A Measure for σ-Donor and π-Acceptor Properties of Diiminepyridine-Type Ligands

Relative metal−ligand binding strengths for the ZnCl2 and low-spin CoIMe fragments and a variety of diiminepyridine (DIP)-like ligands have been calculated at the DFT level. The assumption of a linear energy relation ΔEstab(F,L) = αFσL + βFπL for the relative binding energy of fragment F to ligand L was used to derive scales for ligand parameters σL and πL representing the σ-donor and π-acceptor qualities of these ligands. The results show that DIP ligands in general are only fair σ-donors but exceptionally good π-acceptors, being eclipsed only by their bis(diazo)pyridine analogues and bis(carbene)pyridine variations. Bis(phosphinimine)pyridines are much poorer π-acceptors than DIP, but are comparable in σ-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 (αF) and π-basicity (βF). For the series of fragments ScCl2−CuCl2 (all high-spin), results indicate that Lewis acidity increases almost monotonously; π-basicity decreases from ScCl2 to MnCl2 (where it vanishes), is significant again for FeCl2, but negligible for CoCl2−CuCl2.