Quantum Mechanical Exchange Coupling in Iridium Trihydride Complexes

Abstract

Cationic trihydride complexes of the form [(η-C5R5)Ir(L)H3]BF4 (R = H, Me; L = various phosphines) have been studied. The 1H NMR spectra of these complexes at low temperature display line patterns in the hydride region consistent with AB2X or A2BX spin systems (X = 31P). The values for the HA−HB coupling constant (JAB) derived by computer simulation of the observed spectra are large, ranging from 20−830 Hz. In general, JAB is inversely proportional to the basicity of the ligand L and strongly temperature dependent. These unusual coupling constants have been attributed to quantum mechanical exchange coupling of the hydride ligands. All of the complexes have been partially deuterated and tritiated at the hydride sites and studied by both 1H and 3H NMR spectroscopy. In contrast to JAB, the values of JHT and JTT are independent of temperature. The observed values for JHT have been used to ascertain the contribution of the magnetic H−H coupling to JAB. The contributions of the exchange coupling to JAB have bee...