PRINCIPAL RESONANCE CONTRIBUTORS TO HIGH-VALENT, TRANSITION-METAL ALKYLIDENE COMPLEXES
PRINCIPAL RESONANCE CONTRIBUTORS TO HIGH-VALENT, TRANSITION-METAL ALKYLIDENE COMPLEXES
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DOI：
10.1021/ja00014a015
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发表时间：
1991-07-03
影响因子：
15
通讯作者：
GORDON, MS
中科院分区：
文献类型：
--
作者：
CUNDARI, TR;GORDON, MS
报告了原型高价亚烷基配合物的从头算计算结果。对每个势能面上的驻点进行表征，并与可用的实验信息进行比较；只要使用适当灵活的价基组，理论计算的几何形状和实验确定的几何形状之间就会获得良好的一致性。感兴趣的配合物包括具有氢化物配体的IVB族（Ti、Zr和Hf）和VB族（Nb和Ta）亚烷基以及最近合成和表征的四配位烯烃复分解催化剂（Mo-、W-和Re-亚烷基）的模型。鉴于有关过渡金属卡宾配合物反应性的许多讨论都是根据 M-C sigma 和 pi 轨道中电子重排所产生的共振贡献者提出的，因此从研究的第一部分获得的最小值将受到进一步的程序来计算这些贡献。其中碳是 M-C 键负端的共振结构（即亲核共振结构）对这些配合物的基态波函数贡献了 50%。其中碳呈中性的占剩余部分的大部分（45%）。只有 5% 由亲电共振结构组成，即碳是 M-C 键正端的结构。此外，金属-碳双键主要由五个共振结构组成。其中四个共振结构对应于先前文献中讨论过的卡宾键合模型。另一种共振结构对基态波函数贡献约 33%，迄今为止在研究卡宾的化学反应性时尚未考虑到。这种巨大的共振贡献者可以描述为由配位碳-金属 σ 键加上共价 M-C π 键产生。
The results of ab initio calculations are reported for prototypical high-valent, alkylidene complexes. Stationary points on each potential energy surface are characterized and compared to experimental information where available; as long as a suitably flexible valence basis set is used, good agreement between theoretically calculated and experimentally determined geometries is obtained. The complexes of interest include group IVB (Ti, Zr and Hf) and group VB (Nb and Ta) alkylidenes with hydride ligands as well as models for the four-coordinate, olefin metathesis catalysts (Mo-, W-, and Re-alkylidenes) which have been recently synthesized and characterized. In light of the fact that much of the discussion concerning the reactivity of transition-metal carbene complexes has been presented in terms of the resonance contributors derived from rearranging the electrons in the M-C sigma and pi-orbitals, the minima obtained from the first portion of the study are then subjected to a further procedure to calculate these contributions. Resonance structures in which the carbon is the negative end of the M-C bond (i.e., nucleophilic resonance structures) contribute 50% to the ground-state wave function of these complexes. Those in which the carbon is formally neutral account for much of the remainder (45%). Only 5% is comprised of electrophilic resonance structures, i.e., those in which the carbon is the positive end of the M-C bond. Furthermore, the metal-carbon double bond is predominantly comprised of five resonance structures. Four of these resonance structures correspond to models of carbene bonding which have been discussed previously in the literature. The other resonance structure, which contributes roughly 33% to the ground-state wave function, has hitherto not been considered when examining the chemical reactivity of carbenes. This large resonance contributor can be described as arising from a dative carbon-to-metal sigma-bond plus a covalent M-C pi-bond.