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 = Et 2 PCH 2 N(Me)CH 2 PEt 2 )

The complexes M(PNP) 2 2+ (M = Pd, Pt, PNP = Et 2 PCH 2 N(Me)CH 2 PEt 2 ) were synthesized by addition of PNP to Pd(CH 3 CN) 4 (BF 4 ) 2 and (COD)PtCl 2, respectively. Pd(PNP) 2 was synthesized by reaction of Pd(PNP) 2 2+ with H 2 and tetramethylguanidine (TMG) in CH 3 CN. The thermodynamic hydride donor ability, Δ G ° H -, for HPt(PNP) 2 + (54.7 kcal/mol) was measured by heterolytic cleavage of hydrogen in the presence of NEt 3 in CH 3 CN. 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 p K 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.