geometry; the co-ordination angles at iron approach trigonal bipyramidal with the PPh3 ligand in one apical site and the other apical site vacant. The complex [Fe(OPPh3)2(NO)2]+4+ resulted from the reaction between [{Fe(µ-I)(NO)2}2] and OPPh3 in the presence of TlPF6. An analysis of the ESR spectra of the paramagnetic cations 1b+, 3+ and 4+, together with extended-Huckel MO calculations on models of 1b+ and 3+, suggest that the complex catalysts formed from [{Fe(µ-Cl)(NO)2}2] and Ag+ or Tl+ are also four-co-ordinate 17-electron radicals. A crystallographic database study of four-co-ordinate dinitrosyl complexes of iron and other metals confirms that the N–Fe–N and O ⋯ Fe ⋯ O angles are linearly related. Consideration of these geometric effects, and those resulting from oxidation of 1b, in the light of a model proposed by Summerville and Hoffmann provides insight into the bonding in these and related species.

The reaction of [Cu(NCMe)4][PF6] with 2 equivalents of [Mn(CN)Lx]{Lx=(CO)(dppm)2, cis-or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et, dppm = Ph2PCH2PPh2)} in CH2Cl2 gave [Cu{µ-NC)MnLx}2][PF6]. With 2 equivalents of [Mn(CN)Lx] in toluene, AgPF6 gave [Ag{(µ-NC)MnLx}2]+{Lx=cis- or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)} but in CH2Cl2cis-[Mn(CN)(CO)2(PEt3)(dppe)](dppe = Ph2PCH2CH2PPh2) or trans-[Mn(CN)(CO)(dppm)2] and AgX (X = BF4–, PF6– or SbF6–) gave the tricationic manganese(II) complexes [Ag{(µ-NC)Mn(CO)(dppm)2}2][PF6]3 and [Ag{(µ-NC)MnLx}2]X3{Lx=trans-(CO)2(PEt3)(dppe)]; the complexes [Ag{(µ-NC)MnLx}2][PF6]3{Lx=trans-(CO)2(P(OR)3](dppm)(R = Ph or Et)} were prepared directly from Ag[PF6] and trans-[Mn(CN)(CO)2{P(OR)3}(dppm)][PF6](R = Ph or Et) in CH2Cl2. Treatment of [AuCl(tht)](tht = tetrahydrothiophene) with [Mn(CN)Lx] in CH2Cl2 in the presence of Tl[PF6] yielded [Au{(µ-NC)MnLx}2][PF6]{Lx=(CO)(dppm)2, cis- or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)}. X-Ray structural studies on [Ag{(µ-NC)MnLx}2][PF6]{Lx=trans-(CO)2[P(OPh)3](dppm)}, [Au{(µ-NC)MnLx}2][PF6]{Lx=trans-(CO)2[P(OEt)3](dppm)}, and [Ag{(µ-NC)MnLx}2][PF6]3[Lx=(CO)(dppm)2] showed, in each case, near linear Mn–CN–M′–NC–Mn skeletons (M′= Ag or Au); the Mn–P and P–substituent bond lengths are consistent with octahedral MnI and MnII centres in the monocations and trication respectively. Each of the complexes [M′{(µ-NC)MnLx}2][PF6]{M′= Cu or Au, Lx=(CO)(dppm)2; M′= Cu or Ag, Lx=trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)} showed one reversible two-electron oxidation wave at a platinum electrode in CH2Cl2; the trication [Cu{(µ-NC)Mn(CO)(dppm)2}2]3+ was generated in solution by controlled potential electrolysis of [Cu{(µ-NC)Mn(CO)(dppm)2}2]+, and [Au{(µ-NC)Mn(CO)(dppm)2}2][PF6]3 was prepared by chemical oxidation of [Au{(µ-NC)Mn(CO)(dppm)2}2][PF6] with [Fe(cp)2][PF6](cp =η-C5H5) in CH2Cl2. Magnetic and ESR spectroscopic studies provided further evidence for the presence of two isolated low-spin MnII centres in the trications [Ag{(µ-NC)MnLx}2]3+{Lx=(CO)(dppm)2, trans-(CO)2[P(OR)3](dppm)(R = Ph or Et) or trans-(CO)2(PEt3)(dppe)}. By contrast, [Au{(µ-NC)MnLx}2]+{Lx=trans-(CO)2[P(OR)3(dppm)(R = Et or Ph)} showed two reversible one-electron oxidation waves corresponding to the stepwise formation of di- and tri-cations. Electrolytic oxidation of [Au{(µ-NC)MnLx}2]+ in tetrahydrofuran, or chemical oxidation with [N(C6H4Br-p)3]+ or [Fe(η-C5H4COMe)(cp)]+ in CH2Cl2, gave solutions of [Au{(µ-NC)MnLx}2]2+{Lx=trans-(CO)2[P(OEt)3](dppm)}, IR spectroscopic and voltammetric studies on which are compatible with weak interaction between the two manganese centres in the mixed-valence dication.

