Molecular and Electronic Structural Studies of d³-d³ and d⁵-d⁵ M₂R₆ Complexes (M = Mo, Ru; R = CH₃, CH₂CMe₃): On the Interplay of Metal–Metal Bond Order and MR₃ Pyramidalization

Hexakis(neopentyl)diruthenium(III,III) [Ru2(CH2CMe3)6 or Ru2Np6], first synthesized in 1984, is a d5-d5 analogue of the classic d3-d3 M2X6 Chisholm-type, unsupported metal-metal triply-bonded Group 6 complexes. We report an alternative synthetic route to Ru2Np6 and an updated low-temperature crystal structure. The Ru-Ru bond length (2.3141(3) Å) is only 0.15 Å longer than the Mo-Mo bond in Mo2Np6, less than might be expected upon adding four electrons. The Ru-Ru bond was originally proposed to be a triple bond, which seemed inconsistent with the usual M-M bonding model. We use DFT and TD-DFT calculations on Mo2R6 and Ru2R6 (R = Me, Np) to investigate the differences in metal-metal and metal-ligand bonding between the d3-d3 and d5-d5 systems. In the Ru2R6 systems, the two RuR3 fragments adopt a pyramidalized geometry to maximize ligand-to-metal donation and to shift electron density from the strongly antibonding Ru-Ru π* orbital to the weakly bonding Ru-Ru δ orbital, thus preserving some of the Ru-Ru π bonding. In contrast, the MoR3 fragments in Mo2R6 adopt a more trigonal planar geometry to preserve the Mo-Mo π bonding. The calculated and experimental UV-vis spectra are near band-for-band matches, and the energies and orbital characters of the excitations are presented.