Cavigliasso Germán, Stranger Robert
Department of Chemistry, Faculty of Science, Australian National University, Canberra ACT 0200, Australia.
Inorg Chem. 2008 Apr 21;47(8):3072-83. doi: 10.1021/ic702070z.
The molecular and electronic structures of trinuclear face-shared [M3X12]3-species of Mo (X = F, Cl, Br, I) and W (X = Cl), containing linear chains of metal atoms, have been investigated using density functional theory. The possibility of variations in structure and bonding has been explored by considering both symmetric (D3d) and unsymmetric (C3v) forms, the latter having one long and one short metal-metal distance. Analysis of the bonding in the structurally characterized [Mo3I12]3- trimer reveals that the metal-metal interaction qualitatively corresponds to a two-electron three-center sigma bond between the Mo atoms and, consequently, a formal Mo-Mo bond order of 0.5. However, the calculated spin densities suggest that the electrons in the metal-metal sigma bond are not fully decoupled and therefore participate in the antiferromagnetic interactions of the metal cluster. Although the same observation applies to [Mo3X12]3- (X = Br, Cl, F) and [W3Cl12]3-, both the spin densities and shorter distances between the metal atoms indicate that the metal-metal interaction is stronger in these systems. The broken-symmetry approach combined with spin projection has been used to determine the energy of the low-lying spin multiplets arising from the magnetic coupling between the metal centers. Either the symmetric and unsymmetric S = 3/2 state is predicted to be the ground state for all five systems. For [Mo3X12]3- (X = Cl, Br, I), the symmetric form is more stable but the unsymmetric structure, where two metal centers are involved in a metal-metal triple bond while the third center is decoupled, lies close in energy and is thermally accessible. Consequently, at room temperature, interconversion between the two energetically equivalent configurations of the unsymmetric form should result in an averaged structure that is symmetric. This prediction is consistent with the reported structure of [Mo3I12]3-, which, although symmetric, indicates significant movement of the central Mo atom toward the terminal Mo atoms on either side. In contrast, unsymmetric structures with a triple bond between two metal centers are predicted for [Mo3F2]3- and [W3C12]3-, as the symmetric structure lies too high in energy to be thermally accessible.
利用密度泛函理论研究了含金属原子线性链的钼（X = F、Cl、Br、I）和钨（X = Cl）的三核面共享[M₃X₁₂]³⁻物种的分子和电子结构。通过考虑对称（D₃d）和非对称（C₃v）形式来探索结构和键合变化的可能性，后者具有一个长的和一个短的金属-金属距离。对结构已表征的[Mo₃I₁₂]³⁻三聚体的键合分析表明，金属-金属相互作用在定性上对应于钼原子之间的两电子三中心σ键，因此，形式上的Mo-Mo键级为0.5。然而，计算得到的自旋密度表明，金属-金属σ键中的电子并未完全解耦，因此参与了金属簇的反铁磁相互作用。尽管相同的观察结果适用于[Mo₃X₁₂]³⁻（X = Br、Cl、F）和[W₃Cl₁₂]³⁻，但自旋密度和金属原子之间较短的距离都表明，这些体系中的金属-金属相互作用更强。结合自旋投影的破缺对称方法已被用于确定由金属中心之间的磁耦合产生的低能自旋多重态的能量。预测对称和非对称S = 3/2态都是所有五个体系的基态。对于[Mo₃X₁₂]³⁻（X = Cl、Br、I），对称形式更稳定，但非对称结构（其中两个金属中心参与金属-金属三键，而第三个中心解耦）能量相近且热可及。因此，在室温下，非对称形式的两个能量相当的构型之间的相互转化应导致一个对称的平均结构。这一预测与报道的[Mo₃I₁₂]³⁻结构一致，该结构虽然对称，但表明中心钼原子向两侧末端钼原子有显著移动。相比之下，对于[Mo₃F₂]³⁻和[W₃C₁₂]³⁻，预测存在两个金属中心之间有三键的非对称结构，因为对称结构能量过高而热不可及。