Treatment of [W(CO)(MeC2Me)2(η-C5H5)][PF6] with ONMe3 in acetonitrile yields [W(NCMe)(MeC2Me)2(η-C5H5)][PF6] which undergoes irreversible reduction at a Pt electrode in THF. Sodium amalgam reduction of [W(NCMe) (MeC2Me)2(η-C5H5)][PF6] gives orange crystals of [W2(µ-σ,σ,η 2,η 2-C4Me4)2 (η-C5H5)2] X-ray studies on which reveal pairwise alkyne coupling and a novel bis(metallacyclopentadiene) structure.

The reaction of [Au2{µ-(CH2)2PPh2}2] or [Au2(µ-L–L)2][ClO4]2[L–L = dppm (Ph2PCH2PPh2) or dppe (Ph2PCH2CH2PPh2)] with [Aun(µ-C5H4NS)n] led to the heterobridged dinuclear complexes [Au2{µ-(CH2)2PPh2}(µ-C5H4NS)] or [Au2(µ-C5H4NS)(µ-L–L)]ClO4. The structure of [Au2{µ-(CH2)2PPh2}(µ-C5H4NS)] has been established by X-ray crystallography and exhibits a short intermolecular gold–gold distance of 2.8623(7)A, whereas the shortest intermolecular distance is 3.984 A. The complex crystallized in the monoclinic space group P21/n with a= 9.430(2), b= 8.819(2), c= 22.786(5)A, β= 99.08(3)°, Z= 4. Cyclic voltammograms showed that this complex is irreversibly oxidised at a platinum electrode; chemical oxidation with the ferrocenium ion in the presence of halide or pseudohalide ions gave the gold(II) complexes [Au2{µ-(CH2)2PPh2}(µ-C5H4NS)X2](X = Cl, Br, I or SCN). The same derivatives can be obtained by oxidative addition of halogen and/or by subsequent substitution with other halogens or pseudohalogens. The complex [Au2{µ-(CH2)2PPh2}(µ-C5H4NS)Br2] crystallized in the triclinic space group P with a= 14.810(10), b= 14.908(10), c= 17.041(12)A, α= 70.20(5), β= 69.77(5), γ= 89.19(5)°, Z= 6 (at –100 °C). The Au–Au bond lengths in the three independent molecules are 2.564(4), 2.548(4) and 2.547(4)A.

The X-ray crystal structure analyses of the three redox pairs [Rh 2 (CO) 2 LL'{μ-PhNC(Me)NPh)} 2 ] z [z=0,1; L=L'=PPh 3 (1, 1 + ); L=L'=P(OPh) 3 (1, 1 + ); L=PPh 3 , L'= P(OPh) 3 (3, 3 + )] are reported and the molecular structures compared with those of [Rh 2 (CO) 2 (PPh 3 ) 2 (μ-RNNNR) 2 ] z [z=0,1; R=p-tolyl (4, 4 + )]

The reaction of [Au2{µ-(CH2)2PPh2}2] with [Au2(µ-L–L)2]n+[n= 0, L–L = S2CNMe2, S2CNEt2 or S2CN(CH2Ph)2; n= 2, L–L = Ph2PCH2PPh2(dppm), Ph2P(CH2)2PPh2 or Ph2PNHPPh2] led to heterobridged dinuclear complexes [Au2{µ-(CH2)2PPh2}(µ-L–L)]n+(n= 0 or 1). The same complexes can also be obtained by reaction of [N(PPh3)2][(AuCl)2{µ-(CH2)2PPh2}] with the silver compounds [Ag(S2CNMe2)]6 or [Ag2(OClO3)2(dppm)3] or by reaction of [(AuPPh3)2{µ-(CH2)2PPh2}][ClO4] with [{Au(C6F5)}2(µ-L–L)](L–L = diphosphines or o-Ph2PC5H4N). The structure of [Au2{µ-(CH2)2PPh2}(µ-S2CNEt2)] has been established by X-ray crystallography. Two molecules are bonded through an intermolecular gold–gold interaction, thus forming a linear chain of four gold atoms with Au–Au (intramolecular) 2.867, 2.868, (intermolecular) 2.984 A.

