Kinetic hydricities

HPt(dmpp), has a thermodynamic and kinetic hydricity that is sufficient for the transfer of to coordinated GO in GpRe(NO)(GO)2 and Gp Re(NO)(GO)2 to give the respective Re formyl complexes (Equation (28)). The overall reaction is in principle catalytic with respect to Pt. Although the yields were low (9%), the design of this system demonstrated how access to relevant thermodynamic data may facilitate rational catalyst design. 

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. ... 

Thermochemical studies of acidity and hydricity are extremely valuable, since they can help determine the energies of potential intermediates in catalytic cycles, and can thus guide the choice of complexes proposed as catalysts. But, since catalysis is a kinetic phenomenon, the kinetics of delivery of a proton or hydride are also important. The kinetics of proton transfer from metal hydrides to amines [21] or metal alkynyl complexes [22], as well as degenerate proton transfers between metal hydrides and metal anions [21] led to the conclusion that proton transfers from metal hydrides have a high intrinsic barrier. 

Almost simultaneously with the appearance of Guldberg and Waage s first paper of 1864, Harcourt and Esson commenced their work on reaction rates. Whereas the French and Norwegian workers had been concerned with both equilibria and reaction rates, Harcourt and Esson chose reactions which were effectively irreversible, and they were therefore able to concentrate on chemical kinetics alone. They made no claim to be measuring affinity or chemical force. They initially studied the reaction between potassium permanganate, oxalic acid and sulphuric acid, but this proved to be rather complicated. They therefore switched to the reaction between hydric peroxide and hydric iodide in acidified solution ... 

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