Structural consequences of the one-electron reduction of d4 [Mo(CO)2(η-PhC≡CPh)Tp']+ and the electronic structure of the d5 radicals [M(CO)L(η-MeC≡CMe)Tp'] {L = CO and P(OCH2)3CEt}

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.