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

用高压核磁共振方法研究了几种氧钒(IV)络合物VOL_5、L~(2+)和G~(2+)的交换反应动力学，其中L=二甲亚砜、N，N-二甲基乙酰胺、N，N-二甲基甲酰胺、甲烷和水。测定了不同外加压力下的一级速率常数k_(？)OBSD；；并绘制了k_&lt；OBSD&>；与压力的关系图，得到了活化&lt；DELTA&gt；V^&lt；&lt；thermodynamics&gt；&gt；.的体积DMSO、DMA、DMF、CH3CN和H2O的&lt；Delta&gt；V^&lt；&lt；热力学&gt；和gt；分别为-13.1、-9.9、-6.7、-0.4和+1.9 cm^3mol^-1&gt；有趣的是，~lt；Delta&gt；V^&lt；&lt；热力学&gt；&gt；的值随着交换配体的不同而变化。这意味着VO^&lt；2+&gt；络合物的配体交换机理是不同的。对于DMSO、DMA和DMF的交换，&lt；Delta&gt；V^&lt；&lt；热力学&gt；&gt；的大负值表明了缔合交换机制。另一方面，从&lt；DELTA&gt；V^&lt；&lt；thermodynamics&gt；&gt；.的轻微正值来看，解离交换机制似乎对H_2O的交换起作用

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>>.