Computational Investigations of Metal–Metal Bonding in Molecular Thorium Compounds and Ce and Group IV Analogues
来源：ACS Publications
We report quantum chemical investigations of metal–metal bonding in molecular thorium clusters and compare them with cerium and group IV transition metal analogues. We explore periodic trends in metal–metal bonding and the roles of electron delocalization, orbital diffuseness, and oxidation state. As cluster size increases in the series [{Th(η8-COT)Cl2}nK2] (COT = C8H8, n = 2–5, 2–5) and [Th4Cl4(η8-COT)4]2+ (T), n-center-2-electron bonding weakens. Quantum theory of atoms in molecules (QTAIM) analysis finds Th–Th bond paths only in 2 and 3, while T exhibits a non-nuclear attractor, indicating charge concentration in the [Th4Cl4]10+ core. In the Ce analogues, Ce–Ce bonding is observed only in oxidation states below +3. Calculations on [Ce3Cl6]z (z = 1–3) and Cp-stabilized analogues show Ce–Ce bond shortening with increasing population of the 3-center MOs. The QTAIM confirms Ce–Ce bond paths in [Ce3Cl6]+, [Ce3Cl6(η5-Cp)3]−, and [Ce3Cl6(η5-Cp)3K2]. Group IV analogues reveal variations in metal–metal bonding on progressing from the contracted 3d AOs of Ti to the more diffuse 6d AOs of Rf. In [M3Cl6(η8-COT)3K2] (3-M), only 3-Rf exhibits QTAIM bond paths similar to the Th analogue, suggesting that only the 6d orbitals are sufficiently diffuse as to support such interactions.
In this contribution, we extend our studies of the trithorium cluster [Th3Cl6(η8-COT)3K2]─here labeled 3─to other systems which could display delocalized Th–Th bonding. We examine analogues with two, four, and five Th atoms, i.e., [{Th(η8-COT)Cl2}nK2] (n = 2 (2), 4 (4), and 5 (5)), each of which features an n-center 2-electron HOMO. Additionally, we probe [Th4Cl4(η8-COT)4]2+ (T), which features a tetrahedral Th4 core with a 4c–2e HOMO, contrasting with the planar cyclic arrangements of 3–5. T was inspired by the erbium complex [Er(η8-hdcCOT)I]4 (hdcCOT is a hexahydrodicyclopenta-substituted COT ligand), although this complex does not exhibit metal–metal bonding. Then, we probe Ce–Ce bonding in dimetallic and trimetallic analogues of the thorium targets and compare the 4f vs 5f systems. Finally, we study the M–M bonding in dimetallic and trimetallic systems of the group IV transition metals (Ti, Zr, Hf, Rf), which we once again compare and contrast with the Th systems, allowing a survey of metal–metal bonding in elements with valence 3d, 4d, 5d, and 6d (Th and Rf) orbitals, and all with the same group valence. This bonding is interrogated via optimized geometries, orbital compositions, metal–metal bond orders, metal oxidation states, QTAIM analyses, and ELF studies.