Synthesis, Structural, and Theoretical Analysis of Ru4(CO)12(μ-PF2)(μ4-P):  Is the μ4-P Atom a Three-Electron or a Five-Electron Donor?

The X-ray molecular structure of Ru4(CO)12(μ-PF2)(μ4-P) exhibits a rather open Ru4 butterfly with no Ru−Ru bond along the butterfly hinge, which is bridged by a μ-PF2 ligand. The μ4-P atom unsymmetrically bridges the Ru4 butterfly with two very different Ru−P−Ru angles: 148.71(15) and 95.43(10)°. The peculiar coordination mode of the μ4-P atom raises the question of how many electrons (three or five) it gives to the metal framework. DFT calculations on the [Ru4(CO)12(μ-PF2)(μ4-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 atomic size and of the pinch effect of the μ-PF2 ligand. The bonding in this cluster has been analyzed and compared to that of the related series [Ru4(CO)12(μ-PF2)(μ4-N)]0/2+ and the [M4(CO)12(μ4-E)]-/3- (M = Fe, Ru; E = N, P). A very good agreement between the optimized geometries and the available X-ray data is observed.