text: The hydroformylation of alkenes to give aldehydes is an important industrial process.  Almost 6 million tons of aldehydes are produced by this method annually.
  Rhodium complexes containing diphosphine ligands are active hydroformylation catalysts.  
  The ratio of linear to branched aldehyde product depends on the structure of the catalyst.
  One intermediate, [Rh(H)(alkene)(CO)L], exists in two different isomers, depending on the position of phosphine ligands (Figure 4). 
  Diphosphine ligands such as dppe, which has a bite angle of about 90°, span the equatorial and apical positions (AE isomer). Diphosphines with larger bite angles (above 120°) preferentially occupy a pair of equatorial positions (EE isomer). It is believed that the EE isomer favors formation of linear aldehydes, the desired product. In an effort to create rhodium complexes in which the phosphine ligands preferentially occupy the equatorial positions, the use of diphosphine ligands with wide bite angles such as BISBI (Figure 5) has been investigated.
  With a bite angle of approximately 113°, BISBI spans sites on equatorial plane of the trigonal bipyramidal intermediate complex (Figure 6).
  The structure of the intermediate [Rh(H)(diphosphine)(CO)] does not however determine the regioselectivity of the hydroformylation. Instead, the formation of the linear vs. branched aldehydes is determined upon formation of [Rh(H)(diphosphine)CO(alkene)] and the subsequent hydride migration step. The bite angle affect the steric crowding at the Rh atom that results from the interactions of the bulky backbone of the ligand with substrate. The wide bite angle that results from the backbone allows the five-coordinate [Rh(H)(diphosphine)CO(alkene)] intermediate to adopt a structure that relieves steric hindrance. Thus, BISBI occupies the equatorial positions, where it has the most space. This preference of a transition state that relieves steric hindrance favors the formation of the linear aldehyde. The regioselectivity is also controlled by the hydride migration, which is usually irreversible in the formation of linear aldehydes.
  Furthermore, studies using Xantphos ligands (ligands with bulky backbones) in hydroformylation have indicated an increase in the rate of catalysis in metal complexes that contain diphosphine ligands with larger bite angles. The electronic effect of this increase in reaction rate is uncertain since it mainly depends on the bonding between the alkene and rhodium. Large bite angles promote alkene to rhodium electron donation, which results in an accumulation of electron density on the rhodium atom. This increased electron density would be available for π-donation into the anti-bonding orbitals of other ligands, which could weaken other M-L bonds within the catalyst, leading to higher rates.
  The application of diphosphine ligands to catalysts is not limited to the process of hydroformylation. Hydrocyanation and hydrogenation reactions also implement phosphine-mediated catalysts.