Tetracarbonylmolybdenum complexes of 2-(phenylhydrazino)pyridine ligands.: Correlations of spectroscopic data with pyridyl substituent effects

The 2-(phenylhydrazino)pyridine (2-PHP) complexes cis-Mo(CO)4(X2-(phenylhydrazino)pyridine) (X=4-CH3O, 4-CH3, H, 4-Cl, 5-Br, 6-CH3, 4,6-(CH3)2) and cis-Mo(CO)4(2-(2-CH3phenylhydrazino)pyridine) have been synthesized and characterized. The properties of these complexes are compared with those of the analogous 2-(phenylazo)pyridine (2-PAP) complexes. The lack of the π-accepting azo group in the 2-PHP ligands leads to less stable complexes, including the inability even to isolate the complex with X=CF3. The 2-PHP complexes show very good correlations among the 95Mo-NMR chemical shift, the sum of the carbonyl stretching frequencies, and the Hammett σ parameter for the pyridyl substituents. There is also an excellent correlation (r=0.978, n=7) of the 95Mo chemical shift of the 2-PHP complexes with the shift for the 2-PAP complexes. The failure of the complexes with X=6-CH3 or 4,6-(CH3)2 or the complex cis-Mo(CO)4(2-(2-CH3phenylhydrazino)pyridine) to fit some of the correlations is attributed to steric or electronic effects. The 2-hydrazinopyridine complex cis-Mo(CO)4(H2NHNC5H4N) also was characterized.