Periodic trends in metal hydride donor thermodynamics:: Measurement and comparison of the hydride donor abilities of the series HM(PNP)2+ (M = Ni, Pd, Pt; PNP = Et2PCH2N(Me)CH2PEt2)

The complexes M(PNP)(2)(2+) (M = Pd, Pt, PNP = Et2PCH2N(Me)CH2PEt2) were synthesized by addition of PNP to Pd(CH3CN)(4)(BF4)(2) and (COD)PtCl2, respectively. Pd(PNP)(2) was synthesized by reaction of Pd(PNP)(2)(2+) with H-2 and tetramethylguanidine (TMG) in CH3CN. The thermodynamic hydride donor ability, DeltaGdegrees(H)(-), for HPt(PNP)(2)(+) (54.7 kcal/mol) was measured by heterolytic cleavage of hydrogen in the presence of NEt3 in CH3CN. The hydride donor ability of HPd(PNP)(2)(+) (51.1 kcal/mol) was determined by measuring the equilibrium constant for direct hydride transfer to Pt(PNP)(2)(2+). The M(II) complexes undergo reversible, two-electron reductions in benzonitrile. The measured reduction potentials were used in a thermodynamic cycle to estimate pK(a) values of 22.1 and 27.6 for deprotonation of HPd(PNP)(2)(+) and HPt(PNP)(2)(+), respectively. The palladium hydride has an acidity equal to that of the nickel hydride and is a better hydride donor than both the nickel and platinum analogues. This shows that it is possible for transition metal hydrides to act simultaneously as both acids and hydride donors.