The crystal and molecular structure of the four title compounds from four circle have been determined. Tri-o-tolylphosphine (I), triclinic, a= 12.081(5), b= 10.915(5), c= 14.348(7)Å; α= 91.95(5)°, β= 110.58(4)°, γ= 98.22(5)°, space group P, Z= 4; R 0.045, 4 698 independent reflections. Tri-o-tolylphosphine oxide (II), monoclinic, a= 9.039(5), b= 17.282(6), c= 35.192(12)Å, β= 95.12(1)° space group P2₁/c, Z= 12; R 0.075, 5 428 independent reflections. Tri-o-tolylphosphine sulphide (III), triclinic, a= 15.377(4), b= 15.346(4), c= 7.979(4)Å; α= 103.1(1)°, β= 86.6(1)°, γ= 96.5(1)°, space group P, Z= 4; R 0.094, 2 890 independent reflections. Tri-o-tolylphosphine selenide (IV), monoclinic, a= 33.497(10), b= 8.004(2), c= 14.701(10)Å, β= 110.67(4)°, space group C2/c, Z= 8; R 0.067, 2 343 independent reflections.

The differences in P–C bond lengths appear to be determined by charge density on the aromatic group and the phosphorus atom, and by steric hindrance and crystal packing forces, and not by any extension of the π system of the aromatic groups to the phosphorus atom.