DIHYDRIDE COMPLEXES OF THE COBALT AND IRON-GROUP METALS - AN INVESTIGATION OF STRUCTURE AND DYNAMIC BEHAVIOR

The previously reported cationic dihydride complexes (PP3)MH(2)(+) (M = Co, Ph and Ir; PP3 = P(CH(2)CH(2)PPh(2))(3)) have been prepared usin g improved synthetic methods. Variable-temperature H-1 and P-31 NMR sp ectra of these complexes reveal complex dynamic behavior. The hydride region H-1 NMR spectra have been accurately simulated at all temperatu res using a simple site permutation model after taking into considerat ion the opposite signs of the cis and trans H-P coupling constants. Pa rtial deuteration of the hydride ligands in the rhodium and cobalt com plexes is achieved by exposure to D-2. In the partially deuterated sam ples, no evidence is found for a bound dihydrogen ligand, but the invo lvement of a dihydrogen species in the dynamic process which interchan ges the two hydride positions remains a mechanistic possibility, as in dicated by a kinetic isotope effect k(H)/k(D) = 1.3(1) The partially d euterated samples exhibit large and temperature-dependent isotope effe cts on the H-1 NMR chemical shifts observed for the hydride resonances , which are attributed to isotopic perturbation of resonance, This ari ses 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)FeH2.