Formation of Cationic Rh I Complexes

A cyclic diene ligand such as norboraadiene involved in a Rh(I) bis-phosphine complex is easily hydrogenated by and removed from the coordination sphere. When the reaction is conducted in a donor solvent such as MeOH, MeCN, and THF, the vacated coordination sites are occupied by solvent molecules. The dominant products of the reaction, however, depend on the nature of the phosphine ligands. A complex having two unidentate phosphines requires three molecules of H2, affording an octahedral dihydrido Rh(III) complex [Eq. (6)]. On the other hand, a complex having a chelating diphosphine absorbs two molecules of H2 to yield a square-planar Rh(I) complex [Eq. (7)]  

The difference between these two reactions may be caused by the trans influences of the hydride and phosphine ligands. Usually, phosphines avoid becoming trans to hydrides when possible. Therefore, complex 7 resists the reaction with another molecule of H2, since the oxidative addition is cis, requiring at least one site trans to P. 

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