The potassium salts of the new S(2)N-heteroscorpionate ligand hydrobis(methimazolyl)(3,5-dimethylpyrazolyl)borate [HB(mt)(2)(pz(3,5-Me))](-) and its known analogue hydrobis(methimazolyl)(pyrazolyl)borate [HB(mt)(2)(pz)](-) (prepared from KTp' or KTp and methimazole, Hmt), and the adduct KTp·Hmt have polymeric structures in the solid state (the first a ladder and the other two chains). The iridaboratranes [IrHLL'{B(mt)(2)X}] (X = pz(3,5-Me) or pz), prepared from the heteroscorpionate anion and [{Ir(cod)(μ-Cl)}(2)] (LL' = cod), subsequent carbonylation [LL' = (CO)(2)] and then reaction with phosphine [LL' = (CO)(PR(3)), R = Ph or Cy], have a pendant pyrazolyl ring and a bicyclo-[3.3.0] cage formed by an S(2)-bound B(mt)(2) fragment. The binuclear species [(cod)HIr{μ-B(mt)(3)}IrCl(cod)], the only isolated product of the reaction of KTm with [{Ir(cod)(μ-Cl)}(2)], also has an S(2)-bound iridaboratrane unit but with the third mt ring linked to square planar iridium(I).

Molecular orbital calculations at the extended Huckel level have been carried out on the model binuclear cyanobridgedcationiccomplexesofthe type [[Mn]-CN-[Mn]) + ,where [Mn]=cis- or trans-mer-Mn(CO) 2 (PH 3 ) 3 + . The results are consistent with a weak Mn- - -Mn interaction mediated by the CN bridge. The energy and composition of the highest occupied orbitals are determined by the stereochemistry and so is the Mn atom at which the HOMO is localized.

1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 1. Radical Cations 891 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich Compounds 894

Monometallic complexes of the tetrakis(pyrazolyl)borate ligand [ML(2){B(pz)(4)}] {M = Rh, Ir; L(2) = eta-cod, eta-nbd, (CO)(2), (CO)(PPh(3))} have two free pyrazolyl rings which can be coordinated to a second ML(2) unit to give the dimeric compounds [L(2)M{mu-B(pz)(4)}ML(2)](+), and to a metal halide to give heterobimetallic species [L(2)M{mu-B(pz)(4)}M'Cl(2)]. (1)H NMR spectroscopy shows that [(eta-cod)Rh{mu-B(pz)(4)}Rh(eta-cod)](+) 1(+), [(eta-nbd)Rh{mu-B(pz)(4)}Rh(eta-nbd)](+) 2(+), [(eta-cod)Ir{mu-B(pz)(4)}Ir(eta-cod)](+) 3(+) and [(CO)(2)Rh{mu-B(pz)(4)}Rh(CO)(2)](+) 4(+) are fluxional at room temperature. Cooling a solution of [(eta-cod)Rh{mu-B(pz)(4)}Rh(eta-cod)](+) 1(+) to -90 degrees C slows the fluxional process, which involves inversion of the two B-(N-N)(2)-M six-membered rings. Attempts to synthesise the asymmetric complexes [(eta-cod)Rh{mu-B(pz)(4)}Rh(eta-nbd)](+) 7(+), [(eta-cod)Rh{mu-B(pz)(4)}Ir(eta-cod)](+) 8(+) and [(eta-cod)Rh{mu-B(pz)(4)}Rh(CO)(2)](+) 9(+) produced a mixture of [L(2)M{mu-B(pz)(4)}ML(2)](+), [L'(2)M'{mu-B(pz)(4)}M'L'(2)](+) and the desired species. The heterobimetallic complexes [L(2)Rh{mu-B(pz)(4)}M'Cl(2)] (M' = Co, L(2) = eta-cod 10; M' = Co, L(2) = eta-nbd 11; M' = Co, L = CO 12; M' = Co, L(2) = (CO)(PPh(3)) 13; M' = Zn, L(2) = eta-cod 14; M' = Zn, L(2) = eta-nbd 15; M' = Pd, L(2) = eta-cod 16) possess square planar Rh(I) linked to square planar Pd(II) or tetrahedral Zn(II) and Co(II) centres. The paramagnetic Co(II) complexes give (1)H NMR spectra with signals shifted over a range of 75 ppm. The UV-Vis spectra of 10-13 show four bands, one MLCT at Rh and three d-d transitions arising from the splitting of the (4)T(1)(P) excited state due to approximate C(2v) symmetry at Co.

