A molecular-orbital study of some di-µ-phosphido-bis(tricarbonyliron) complexes. The importance of metal–bridging ligand interactions in determining molecular geometry

Abstract

Extended Hückel molecular orbital (m.o.) calculations are used to examine the structures of [Fe₂(CO)₆(µ-AX₂)₂] systems (A = N, P, or As: X = a variety of organic ligands). The experimentally observed dramatic structural change, with a startling increase in metal–metal distance in the phosphido-bridged dimers when CF₃ replaces Me, H, or Ph as a bridging substituent (for A = P), is found to be due to the nature of the energetically highest-occupied m.o. (HOMO). This orbital, whilst metal–metal bonding, is metal–phosphorus antibonding, and the balance between these two forces controls the equilibrium geometry to a large extent. Replacement of X by CF₃leads to stronger Fe-P repulsions in the HOMO and a longer Fe–Fe distance. The importance of Fe–P interactions stands out clearly in the calculations and these should be considered together with Fe–Fe interactions when viewing the structures of this and similar molecules. The geometry of these dimeric ‘metal–metal-bonded’ systems is thus not determined purely by metal–metal interactions, as most previous studies have emphasised.