Carbonyl hydrido tris(triphenylphosphine)rhodium(I) [Carbonyl(hydrido)tris(triphenylphosphane)rhodium(I)] is an organorhodium compound with the formula [RhH(CO)(PPh<sub>3</sub>)<sub>3</sub>] (Ph = C<sub>6</sub>H<sub>5</sub>). It is a yellow, benzene-soluble solid, which is used industrially for hydroformylation.
[RhH(CO)(PPh<sub>3</sub>)<sub>3</sub>] was first prepared by the reduction of [RhCl(CO)(PPh<sub>3</sub>)<sub>2</sub>], e.g. with sodium tetrahydroborate, or triethylamine and hydrogen, in ethanol in the presence of excess triphenylphosphine:
It can also be prepared from an aldehyde, rhodium trichloride and triphenylphosphine in basic alcoholic media.
The complex adopts a trigonal bipyramidal geometry with trans CO and hydrido ligands, resulting in pseudo-C<sub>3v</sub> symmetry. The Rh-P, Rh-C, and Rh-H distances are 2.32, 1.83, and 1.60 ÃÂ , respectively. This complex is one of a small number of stable pentacoordinate rhodium hydrides.
This precatalyst was uncovered in attempts to use tris(triphenylphosphine)rhodium chloride as a hydroformylation catalyst. It was found that the complex would quickly carbonylate and that the catalytic activity of the resulting material was enhanced by a variety of additives but inhibited by halides. This inhibition did not occur in the presence of base, suggesting that the hydrido-complex represented the catalytic form of the complex.
[RhH(CO)(PPh<sub>3</sub>)<sub>3</sub>] is a catalyst for the selective hydroformylation of 1-olefins to produce aldehydes at low pressures and mild temperatures. The selectivity for n-aldehydes increases in the presence of excess PPh<sub>3</sub> and at low CO partial pressures. The first step in the hydroformylation process is the dissociative substitution of an alkene for a PPh<sub>3</sub>. The migratory insertion of this 18-electron complex can result in either a primary or secondary rhodium alkyl. This step sets the regiochemistry of the product, however it is rapidly reversible. The 16-electron alkyl complex undergoes migratory insertion of a CO to form the coordinately unsaturated acyl. This species once again gives an 18-electron acyl complex. The last step involves ò-H elimination via hydrogenolysis which results in the cleavage of the aldehyde product and regeneration of the rhodium catalyst.