(Acetylacetonato-κ2 O,O′)[(2-bromophenyl)diphenylphosphane-κP]carbonylrhodium(I)

In the title compound, [Rh(C5H7O2)(C18H14BrP)(CO)], the RhI atom adopts a slightly distorted square-planar geometry involving two O atoms [Rh—O = 2.077 (2) and 2.033 (2) Å] of the acetylacetonate ligand, one carbonyl C atom [Rh—C = 1.813 (2) Å] and one P atom [Rh—P = 2.242 (5) Å] of the PPh2(2-BrC6H4) phosphane ligand. Difference electron density maps indicate a disorder of the Br atom over two positions in an approximate 0.95:0.05 ratio. However, this disorder could not be resolved satisfactorily with the present data.

In the title compound, [Rh(C 5 H 7 O 2 )(C 18 H 14 BrP)(CO)], the Rh I atom adopts a slightly distorted square-planar geometry involving two O atoms [Rh-O = 2.077 (2) and 2.033 (2) Å ] of the acetylacetonate ligand, one carbonyl C atom [Rh-C = 1.813 (2) Å ] and one P atom [Rh-P = 2.242 (5) Å ] of the PPh 2 (2-BrC 6 H 4 ) phosphane ligand. Difference electron density maps indicate a disorder of the Br atom over two positions in an approximate 0.95:0.05 ratio. However, this disorder could not be resolved satisfactorily with the present data.

Experimental
Crystal data [Rh(C 5 (Bonati and Wilkinson, 1964). This work is part of an on-going investigation aimed at determining the steric effects induced by various phosphane ligands on a rhodium(I) metal centre. Previous work illustrating the catalytic importance of the rhodium(I) square-planar moieties has been conducted on rhodium mono-and di-phosphane complexes containing the symmetrical bi-dentate ligand acac (acac = acetylacetonate) (Moloy and Wegman, 1989). Symmetrical di-phosphane ligands result in the production of acetaldehyde, whereas unsymmetrical di-phosphane ligands are more stable and efficient catalysts for the carbonylation of methanol to acetic acid (Carraz et al., 2000).

Refinement
The aromatic, methine, and methyl H atoms were placed in geometrically idealized positions (C-H = 0.95-0.98) and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C) for aromatic and methine H atoms, and U iso (H) = 1.5U eq (C) for methyl H atoms respectively. Methyl torsion angles were refined from electron density. The Br atom was modelled disorderd over two positions in a 95:5 ratio. This resulted in an unacceptably short C26-Br1B distance of 1.657 Å for the minor component.
Applying a distance restraint (SADI or DFIX in SHELXL) to the minor C26-Br1B component resulted in a severe distortion of the phenyl ring. In addition, this resulted in an unstable refinement.
Modelling the complete ring (C21-C26, Br1B) as disorderd over two positions resulted in a 96:4 ratio. This disorder provides a chemically acceptable explanation of the low occupancy of the minor disorder, as it results in distortion of the P coordination sphere. This behaviour is, however, expected in solution at room temperature. Unfortunately modelling the complete ring as a disorder resulted in an unstable refinement (results file in the supplementary information at the end of the cif file).   Special details Experimental. The intensity data was collected on a Bruker Apex DUO 4 K CCD diffractometer using an exposure time of 10 s/frame. A total of 3977 frames were collected with a frame width of 1.5° covering up to θ = 66.56° with 98.4% completeness accomplished. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (