A measure for σ-donor and π-acceptor properties of diiminepyridine-type ligands

Relative metal-ligand binding strengths for the ZnCl2 and low-spin (CoMe)-Me-I fragments and a variety of diiminepyridine (DIP)-like ligands have been calculated at the DFT level. The assumption of a linear energy relation Delta E-stab(F,L) = alpha(F)sigma(L) + beta(F)pi(L) for the relative binding energy of fragment F to ligand L was used to derive scales for ligand parameters sigma(L) and pi(L) representing the sigma-donor and pi-acceptor qualities of these ligands. The results show that DIP ligands in general are only fair sigma-donors but exceptionally good pi-acceptors, being eclipsed only by their bis(diazo)pyridine analogues and bis(carbene)pyridine variations. Bis(phosphinimine)pyridines are much poorer pi-acceptors than DIP, but are comparable in a-donor capacity. Introduction of substituents at the pyridine or N-aryl rings of DIP results in changes that are much smaller than the above-mentioned replacement of the ligand side arms. The analysis method also puts metal fragments on a scale for Lewis acidity (alpha(F)) and pi-basicity (beta(F)). For the series of fragments ScCl2-CuCl2 (all high-spin), results indicate that Lewis acidity increases almost monotonously; pi-basicity decreases from ScCl2 to MnCl2 (where it vanishes), is significant again for FeCl2, but negligible for CoCl2-CuCl2.