The bite angle makes the difference: a practical ligand parameter for diphosphine ligands

Over the past twenty years, a correlation between the P–M–P bite angle in diphosphine complexes and selectivity has been observed in various catalytic reactions such as hydroformylation, hydrocyanation and cross coupling. The large number of examples indicates that this correlation is not fortuitous. In order better to understand the underlying principles of the bite angle effect, we have first analysed crystal structures available in the Cambridge Crystallographic Database. Systematic searches indicate that for many bidentate diphosphine ligands the P–M–P angles concentrate in surprisingly small ranges, even if complexes of different metals in various oxidation states are considered. Several examples in the literature show that continuous electronic changes associated with changing bite angles cannot only be verified by different spectroscopic techniques, but also explained on a theoretical level (Walsh diagrams). The ligand bite angle is a useful parameter for the explanation of observed rates and selectivities and likewise for the design of ligands for new catalytic reactions.