Metal cluster chemistry: Structure and stereochemistry in the polynuclear rhodium hydrides H(n)Rh(n)[P(OR)(3)](2n).

Crystallographic analyses of x-ray and neutron diffraction data have provided a definitive structural representation of {HRh[P(O-i-C(3)H(7))(3)](2)}(2) and {HRh[P(OCH(3))(3)](2)}(3). These polynuclear hydrides are generated from square planar H(2)Rh[P(OR)(3)](2) 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-C(3)H(7)O)(3)P](2)RhH(3)Rh [P(O-i-C(3)H(7))(3)](2) and H(5)Rh(3)[P(OCH(3))(3)](6), respectively. NMR spectral information has served to define the stereochemical features of these polyhydrides. The significance of this chemistry in the metal cluster-metal surface analogy is described.