Metal cluster chemistry: Structure and stereochemistry in the polynuclear rhodium hydrides H_nRh_n[P(OR)₃]_2n

Crystallographic analyses of x-ray and neutron diffraction data have provided a definitive structural representation of fHRh[P(O-i-C3H7)3J2j2 and IHRhIP(OCH3)31213. These polynuclear hydrides are generated from square planar H2RhIP(OR)3J2 units by edge (hydrogen atom) sharing and by vertex (hydrogen atom) sharing to form the dimeric and trimeric structures, respectively. The square-planar units are held together through four-center and three-center two-electron Rh-H-Rh bonds in the dimer and trimer, respectively. The dimer and trimer molecules each add one molecule of hydrogen to form H[(i-C3H70)3P]2RhH3Rh[P(O-i-C3H7)3J2 and HRh3[P(OCH3)3J6, respectively. NMR spectral information has served to define the stereochemical features of these polyhydrides. The significance of this chemistry in the metal clustermetal surface analogy is described. Discrete molecular metal clusters can be considered as simple models of metal surface coordination chemistry (1-8). This modeling approach can yield information about structure, ligand migration, chemistry, and catalytic chemistry that may be of substantive value in the analysis of metal surface chemistry (7, 8). One limitation in this formal analogy is the tendency of some molecular clusters to fragment into mononuclear metal complexes in chemical and catalytic reactions. By using coordinately unsaturated metal clusters, the probability of cluster fragmentation in reactions can be substantially decreased. We describe here the structural features of a set of coordinately unsaturated polynuclear rhodium hydrides that exhibit high catalytic activity (9, 10) in olefin hydrogenation reactions and that do not significantly fragment in these catalytic reactions.