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.