The crystal structures of (p-ClPh)3PO and (p-OMePh)3PO, including an analysis of the P-O bond in triarylphosphine oxides

The crystal structures of the compounds tris(para-chlorophenyl)phosphine oxide {(p-ClPh)₃PO} and tris(para-methoxyphenyl)phosphine oxide {(p-OMePh)₃PO} were determined by X-ray diffraction methods. (p-ClPh)₃PO crystallizes in the space group P-1 (no. 2) with a = 11.828(2), b = 12.645(2), c = 14.072(2) Å, = 97.90(1), = 109.45(1), = 115.43(1), V = 1692.3(2) ų and Z = 4. The mean O–P and C–P distances are 1.481(6) and 1.806(2) Å, respectively, and the mean C–P–C angle is 106.5(1.1). (p-OMePh)₃PO crystallizes in the space group P2₁/c (no. 14) with a = 18.8642 (10), b = 10.3999(5), c = 21.3462(16) Å, = 115.414(6)°, V= 3782.6(4) ų, and Z = 8. The mean O–P and C–P distances are 1.484(5) and 1.798(4) Å, respectively, and the mean C–P–C angle is 106.5(1.0). These two structures were analyzed along with the previously determined structures of triphenylphosphine oxide {Ph₃PO} and tri-p-tolylphosphine oxide {(p-MePh)₃PO}, and IR data were collected on all four compounds. Both the observed P–O distances and the IR stretching frequencies for these triarylphosphine oxides support the interpretation of the P–O bond as having substantial multiple-bond character, with a bond order between 1.7 and 1.8. The para-substituents on these triarylphosphines were shown to have a statistically insignificant effect on the P–O bond.