Time-resolved IR absorption spectroscopy is used to investigate substitution of the cyclohexane (CyH) molecule of the photolytically generated alkane-solvated transient intermediate Cr(CO)5(CyH) by heterocyclic ligands C4HnE (n = 4, 8; E = O, NH, S). From the concentration and temperature dependences of the pseudo-first order rate constants, we obtain activation parameters for the reactions, and find that they are consistent with an associative (A) or interchange (I) mechanism. As was the case with ligand substitution reactions at W(CO)5(CyH), a ligand's reactivity depends both on its electron-donating ability and on its polarizability. We also find that for a reaction with a given ΔH‡, the activation entropy is higher for reaction of Cr(CO)5(CyH) than it is for reaction of W(CO)5(CyH). Comparison of the present results with ligand substitution reactions of W(CO)5(CyH), CpMn(CO)2(CyH), and Cr(CO)5(n-heptane) indicates that for ligand substitution reactions at alkane-solvated transition-metal intermediates, the solvent's effect upon the reaction rate is primarily entropic.
利用时间分辨红外吸收光谱研究了杂环
配体 C4HnE(n = 4,8;E = O,NH,S)对光解生成的烷溶瞬时
中间体 Cr(CO)5(CyH)的
环己烷(CyH)分子的取代作用。根据伪一阶速率常数的浓度和温度相关性,我们得到了反应的活化参数,并发现它们与缔合(A)或互换(I)机理相一致。与 W(CO)5(CyH)
配体置换反应的情况一样,
配体的反应活性既取决于其供电子能力,也取决于其极化能力。我们还发现,对于给定 ΔH‡ 的反应,Cr(CO)5(CyH)反应的活化熵高于 W(CO)5(CyH)反应的活化熵。将本研究结果与 W(CO)5(CyH)、CpMn(CO)2(CyH)和 Cr(CO)5(
正庚烷)的
配体置换反应进行比较后发现,在烷溶过渡
金属
中间体的
配体置换反应中,溶剂对反应速率的影响主要是熵效应。