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)

Abstract

The complexes M(PNP)22+ (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)22+ with H2 and tetramethylguanidine (TMG) in CH3CN. The thermodynamic hydride donor ability, ΔG°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)22+. The M(II) complexes undergo reversible, two-electron reductions in benzonitrile. The measured reduction potentials were used in a thermodynamic cycle to estimate pKa 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.