Molecular orbital study of bonding and conformations in dinuclear cyclopentadienyldicarbonyl complexes of manganese and chromium containing germanium, sulfide, dinitrogen or phosphinidene bridges

Abstract

We carried out nonempirical molecular orbital calculations on syn, gauche and anti conformations of four dinuclear complexes of Mn and Cr with various bridging ligands: [CpM(CO) 2] 2(μ 2-L) where M is Mn, L is C (as a model for Ge), N 2 or PPh; and M is Cr, L is S. Contrary to a recent claim, [CpMn(CO) 2] 2Ge does not seem to contain Mn Ge double bonds and is not analogous to allenes. The Mn Ge bonds are partially triple so that internal rotation about the Mnz.sbnd;Ge Mn axis is facile, in accord with the infrared spectra. Bonding in [CpCr(CO) 2] 2S and in [CpMn(CO) 2] 2N 2 is very similar to that in the Ge-containing complex. We explain the observed nonrigidity of the Cr complex. Reported infrared data lead to mutually inconsistent conclusions about conformation of the N 2-containing complex in solution. On the basis of calculations, we suggest that these complex molecules are not centrosymmetric in solution. The N 2-bridged dinuclear molecules are better viewed as containing an N 2 molecule rather than two N atoms. The π-antibonding orbitals of N 2 are crucial for its bonding to metals; the filled π-bonding orbitals do not seem to donate electrons to the metal atoms. The calculations show substantial Mn P π-bonding in [CpMn(CO) 2] 2PPh and this molecule is best viewed as a three-center, four-electron system. We critically examine several other qualitative and intuitive explanations of bonding in these and similar complexes and discuss conclusions and predictions based on such explanations.