The η2(3e)-vinyl complex is oxidized by [FeCp2]+, [CPh3]+, or B(C6F5)3 to form the 17-electron cation which on warming loses H to form the cationic η2(4e)-alkyne complex [Mo(η2-PhC⋮CPh){P(OMe)3}2Cp]+. In the case of the borane there is evidence for a competing reaction between the η2-vinyl complex and the acid (H2O)B(C6F5)3, resulting in the formation of a labile trans-stilbene complex.

The cyclic voltammograms of the alkyne complexes [M(SR)L(eta-R'C[triple bond, length as m-dash]CR')(eta-C(5)H(5))] (M = Mo or W, R = Me or Ph, R' = Me or Ph) show two oxidation processes. Both are irreversible for the stereochemically rigid carbonyls (L = CO) but the first is reversible for the fluxional phosphites {L = P(OMe)(3)}; the paramagnetic monocations [M(SPh){P(OMe)(3)}(eta-MeC[triple bond, length as m-dash]CMe)(eta-C(5)H(5))](+) were detected by ESR spectroscopy after in situ chemical one-electron oxidation. By contrast, the hydrotris(pyrazolyl)borate analogues [W(SR)(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'] {R = Me or Ph, Tp' = hydrotris(3,5-dimethylpyrazolyl)borate} are oxidised in two reversible steps to the corresponding mono- and dications; the redox pair [W(SPh)(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'](z) (z = 0 and 1+) has been structurally characterised. A comparison of the redox potentials for the oxidation of [W(SR)(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'] with those of the halide analogues [WX(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'] suggests that the factors which give rise to the inverse halide order for the latter may not operate for the thiolates, which appear to be the better pi-donors in all three redox states [WL(CO)(eta-PhC[triple bond, length as m-dash]CPh)Tp'](z) (L = halide or thiolate, z = 0, 1+ and 2+).

The structures adopted by a range of hydrotris(3,5-dimethylpyrazolyl)methane complexes [ML(2){HC(pz')(3)}](+) (M = Rh, Ir; L(2) = diene) have been investigated. There is low steric hindrance between ligands in [Rh(eta-nbd){HC(pz')(3)}](+) (nbd = norbornadiene) and [Rh(eta-dmbd){HC(pz')(3)}](+) (dmbd = 2,3-dimethylbuta-1,3-diene) resulting in kappa(3) co-ordination of the pyrazolylmethane. The complexes [M(eta-cod){HC(pz')(3)}](+) (cod = cycloocta-1,5-diene) (M = Rh, Ir) are kappa(2) co-ordinated with the free pyrazolyl ring positioned above and approximately parallel to the square plane about rhodium or iridium. The HC(pz')(3) complexes undergo fast exchange of the co-ordinated and unco-ordinated pyrazolyl rings on the NMR spectroscopic timescale. However, for [Rh(eta-dmbd){HC(pz')(3)}](+), the fluxional process is slowed at low temperatures, so that inequivalent pyrazolyl rings may be observed. A mechanism for the fluxional process is proposed involving dynamic interconversion between isomeric forms in solution. The bonding mode of the HC(pz')(3) ligand can be determined by (13)C NMR spectroscopy. The (13)C chemical shifts (for the sp(3) hybridised carbon of the ligand) show the general pattern, kappa(3)71.5 ppmkappa(2). The electrochemical behaviour of the pyrazolylmethane complexes is related to the degree of structural change, which occurs on electron transfer and is compared with that of the pyrazolylborate analogues.