Energy analysis of metal-ligand bonding in transition metal complexes withterminal group-13 diyl ligands (CO)(4)Fe-ER, Fe(EMe)(5) andNi(EMe)(4) (E =B-Tl; R = Cp, N(SiH3)(2), Ph, Me) reveals significant pi bonding in homoleptical molecules

The metal-ligand bonds of the title compounds have been investigated with t he help of an energy partitioning analysis at the DFT level. It was found t hat the attractive orbital interactions between Fe and ER in (CO)(4)Fe-ER a rise mainly from Fe <-- ER sigma donation. Only the boron diyl complexes (C O)(4)Fe-BR have significant contributions by Fe --> ER pi back-donation, bu t the Fe <-- BR sigma -donation remains the dominant orbital interaction te rm. The relative contributions of Fe-ER sigma donation and pi back-donation are only slightly altered when R changes from a good pi donor to a poor pi donor. Electrostatic forces between the metal fragment and the diyl ligand are always attractive, and they are very strong. They arise from the attra ction between the local negative charge concentration at the overall positi vely. charged donor atom E of the, Lewis base ER and the positive charge of the iron nucleus. Electrostatic interactions and covalent interactions in (CO)(4)Fe-ER complexes have a similar strength when E is Al-Tl and when R i s a good pi donor substituent. The Fe-BR bonds of the boron carbonyldiyl co mplexes have a significantly higher ionic character than the heavier group- 13 analogues. Weak pi donor substituents R enhance the ionic character of the (CO)(4)Fe-ER bond. The metal-ligand bonds in the homoleptic complexes F e(EMe)(5) and Ni(EMe)(4) have a higher ionic character than in (CO)(4)Fe-ER . The contribution of the TM --> ER pi back-donation to the DeltaE(orb) ter m becomes clearly higher and contributes significantly to the total orbital interactions in the homoleptic complexes where no other pi acceptor ligand s are present. The ligand BMe is nearly as strong a pi acceptor in Fe(BMe)( 5) as CO is in Fe CO)(5).