Synthesis, structural, and theoretical analysis of Ru-4(CO)(12)(mu-PF2)(mu(4)-P): Is the mu(4)-P atom a three-electron or a five-electron donor?

The X-ray molecular structure of Ru-4(CO)(12)(mu -PF2)(mu (4)-P) exhibits a rather open Ru-4 butterfly with no Ru-Ru bond along the butterfly hinge, w hich is bridged by a mu -PF2 ligand. The mu (4)-P atom unsymmetrically brid ges the Ru-4 butterfly with two very different Ru-P-Ru angles: 148.71(15) a nd 95.43(10)degrees. The peculiar coordination mode of the mu (4)-P atom ra ises the question of how many electrons (three or five) it gives to the met al framework. DFT calculations on the [Ru-4(CO)(12)(mu -PF2)(mu (4)-P)](2-/ 0/2+) series indicate that the phosphorus atom is better described as being a three-electron donor. Its unexpected pyramidalization is the result of a tomic size and of the pinch effect of the mu -PF2 ligand. The bonding in th is cluster has been analyzed and compared to that of the related series [Ru -4(CO)(12)(mu -PF2)(mu (4)-N)](0/2+) and the [M-4(CO)(12)(mu (4)-E)](-/3-) (M = Fe, Ru; E = N, P). A very good agreement between the optimized geometr ies and the available X-ray data is observed.