Computational Investigations of Metal-Metal Bonding in Molecular Thorium Compounds and Ce and Group IV Analogs

We report quantum chemical investigations of metal–metal bonding in molecular thorium clusters, and compare them with cerium and group IV transition metal analogs. 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 analogs, Ce–Ce bonding is observed only in oxidation states below +3. Calculations on [Ce₃Cl₆]^z (z = 1–3) and Cp-stabilized analogs 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 analogs 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 analog, suggesting that only the 6d orbitals are sufficiently diffuse as to support such interactions.