Bahn Christian S., Tan Agnes, Harris Suzanne
Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071-3838.
Inorg Chem. 1998 Jun 1;37(11):2770-2778. doi: 10.1021/ic9712378.
The results of Fenske-Hall molecular orbital calculations for cubane-type clusters having Mo(3)M'S(4) cores show that the electronic structures of the clusters depend on the nature of M'. When M' is a main group metal, as in Mo(3)(SnCl(3))S(4)(NCS)(9)(-), Mo(3)SnS(4)(S(2)PEt(2))(6), or Sn(Mo(3)S(4)(H(2)O)(9))(2), the heterometal is oxidized upon incorporation into the cluster; no M'-Mo bonds are formed, and electrons are transferred from M' to an orbital localized on the Mo(3)S(4) incomplete cubane core. When M' is a transition metal, as in Mo(3)NiS(4)(CO)(H(2)O)(9), Mo(3)PdS(4)(CO)(tacn)(3) (tacn = 1,4,7-triazacyclononane), or Mo(3)CoS(4)(CO)(Cp')(3) (Cp' = methylcyclopentadiene), M' is not oxidized but instead shares electron density with the Mo(3)S(4) core through the formation of metal-metal bonds with the Mo(3) triangle. The relatively high stretching frequencies observed for CO ligands bound to the Ni and Pd centers in the Mo(3)NiS(4) and Mo(3)PdS(4) clusters arise from the nature of the bonding in the clusters, not from the oxidation of the Ni and Pd atoms. Since the same heterometal orbitals are used both to form the M'-Mo bonds and to back-donate to the CO ligand, the Mo(3) orbitals and the CO pi orbitals compete for M' electron density. The CO orbitals do not compete effectively for metal electron density in the Ni and Pd clusters, and this results in weak back-donation to the CO pi orbitals and relatively high CO stretching frequencies. Although it has been proposed that the Mo(3)NiS(4) cluster may serve as a model for NiMoS hydrodesulfurization (HDS) catalysts, the fact that the Ni center in this cluster is not electron rich suggests that it may not provide a suitable model. The electron density at the heterometal can be increased slightly by increasing the donor ability of the ligands attached to the Mo atoms.
对具有Mo(3)M'S(4)核心的立方烷型簇合物进行Fenske-Hall分子轨道计算的结果表明，这些簇合物的电子结构取决于M'的性质。当M'是主族金属时，如在Mo(3)(SnCl(3))S(4)(NCS)(9)(-)、Mo(3)SnS(4)(S(2)PEt(2))(6)或Sn(Mo(3)S(4)(H(2)O)(9))(2)中，杂金属在并入簇合物时被氧化；不形成M'-Mo键，电子从M'转移到位于Mo(3)S(4)不完全立方烷核心上的一个轨道。当M'是过渡金属时，如在Mo(3)NiS(4)(CO)(H(2)O)(9)、Mo(3)PdS(4)(CO)(tacn)(3)（tacn = 1,4,7-三氮杂环壬烷）或Mo(3)CoS(4)(CO)(Cp')(3)（Cp' = 甲基环戊二烯）中，M'不会被氧化，而是通过与Mo(3)三角形形成金属-金属键与Mo(3)S(4)核心共享电子密度。在Mo(3)NiS(4)和Mo(3)PdS(4)簇合物中，与Ni和Pd中心相连的CO配体观察到的相对较高的伸缩频率源于簇合物中的键合性质，而不是Ni和Pd原子的氧化。由于相同的杂金属轨道既用于形成M'-Mo键，又用于向CO配体进行反馈π电子给予，Mo(3)轨道和CO π轨道争夺M'的电子密度。在Ni和Pd簇合物中，CO轨道不能有效地争夺金属电子密度，这导致对CO π轨道的反馈π电子给予较弱，以及相对较高的CO伸缩频率。尽管有人提出Mo(3)NiS(4)簇合物可以作为NiMoS加氢脱硫（HDS）催化剂的模型，但该簇合物中Ni中心电子不丰富这一事实表明它可能不是一个合适的模型。通过增加与Mo原子相连的配体的给体能力，可以略微增加杂金属处的电子密度。
