SOLVENT EFFECTS ON ELECTRON DELOCALIZATION IN PARAMAGNETIC ORGANOMETALLIC COMPLEXES - SOLVENT MANIPULATION OF THE AMOUNT OF 19-ELECTRON CHARACTER IN CO(CO)3L2 (L(2) = A CHELATING PHOSPHINE)

Infrared, ESR, and electronic absorption spectroscopic studies are reported on the 18+delta Co(CO)3L2 complex. (L2 is the chelating phosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride. 18+delta 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(pi*) 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-degrees) = (7.46 +/- 0.04) x 10(-2) s-1; k(CH2Cl2,25-degrees) = (5,47 +/- 0.03) x 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 nu(C=O) frequencies decrease. An SCF-X-alpha-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 pi* orbital.