Studies of oxalate-bridged MM quadruple bonds and their radical cations (M = Mo or W): On the matter of linkage isomers

Electronic structure calculations employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been carried out on the model complexes {[(HCO₂)₃M₂] ₂(μ-O₂CCO₂)}^0/+ (M = Mo or W) in D_2h symmetry, where the oxalate bridge forms either five- or six-membered rings with the M₂ centres; the complexes are hereafter referred to as μ(5,5)^0/+ and μ(6,6)^0/+, respectively. The calculations predict that the neutral complexes should exist as the μ(5,5) linkage isomer, while the radical cations favour the μ(6,6) isomer by ca. 4-6 kJ mol^-1. For the μ(5,5) isomers, the rotational barriers about the oxalate C-C bond have been calculated to be 15.9 and 27.2 kJ mol^-1 for M = Mo and W, respectively. For the cationic μ(5,5)⁺ isomers the barrier is higher, being 36.8 and 50.6 kJ mol^-1 for M = Mo and W, respectively. The calculated Raman and visible near-IR spectra for the μ(5,5)^0/+ and μ(6,6) ^0/+ are compared with experimental data obtained for the {[( ^tBuCO₂)₃M₂]₂(μ-O ₂CCO₂)}^0/+ complexes, hereafter referred to as M₄OXA^0/+ (M = Mo or W). The experimental data more closely correlate with that calculated for the μ(5,5)^0/+ linkage isomers, and the ¹³C-NMR spectrum of the mixed metal complex Mo ₂W₂OXA indicates the presence of the 5-membered oxalate-bridged species (J_cc = 100 Hz).