The crystal and molecular structure of trans-tetracarbonylbis(triphenyl-phosphine)chromium(0) in a new unit cell: Is the trans conformer more stable than the cis?

The crystal and molecular structure of trans-Cr(CO)₄(PPh₃)₂ has been determined by single crystal X-ray diffraction. The structure differs from one previously reported, packing in a different unit cell with no molecular center of symmetry. This asymmetry, coupled with the noncoplanarity of the four equatorial carbonyl ligands indicates that the structure is dominated by steric interactions. In spite of this, density functional calculations oncis-M(CO)₄(PPh₃)₂ vs. trans-M(CO)₄(PPh₃)₂, M = Cr, Mo, and W, predict that the gas phase cis conformer is more stable than the trans conformer for all three metals. The complex crystallizes in the triclinic space group with lattice parameters a = 10.139(5) Å, b = 10.583(5) Å, c =17.911(5) Å, = 75.860(5)°, = 77.410(5)°, = 65.930(5)°, V =1686.3(12) ų, Z =2.