The metal-ligand bonds of the title compounds have been investigated with the help of an energy partitioning analysis at the DFT level. It was found that the attractive orbital interactions between Fe and ER in (CO)(4)Fe-ER arise mainly from Fe <-- ER sigma donation. Only the boron diyl complexes (CO)(4)Fe-BR have significant contributions by Fe --> ER pi back-donation, but the Fe <-- BR sigma -donation remains the dominant orbital interaction term. 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 attraction between the local negative charge concentration at the overall positively. 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 is a good pi donor substituent. The Fe-BR bonds of the boron carbonyldiyl complexes 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 Fe(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) term becomes clearly higher and contributes significantly to the total orbital interactions in the homoleptic complexes where no other pi acceptor ligands are present. The ligand BMe is nearly as strong a pi acceptor in Fe(BMe)(5) as CO is in Fe CO)(5).