Tetracarbonylmolybdenum complexes of 2-(phenylazo) pyridine ligands. Correlations of molybdenum-95 chemical shifts with electronic, infrared, and electrochemical properties

The complexes cis-Mo(CO)(4)(X-2-(phenylazo)pyridine) (X = 4-CH3O, 4-CH3, H, 4-Cl, 5-Br, 5-CF3, 6-CH3) and cis-Mo(CO)(4)(2-(2-CH (3)-phenylazo)pyridine) have been synthesized and characterized by cyclic voltammetry, 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 sigma parameters for the pyridyl substituents. The failure of the complexes cis-Mo(CO)(4)(6-CH3-2-(phenylazo)pyridine) and cis-Mo(CO)(4)(2-(2-CH3-phenylazo)pyridine) to fit some of the correlations is attributed to steric or electronic effects. The effect of a substituent on the pyridyl ring of 2-(phenylazo)pyridine appears to be entirely an inductive one operating through the sigma bonding. It is suggested that the 2-(phenylazo)pyridines might be appropriately viewed as ligands whose strong pi-acceptor ability resides with the azo group, while the pyridyl group acts primarily as a pyridine whose basicity has been decreased by the strong electron-withdrawing 2-phenylazo substituent.