The natural bite angle — Seen from a ligand's point of view

The natural bite angle concept is examined using N,N′-bisphosphino urea ligands as rigid scaffolds. The ligand has an upper limit of about 95° for the observed bite angle in chelate complexes, but prefers a much lower one. The ligand can be described as possessing downward flexibility. The dependence of the bite angle on the P—P distance within the ligand and the M—P bond length is illustrated. The metal tries to force the ligand into its own preferred structure, whereas the ligand wants to achieve a short P—P distance. A truly rigid ligand such as the N,N′-bisphosphino urea family is thus seen to clearly discriminate between metal atoms according to their individual assertiveness, using the P—P distance in the complex as a measure. Although the natural bite angle concept is valid and helpful in determining the possible bite-angle range for ligands before it is actually synthesised, its practical applicability seems to be limited to those cases where the flexibility range of the ligand allows for only one metal-preferred bite angle to be realized.Key words: natural bite angle, ligand effects, ligand design.