Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump
Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump
复制标题
通过超快 T 跳跃解决不稳定钴配合物中配体取代的动力学
DOI：
--
复制
发表时间：
2008
影响因子：
11.1
通讯作者：
A. Zewail
中科院分区：
文献类型：
--
作者：
Hairong Ma;C. Wan;A. Zewail
水合金属配合物的配体交换在化学和生物系统中很常见。使用超快 T 跃迁，我们研究了这个过程，特别是水钴 (II) 配合物向完全卤化物质的转化。结果揭示了一种逐步机制，时间尺度从数百皮秒到纳秒不等。当结构被保留时，动力学明显更快，但当八面体到四面体的结构变化成为转换的瓶颈时，动力学会受到速率限制。从双分子动力学和能量学（焓和熵）出发，提供了配体协助水分子置换的反应，同时使进入的配体保持在激活状态。反应时间尺度与离子扩散的时间尺度偏差一到两个数量级，表明离子和更大的络合物之间存在碰撞势垒。
Ligand exchange of hydrated metal complexes is common in chemical and biological systems. Using the ultrafast T-jump, we examined this process, specifically the transformation of aqua cobalt (II) complexes to their fully halogenated species. The results reveal a stepwise mechanism with time scales varying from hundreds of picoseconds to nanoseconds. The dynamics are significantly faster when the structure is retained but becomes rate-limited when the octahedral-to-tetrahedral structural change bottlenecks the transformation. Evidence is presented, from bimolecular kinetics and energetics (enthalpic and entropic), for a reaction in which the ligand assists the displacement of water molecules, with the retention of the entering ligand in the activated state. The reaction time scale deviates by one to two orders of magnitude from that of ionic diffusion, suggesting the involvement of a collisional barrier between the ion and the much larger complex.