H-1 SPIN-LATTICE NMR RELAXATION STUDIES OF HYDRIDE CARBONYL CLUSTERS - A METHOD TO EVALUATE DISTANCES INVOLVING HYDRIDO LIGANDS

Proton longitudinal relaxation rates of hydride resonances are dominated by the dipolar interaction with other H-1 nuclei in the same molecule and may provide relevant structural information once the molecular correlation times are determined by independent experiments. H-H distances were determined by this route in HOs3(CO)10(O2CH) (I), HOs3(CO)9C2H (II), and HOs3(CO)10OH (III), whose resonances are well separated in their H-1 NMR spectra. When the proton NMR spectrum of compound III is recorded in acetone, it shows a mixture of two different isomers differing in the orientation of the lone pair on the mu-oxygen atom, as revealed by a marked difference in the relaxation times of the hydride ligands. Furthermore, an alternative procedure based on the comparison of the proton relaxation rates between H,H and H,D isotopomers has been applied for the dihydride species H2Os3(CO)10 (IV), H2FeOs3(CO)13 (V), and H2FeRu3(CO)13 (VI), where the hydride ligands are chemically equivalent.