TETRACARBONYLMOLYBDENUM COMPLEXES OF 2-(PHENYLAZO) PYRIDINE LIGANDS -CORRELATIONS OF MO-95 CHEMICAL-SHIFTS WITH ELECTRONIC, INFRARED, AND ELECTROCHEMICAL PROPERTIES

The complexes cis-Mo(CO)(4)(X-2-(phenylazo)pyridine) (X = 4-CH3O, 4-CH 3, H, 4-Cl, 5-Br, 5-CF3, 6-CH3) and cis-Mo(CO)(4)(2-(2-CH (3)-phenylaz o)pyridine) have been synthesized and characterized by cyclic voltamme try, by visible and infrared spectroscopy, and by H-1,C-13, and Mo-95 NMR spectroscopy. The Mo-95 chemical shift correlates with the lowest energy electronic transition, with the sum of the carbonyl stretching frequencies, with the first oxidation potential, and with Hammett sigm a parameters for the pyridyl substituents. The failure of the complexe s cis-Mo(CO)(4)(6-CH3-2-(phenylazo)pyridine) and cis-Mo(CO)(4)(2-(2-CH 3-phenylazo)pyridine) to fit some of the correlations is attributed to steric or electronic effects. The effect of a substituent on the pyri dyl ring of 2-(phenylazo)pyridine appears to be entirely an inductive one operating through the sigma bonding. It is suggested that the 2-(p henylazo)pyridines might be appropriately viewed as ligands whose stro ng pi-acceptor ability resides with the azo group, while the pyridyl g roup acts primarily as a pyridine whose basicity has been decreased by the strong electron-withdrawing 2-phenylazo substituent.