Bite angle

The bite angle is a geometric parameter used to classify chelating ligands in coordination chemistry, including organometallic complexes. Although the parameter can be applied generally to any chelating ligand, it is commonly applied to describe diphosphine ligands, which can adopt a wide range of chelate ring sizes.

Diamines form a wide range of coordination complexes. They typically form 5- and 6-membered chelate rings. Examples of the former include ethylenediamine and 2,2′-bipyridine. Six-membered chelate rings are formed by 1,3-diaminopropane. The bite angle in such complexes is usually near 90°. Longer chain diamines, which are "floppy", tend not to form chelate rings.

Diphosphines are a class of chelating ligands that contain two phosphine groups connected by a bridge (also referred to as a backbone). The bridge, for instance, might consist of one or more methylene groups or multiple aromatic rings with heteroatoms attached. Examples of common diphosphines are dppe, dcpm (Figure 1), and DPEphos (Figure 2). The structure of the backbone and the substituents attached to the phosphorus atoms influence the chemical reactivity of the diphosphine ligand in metal complexes through steric and electronic effects.

Steric characteristics of the diphosphine ligand that influence the regioselectivity and rate of catalysis include the pocket angle, solid angle, repulsive energy, and accessible molecular surface. Also of importance is the cone angle, which in diphosphines is defined as the average of the cone angle for the two substituents attached to the phosphorus atoms, the bisector of the P–M–P angle, and the angle between each M–P bond. Larger cone angles usually result in faster dissociation of phosphine ligands because of steric crowding.

The natural bite angle (β_n) of diphosphines, obtained using molecular mechanics calculations, is defined as the selective chelation angle (P-M-P bond angle) that is determined by the diphosphine ligand backbone (Figure 3).

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