Variable coordination modes realized with a dihydroxyalkyldiphosphane as ahemilabile ligand: A combined Rh-103-NMR and density-functional study

Cationic rhodium complexes of (R,R)-1,4-bis(diphenylphosphanyl)butane-2,3-d iol and cyclic diolefins exhibit temperature-dependent P-31- and Rh-103-NMR spectra which are best explained by a hemilabile coordination of one of th e hydroxy groups to the rhodium center. A complex with this ligand bound in tridentate fashion is in equilibrium with a species with the common square -planar ligand arrangement. The Rh-103-NMR shift of the fivefold coordinate d complex is found almost 500 ppm downfield from that of a fourfold coordin ated species. This effect is characteristic for an increase in coordination number. At gradient-corrected levels of density-functional theory, a corre sponding species with an oxygen-rhodium contact has been located, together with other isomers. The computed trends in energies and Rh-103 chemical shi fts are consistent with the experimental findings.