Periodic trends in metal-metal bonding in cubane clusters, (C5H5)(4)M4E4 [M = Cr, Mo, E = O, S]

Trends in structure and bonding in a series of metal cubane clusters are ex amined using broken-symmetry density functional theory. For the metal-sulfu r clusters, (C5H5)(4)Mo4S4 and (C5H5)(4)Cr4S4, the twelve cluster valence e lectrons are delocalised in six metal-metal single bonds, giving an approxi mately tetrahedral metal core. In (C5H5)(4)Cr4O4, however, no strong Cr-Cr bonds are present, and three cluster valence electrons remain localised on each of the chromium centres. Antiferromagnetic coupling across four of the six edges of the tetrahedron, and ferromagnetic coupling across the remain ing two give rise to a spin-singlet ground state and a distinct rhombic dis tortion. The driving force for the distortion is only 12 kJ mol(-1), and co nsequently inter- and intra-molecular steric effects may play a major role in determining the structure of the cluster in the solid state. Both chromi um clusters have low-lying excited states in which the bonding pattern is c ompletely reversed, with six Cr-Cr bonds present in (C5H5)(4)Cr4O4 but none in (C5H5)(4)Cr4S4. In each case the excited state lies less than 45 kJ mol (-1) above the ground state, despite the fact that a substantial structural rearrangement is involved. Changes in metal-metal bond strength and spin p olarisation energy are found to contribute approximately equally to the per iodic trend towards electron localisation in the chromium clusters.