High Pressure NMR Study on the Kinetics of Ligand Exchange Reactions in Metal Complexes. Activation Volume and Reaction Mechanism.

本文用高压NMR方法研究了钒氧配合物VOL_5^<2+>（L = dmso（二甲基亚砜），dma（N，N-二甲基乙酰胺），dmf（N，N-二甲基甲酰胺），CH_3CN和H_2O）中配体交换反应的动力学。在不同的外加压力下测定一级速率常数k_<obsd>，并且k_对压力的曲线图<obsd>给出活化体积<DELTA>V_<thermodynamics>。配<DELTA><thermodynamics>体dmso，dma，dmf，CH_3CN和H_2O的V_（^）值分别为-13.1，-9.9，-6.7，-0.4和+1.9 cm ~ 3·mol ~<-1>（-1）。有趣的是，<DELTA>V^&lt;<thermodynamics>&gt;的值随交换配体而变化。这意味着VO^&lt;2+&gt;配合物的配体交换机理并不相同。dmso<DELTA><thermodynamics>、dma和dmf交换的V^&lt; &gt;的大负值暗示了缔合交换机制。另一方面，根据V^&lt; &gt;的轻微正值，H_2O的交换可能是解离交换机制<DELTA><thermodynamics>。

The kinetics of ligand exchange reactions in various oxovanadium(IV) complexes, VOL_5^<2+>, where L = dmso (dimethyl sulfoxide), dma(N,N-dimethylacetoamide), dmf (N,N- diemthylfomamide), CH_3CN, and H_2O, by means of the high pressure NMR method. The first-order rate constant, k_<obsd>, were determined at various applied pressure and plots of k_<obsd> vs. pressure gave the volumes of activation <DELTA>V^<<thermodynamics>>. The values of <DELTA>V^<<thermodynamics>> for the ligands, dmso, dma, dmf, CH_3CN, and H_2O, are -13.1, -9.9, -6.7, -0.4 and +1.9 cm^3 mol^<-1>, respectively. It is interesting that the values of <DELTA>V^<<thermodynamics>> change with the exchanging ligands. This means that the mechanism is not the same in the ligand exchange of VO^<2+> complexes. The large negative values of <DELTA>V^<<thermodynamics>> for the exchange of dmso,dma and dmf suggest the associative interchange mechanism. On the other hand, the dissociative interchange mechanism seems operative for the exchange of H_2O in terms of the lightly positive value of <DELTA>V^<<thermodynamics>>.