Hydride complexes thermodynamic hydricity

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

The thermodynamic hydricity of an M-H bond is related to the ionicity of that bond, which can be calculated from the quadrupole coupling constant (available from the NMR spectrum of the M-D analog). Such data can be compared to rate constants for H transfer, or kinetic hydricities. The rate constants for transfer of the hydride in a series of complexes to trityl cation in CH Cl (Equation 3.130 and Table 3.6), - and from a series of CpRu(P-P)H complexes to the iminium cation in Equation 3.131 (Table 3.7) have been measured. 

With each acceptor the "kinetic hydricities" cover a range of about 10 The data in Table 3.6 show that many hydride complexes are better H" donors than EtjSiH, that second- and third-row hydride complexes are faster H" donors than first-row ones, and that electronic factors are more important than steric factors (the substitution of Cp for Cp or PRj for a CO increases the rate). The data in Table 3.7 show that the rate of H transfer from CpRu(P-P)H increases as the size of the chelate ring decreases. (The effect is similar to that of chelate ring size on the thermodynamic hydricity of the five-coordinate hydrides in Table 3.5.) Smaller rings make CpRu(P-P) more pyramidal and raise the energy of its vacant hydride acceptor orbital, making hydride donation from CpRu(P-P)H more favorable. ... 

M—H bond dissociation energies, 1, 287 photochemistry, 1, 251 single crystal neutron diffraction, 1, 578 stability toward disproportionation, 1, 301 Metal—hydrogen bonds bond dissociation energy in 1,2-dichloroethane, 1, 289 stable metal hydrides in acetonitrile, 1, 287 thermochemical cycle, 1, 286 in THF and dichloromethane, 1, 289 olefin insertion thermodynamics, 1, 629 in Zr(IV) bis-Cp complexes, 4, 878 Metal—hydrogen hydricity data, 1, 292... 

---