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

The previously reported cationic dihydride complexes (PP₃)MH₂⁺ (M = Co, Rh and Ir; PP₃ = P(CH₂CH₂PPh₂)₃) have been prepared using improved synthetic methods. Variable-temperature ¹H and ³¹P NMR spectra of these complexes reveal complex dynamic behavior. The hydride region ¹H 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 D₂. 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 ¹H 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 (PP₃)FeH₂.