HRh(dppb)2, a powerful hydride donor

The Rh(I) and Rh(III) hydrides HRh(dppb)(2) and [HRh(dppb)(2)(NCCH3)](BF4)(2) (where dppb is 1,2-(bis(diphenylphosphino)benzene) have been prepared, and a structural study of [HRh(dppb)2(NCCH3)](BF4)2 has been completed. The latter complex is an octahedral complex with a trans arrangement of the hydride and acetonitrile ligands. A pK(a) value of 9.4 was measured for this complex by equilibration of [Rh(dppb)(2)](BF4) with 4-bromoanilinium tetrafluoroborate in acetonitrile. [Rh(dppb)21(BF4) reacts with H-2 in the presence of Pt(dmpp)(2), which acts as a base, to form HRh(dppb)(2) and [HPt(dmpp)(2)](BF4) (where dmpp = 1,2-bis-(dimethylphosphino)propane). An equilibrium constant of 0.42 +/- 0.2 was measured for this reaction. Using this equilibrium measurement and a thermodynamic cycle, the hydride donor ability (DeltaGdegrees(H-)) of HRh(dppb)2 was determined to be 34 kcal/mol. This value indicates that HRh(diphosphine)(2) complexes are powerful hydride donors. Similarly the pKa value of HRh(dppb)2 was calculated to be 35 from a thermodynamic cycle that included the potential of the Rh(I/-I) couple (E-1/2 = -2.02 V vs ferrocene). These results combined with results from the literature suggest the following order of hydricity for five-coordinate, 18-electron hydrides: second row > third row > first row. Similarly an acidity order of second row first row > third row is deduced.