Dihydride Complexes of the Cobalt and Iron Group Metals:  An Investigation of Structure and Dynamic Behavior

Abstract

The previously reported cationic dihydride complexes (PP 3 )MH 2 + (M = Co, Rh and Ir; PP 3 = P(CH 2 CH 2 PPh 2 ) 3 ) have been prepared using improved synthetic methods. Variable-temperature 1 H and 31 P NMR spectra of these complexes reveal complex dynamic behavior. The hydride region 1 H NMR spectra have been accurately simulated at all temperatures using a simple site permutation model after taking into consideration the opposite signs of the cis and trans H−P coupling constants. Partial deuteration of the hydride ligands in the rhodium and cobalt complexes is achieved by exposure to D 2 . In the partially deuterated samples, no evidence is found for a bound dihydrogen ligand, but the involvement of a dihydrogen species in the dynamic process which interchanges the two hydride positions remains a mechanistic possibility, as indicated by a kinetic isotope effect k H / k D = 1.3(1). The partially deuterated samples exhibit large and temperature-dependent isotope effects on the 1 H NMR chemical shifts observed for the hydride resonances, which are attributed to isotopic perturbation of resonance. This arises from non-statistical occupation of the two different hydride sites and also leads to perturbation of the averaged H−P coupling constants. Similar observations have been made for the neutral iron complex (PP 3 )FeH 2 .