Bite angle

In coordination chemistry the bite angle is the ligand–metal–ligand bond angle of coordination complex containing a bidentate ligand. This geometric parameter is used to classify chelating ligands, including those in organometallic complexes. It is most often discussed in terms of catalysis, as changes in bite angle can affect not just the activity and selectivity of a catalytic reaction but even allow alternative reaction pathways to become accessible. Although the parameter can be applied generally to any chelating ligand, it is commonly applied to describe diphosphine ligands, as they can adopt a wide range of bite angles. 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 (βn) of diphosphines, obtained using molecular mechanics calculations, is defined as the preferred chelation angle determined only by ligand backbone and not by metal valence angles (Figure 3). Both steric bite angle effect and the electronic bite angle effects are recognized. The steric bite angle effect involves the steric interactions between ligands or between a ligand and a substrate. The electronic bite angle effect, on the other hand, relates to the electronic changes that occur when the bite angle is modified. This effect is sensitive to the hybridization of metal orbitals. This flexibility range accounts for the diverse conformations of the ligand with energies slightly above the strain energy of the natural bite angle. The bite angle of a diphosphine ligand also indicates the distortion from the ideal geometry of a complex based on VSEPR models. Octahedral and square planar complexes prefer angles near 90° while tetrahedral complexes prefer angles near 110°. Since catalysts often interconvert between various geometries, the rigidity of the chelate ring can be decisive. A bidentate phosphine with a natural bite angle of 120° may preferentially occupy two equatorial sites in a trigonal bipyramidal complex whereas a bidentate phosphine with a natural bite angle of 90° may preferentially occupy apical-equatorial positions. Diphosphine ligands with bite angles of over 120° are obtained using a bulky, stiff diphosphine backbones. Diphosphines of wide bite angles are used in some industrial processes.