trans-Bis[(2-bromophenyl)diphenylphosphane-κP]carbonylchloridorhodium(I)

The title compound, trans-[RhCl(C18H14BrP)2(CO)], has a slightly disordered square-planar geometry with the Rh ionI situated on an inversion the centre and carbonyl–chloride disorder observed as a result of the crystallographic inversion symmetry. Selected geometric parameters include: Rh—P = 2.3430 (8) Å, Rh—Cl = 2.434 (3) Å, Rh—C = 1.722 (8) Å, P—Rh—P = 180.00 (3)°, P—Rh—Cl = 95.40 (7)°, 84.60 (7)° and Rh—C—O = 177.9 (8)°.


Frederick P. Malan, Rehana Malgas-Enus and Reinout Meijboom Comment
A vast range of different Vaska-type complexes have been synthesized, reported and spectroscopically studied (Roodt et al., 2003;Lamb et al.;2009) since the synthesis and correct formulation of the original Vaska complex, trans-[Ir(CO)Cl(C 18 H 15 P) 2 ], by Vaska & Di Luzio (1961). This class of symmetrical square-planar complexes (including Rh, Ir, Pd and Pt) usually crystallizes with the metal atom on a crystallographic inversion centre, resulting in a disordered packing arrangement (Chen et al., 1991;Otto et al., 2000;Otto & Roodt, 2004;Meijboom et al., 2004). The title compound serves as yet another complex to add to the Vaska's complex range with varying Group 5 ligand systems possessing different stereoelectronic properties.
In the title compound, the Rh atom lies at the centre of a slightly distorted square-planar geometric arrangement. The Rh atom crystallizes on a centre of symmetry, a crystallographic inversion centre, and has the carbonyl and chloro-ligands disordered at a 0.5:0.5 ratio. The stereoelectronic property of the phosphane with the bromo-functionality is indicated by the smaller O1-Rh1-P1 angle (85.39 (19)°), which is translated through symmetry to the inverted side of the molecule.
Selected spectroscopic data of the current compound is comparable to other similar complexes reported previously by Roodt et al. (2003) and Otto & Roodt (2004). However, the interesting difference in the magnitude of v(CO) for the solid and solution (in DCM) states of the title compound is ascribed to the packing of the molecules, which slightly distorts the Rh C-O angle. This effect was previously observed and reported for a polymorph of trans-[Rh(CO)Cl{PPh 3 } 2 ] (Kemp et al., 1995).

Experimental
A solution of dichlorotetracarbonyldirhodium (0.050 g, 0.13 mmol) in acetone (3 cm 3 ) was slowly added to a solution of the phosphane, C 18 H 14 BrP (0.176 g, 0.51 mmol) in acetone (3 cm 3 ) at room temperature,after which the mixture was left to crystallize. Slow evaporation of the solvent afforded the title compound as yellow crystals. Spectroscopic analysis: 1973.2 cm -1 .

Refinement
The aromatic H atoms were placed in geometrically idealized positions (C-H = 0.93 Å) and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C) for all H atoms. The highest calculated residual electron density is 0.66 e.Å -3 at 1.597 Å from C15, which bears no physical meaning.

Figure 1
The structure of the title compound, showing 50% probability displacement ellipsoids. All rings have been numbered in the same, systematic manner. H atoms are depicted by arbitrary size spheres. Hashed atoms are generated by symmetry (x + 1, -y, -z + 2). Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.