Solvent effects on electron delocalization in paramagnetic organometallic complexes: Solvent manipulation of the amount of 19-electron character in Co(CO)3L2 (L2 = a chelating phosphine)

Article abstract of DOI:10.1021/ja00042a019

Infrared, ESR, and electronic absorption spectroscopic studies are reported on the 18+δ Co(CO)₃L₂ complex. (L₂ is the chelating phosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride. 18+δ complexes are 19-electron complexes in which the unpaired electron is primarily localized on a ligand.) The spectra are solvent dependent and are interpreted in terms of increased delocalization of the unpaired electron from an L₂(π*) orbital onto the Co(CO)₃ portion of the molecule with decreasing solvent polarity. The relationship between the extent of delocalization onto the Co(CO)₃ and the substitution reactivity of the molecule was studied. Increased delocalization increases the rate of CO loss (and hence dissociatively activated substitution) because the acceptor molecular orbital on the Co(CO)₃ fragment is Co-CO antibonding (k(benzene,25°) = (7.46 ± 0.04) × 10^-2 s^-1; k(CH₂Cl₂,25°) = (5.47 ± 0.03) × 10^-3 s^-1). These substitution results are an exception to the rule of thumb which states that the lability of M-CO bonds decreases as the ν(G≡O) frequencies decrease. An SCF-Xα-SW calculation on the Co(CO)₃L₂′ complex (L₂′ = 2,3-bis(phosphino)maleic anhydride; i.e. L₂′ is L₂ with the phenyl groups replaced by H atoms) confirmed previous ESR spectroscopic results which showed that the SOMO on the Co(CO)₃L₂ complex is primarily an L₂-based π* orbital.