trans-Tetracarbonylbis(triphenylphosphane-κP)molybdenum(0)

The well known title compound, trans-[Mo(C18H15P)2(CO)4], has not been studied previously by X-ray crystallography, unlike its cis isomer. The complex possesses crystallographically imposed inversion symmetry, with the Mo atom residing on an inversion centre (1a Wyckoff position). The two triphenylphosphane groups are arranged in a staggered orientation. Each of the phenyl groups exhibits significantly different Mo—P—C—C torsion angles ranging from 2.6 (2) to 179.4 (1)°, most likely due to steric interactions based upon their positions relative to the carbonyl ligands.

The well known title compound, trans-[Mo(C 18 H 15 P) 2 (CO) 4 ], has not been studied previously by X-ray crystallography, unlike its cis isomer. The complex possesses crystallographically imposed inversion symmetry, with the Mo atom residing on an inversion centre (1a Wyckoff position). The two triphenylphosphane groups are arranged in a staggered orientation. Each of the phenyl groups exhibits significantly different Mo-P-C-C torsion angles ranging from 2.6 (2) to 179.4 (1) , most likely due to steric interactions based upon their positions relative to the carbonyl ligands.

The Department of Chemistry and the College of Liberal
Arts & Sciences at UIC are acknowledged for purchasing a Bruker SMART X2S bench-top diffractometer, and for providing supplies and equipment for the inorganic teaching lab that motivated this study.

Comment
We initiated this study as part of an undergraduate teaching lab using the Bruker SMART X2S bench-top diffractometer.
Students use FT-IR spectroscopy to propose whether unknown samples are either the cis-or trans-isomer, and then use crystallography to test their hypotheses. To our surprise, the trans-isomer had not been reported in the CSD. In our hands, dissolving the trans-isomer in dichloromethane causes reversion to the cis-isomer. Crystallization from chloroform, on the other hand, provides the trans arrangement cleanly.
The 0 1 0 and 0 0 1 reflections were omitted from final refinements because of the suspicion that they were affected by the beamstop. Hydrogen atoms were placed at calculated positions 0.93 angstroms from the phenyl carbons and refined using the standard riding model with U iso (H) set to 1.2 times U eq (C).

Special details
Experimental. For synthesis of the compound, see Cotton (1982). Yellow crystals of the title compound suitable for Xray diffraction were obtained by layering methanol above a chloroform solution of the title compound and allowing the layers to mix gradually. This crystallization method was performed on the bench with reagent grade solvents and without use of an inert atmosphere. 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