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不同氧钒（IV）配合物（VOL_5^<2+>，其中L = dmso（二甲亚砜），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。在不同施加压力下测定了一阶速率常数k_<obsd>, k_<obsd>随压力的变化曲线得到了活化体积<DELTA>V^<<热力学>>。配体dmso、dma、dmf、CH_3CN和H_2O的<DELTA>V^<<热力学>>分别为-13.1、-9.9、-6.7、-0.4和+1.9 cm^ 3mol ^<-1>。有趣的是，<DELTA>V^<<热力学>>随交换配体的变化而变化。这意味着VO^<2+>配合物的配体交换机制不同。dmso、dma和dmf交换的<DELTA>V^<<热力学>>的负值较大，表明存在缔合交换机制。另一方面，从<DELTA>V^<<热力学>>的小正值来看，解离交换机制似乎对H_2O的交换有效。

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>>.