trans-Carbonylchloridobis(tri-o-tolylphosphane-κP)rhodium(I)

In the title compound, [RhCl(C21H21P)2(CO)], the coordination geometry around the RhI atom is slightly distorted square-planar with the phosphane ligands in trans positions with respect to each other. The chloride and carbonyl ligands show positional disorder, and the RhI atom lies on a center of inversion. The effective cone angle ΘE for the title compound is 169.0 (3)°. There are no significant intermolecular interactions.

In the title compound, [RhCl(C 21 H 21 P) 2 (CO)], the coordination geometry around the Rh I atom is slightly distorted square-planar with the phosphane ligands in trans positions with respect to each other. The chloride and carbonyl ligands show positional disorder, and the Rh I atom lies on a center of inversion. The effective cone angle Â E for the title compound is 169.0 (3) . There are no significant intermolecular interactions.

Comment
The complex first synthesized by Angoletta (1959) and later correctly formulated by Vaska (Vaska & Di Luzio, 1961), trans-[IrCl(CO)(PPh 3 ) 2 ] has become known under the name of the latter. This complex and its analogues have been extensively used as catalysts and model compounds (Roodt et al., 2003).
Here we report a rhodium analogue bearing o-tolyl substituents on the phosphane ligands (I). As with many of these 'Vaska complexes', compound (I) crystallized with the metal on a crystallographric centre of symmetry, leading to a 50/50 disorder for the chloride and carbonyl ligands. With this report, Vaska complexes bearing all isomers of tritolylphosphane have been described. Compound (I) crystallizes in a slightly distorted square planar geometry with the phosphane ligands in trans-position to each other (Fig. 1).
The Rh-C and Rh-Cl bonds do not show large deviations from similar complexes, showing that the steric influence of the o-methyl substituents is not so significant as to distort the coordination geometry to a large degree. However, the Rh-P bond is longer than in similar complexes, indicating that the bulky ortho-aryl substituents force the phosphane ligands away from the rhodium. A useful indicator for the steric influence of phosphane ligands is the effective cone angle Θ E . For compound (I) this angle was found to be 169.0 (3) °, significantly larger than differently substituted triarylphosphanes like tri(m-tolyl)-phosphane, for which values of 155° and 160° were reported (Meijboom et al., 2005).

Experimental
Compound (I) was synthesized by slow addition of 4 equivalents of tri(o-tolyl)phosphane to a dimethyl formamide solution of [RhCl(CO) 2 ] 2 . After precipitation with ice water and separation of the product, it was recrystallized by slow evaporation from a 1:5 dichloromethane/hexane mixture. Analysis of the compound showed a CO stretching signal in IR at 1972 cm -1 , and a signal for the phosphane ligands in 31 P NMR at 25.7 p.p.m. with a J Rh-P of 125.9 Hz. These signals are in good agreement with various other rhodium Vaska's complexes.

Refinement
The aromatic and methyl H atoms were placed in geometrically idealized positions (C-H = 0.93 and 0.96 Å, respectively) and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C) for aromatic, and U iso (H) = 1.5U eq (C) for methyl H-atoms. The highest residual electron density was located 0.87 Å from Rh1 and was essentially meaningless.