QUANTUM-MECHANICAL EXCHANGE COUPLING IN IRIDIUM TRIHYDRIDE COMPLEXES

Cationic trihydride complexes of the form [(eta-C(5)R(5))Ir(L)H-3]BF4 (R = H, Me; L = various phosphines) have been studied. The H-1 NMR spe ctra of these complexes at low temperature display line patterns in th e hydride region consistent with AB(2)X or A(2)BX spin systems (X = P- 31). The values for the H-A-H-B coupling constant (J(AB)) derived by c omputer simulation of the observed spectra are large, ranging from 20- 830 Hz. In general, J(AB) is inversely proportional to the basicity of the ligand L and strongly temperature dependent. These unusual coupli ng constants have been attributed to quantum mechanical exchange coupl ing of the hydride ligands. All of the complexes have been partially d euterated and tritiated at the hydride sites and studied by both H-1 a nd H-3 NR IR spectroscopy. In contrast to J(AB), the values of J(HT) a nd J(TT) are independent of temperature. The observed values for J(HT) have been used to ascertain the contribution of the magnetic H-H coup ling to J(AB). The contributions of the exchange coupling to J(AB) hav e been derived and the corresponding temperature dependency accurately modeled. Significant isotope effects on the values of J(AB) and the h ydride chemical shifts were observed upon tritium and deuterium substi tution. The barriers for thermally activated hydride site exchange hav e also been determined. No appreciable kinetic isotope effects on the thermally activated rearrangement process were observed upon substitut ion of D and T into the hydride sites. These results are interpreted i n terms of a new two-dimensional model for quantum mechanical exchange coupling of the hydrides in these cationic complexes.