REACTIONS OF THE ELECTRON-RICH BINUCLEAR HYDRIDE COMPLEXES [(PRI2P(CH2)XPPRI2)RH]2(MU-H)2 (X = 2 OR 3) WITH ZNR2 AND MGR(2

The reaction of dialkylzinc reagents (ZnR2; R = CH2Ph, C5H5, C3H5) wit h the binuclear hydride dimers [P2Rh]2(mu-H)2 (P2 = dippp = 1,3-bis(di isopropylphosphino)propane, dippe = 1,2-bis(diisopropylphosphino)ethan e) yields two products: the first is a mononuclear Rh(I) derivative P2 RhR and the second is a tetranuclear complex of the general formula [P 2Rh]2-(mu-H)2(mu-ZnR)2. Two of the tetranuclear species have been stud ied crystallographically. The molecular structure of [(dippp)Rh]2(mu-H )2(mu-ZnCH2Ph)2 (a = 19.608(7) angstrom, b = 12.891(6) angstrom, c = 2 2.847(6) angstrom, beta = 120.02(2)-degrees, Z = 4, fw = 1073.61, rho( calcd) = 1.426 g cm-3, space group = C2/c) reveals a dirhodium core sy mmetrically bridged by two hydrides and two ZnCH2Ph units. The molecul ar structure of [(dippp)Rh]2(mu-H)2(mu-ZnC5H5)2.CH3C6H5 (a = 12.050(6) angstrom, b = 35.230(4) angstrom, c = 12.414(2) angstrom, beta = 97.8 4(3)-degrees, Z = 4, fw = 1113.68, rho(calcd) = 1.417 g cm-3, space gr oup = P2(1)/n) also shows a dirhodium core, but in this case, there ar e two unsymmetrically bridging ZnC5H5 units, one hydride which bridges the two rhodium centers and one hydride which bridges a rhodium and a ZnC5H5 unit. The reaction of dibenzylmagnesium with [(dippp)Rh]2(mu-H )2 is similar in that the Rh(I) species (dippp)Rh(eta3-CH2Ph) is forme d; however, the second product is believed to be the heterobimetallic complex P2RhH2MgCH2Ph that does not dimerize. The Rh-Zn complexes reac t reversibly with dihydrogen to give binuclear polyhydrides that are i n equilibrium with monomeric Rh(III) complexes of the form P2RhH2ZnR. The Rh-Mg complex reacts reversibly with dihydrogen to give a heterobi nuclear tetrahydride. Mechanistic studies are presented that show the formation of the Rh-Zn tetranuclear species involves fragmentation fol lowed by recombination steps.