Scheins Stephan, Messerschmidt Marc, Gembicky Milan, Pitak Mateusz, Volkov Anatoliy, Coppens Philip, Harvey Benjamin G, Turpin Gregory C, Arif Atta M, Ernst Richard D
Department of Chemistry, University of New York at Buffalo, Buffalo, New York 14260, USA.
J Am Chem Soc. 2009 May 6;131(17):6154-60. doi: 10.1021/ja807649k.
The experimental electron density study of Ti(C(5)H(4)Me)(2)[(CH(2))(2)CMe(2)] provides direct evidence for the presence of (C-C)-->Ti agostic interactions. In accord with the model of Scherer and McGrady, the C(alpha)-C(beta) bond densities no longer show cylindrical symmetry in the vicinity of the Ti atom and differ markedly from those of the other C-C bonds. At the points along the C(alpha)-C(beta) bond where the deviation is maximal the electron density is elongated toward the metal center. The distortion is supported by parallel theoretical calculations. A calculation on an Mo complex in which the agostic interaction is absent supports the Scherer and McGrady criterion for agostic interactions. Despite the formal d(0) electron configuration for this Ti(IV) species, a significant nonzero population is observed for the d orbitals, the d orbital population is largest for the d(xy) orbital, the lobes of which point toward the two C(alpha) atoms. Of the three different basis sets for the Ti atom used in theoretical calculations with the B3LYP functional, only the 6-311++G** set for Ti agrees well with the experimental charge density distribution in the Ti-(C(alpha)-C(beta))(2) plane.
对Ti(C(5)H(4)Me)(2)[(CH(2))(2)CMe(2)]进行的实验电子密度研究为(C-C)-->Ti超共轭相互作用的存在提供了直接证据。与Scherer和McGrady的模型一致，在Ti原子附近，C(α)-C(β)键密度不再呈现圆柱对称性，且与其他C-C键的密度有显著差异。在C(α)-C(β)键上偏差最大的点处，电子密度向金属中心拉长。这种畸变得到了平行理论计算的支持。对一个不存在超共轭相互作用的Mo配合物进行的计算支持了Scherer和McGrady关于超共轭相互作用的判据。尽管该Ti(IV)物种具有形式上的d(0)电子构型，但d轨道仍观察到显著的非零占据数，其中d(xy)轨道的d轨道占据数最大，其瓣指向两个C(α)原子。在使用B3LYP泛函进行理论计算时，用于Ti原子的三种不同基组中，只有Ti的6-311++G**基组与Ti-(C(α)-C(β))(2)平面内的实验电荷密度分布吻合良好。
