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)₂](BF₄) reacts with H₂ 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 (ΔG°_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 pK_a 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.