Thermodynamic Hydricity of Transition Metal Hydrides

Transition metal hydrides play a critical role in stoichiometric
and catalytic transformations. Knowledge of free energies for cleaving
metal hydride bonds enables the prediction of chemical reactivity,
such as for the bond-forming and bond-breaking events that occur in
a catalytic reaction. Thermodynamic hydricity is the free energy required
to cleave an M–H bond to generate a hydride ion (H–). Three primary methods have been developed for hydricity determination:
the hydride transfer method establishes hydride transfer equilibrium
with a hydride donor/acceptor pair of known hydricity, the H2 heterolysis method involves measuring the equilibrium of heterolytic
cleavage of H2 in the presence of a base, and the potential–pKa method considers stepwise transfer of a proton
and two electrons to give a net hydride transfer. Using these methods,
over 100 thermodynamic hydricity values for transition metal hydrides
have been determined in acetonitrile or water. In acetonitrile, the
hydricity of metal hydrides spans a range of more than 50 kcal/mol.
Methods for using hydricity values to predict chemical reactivity
are also discussed, including organic transformations, the reduction
of CO2, and the production and oxidation of hydrogen.

过渡金属氢化物在化学计量和催化转化中扮演着至关重要的角色。了解断裂金属氢化物键的自由能，能够预测化学反应活性，例如在催化反应中发生的键形成和键断裂事件。热力学氢活性是指断裂M-H键以生成氢化物离子（H-）所需的自由能。目前已开发出三种主要方法用于氢活性的测定：氢化物转移法通过与已知氢活性的氢化物供体/受体对建立氢化物转移平衡，H2异裂法涉及在存在碱的情况下测量H2的异裂平衡，以及电位-pKa法考虑逐步转移一个质子和两个电子以实现净氢化物转移。通过这些方法，在乙腈或水中已确定了超过100个过渡金属氢化物的热力学氢活性值。在乙腈中，金属氢化物的氢活性范围超过50 kcal/mol。此外，还讨论了利用氢活性值预测化学活性的方法，包括有机转化、CO2的还原以及氢的生产和氧化。