Breaking an electronically preferred symmetry by steric effects in a series of [Ir(biph)X(QR(3))(2)] compounds (X = Cl or I, Q = P or As)

A hybrid quantum mechanical/molecular mechanics IMOMM (B3LYP : MM3) method has been applied to a series of five-coordinate 16-electron d(6) ML5 Ir-III compounds having a relatively flat potential for a distortion from Y to T geometry and for which crystal structures have been obtained. In this serie s of type [Ir(biph)X(QPh(3))(2)] (biph = biphenyl-2.2'-diyl; Q = P, X = Cl, 2a; Q = As, X = Cl, 2b; Q = P, X = I, 2c), the halide is found to lie in t he (biph)Ir plane but off the C-2 axis of the {Ir(biph)Q(2)} fragment by a variable angular distortion phi. While phi = 0 is preferred electronically for [Ir(C4H4)Cl(PH3)(2)], the steric bulk of the real systems 2a-2c leads t o phi taking experimental values of 8.2-17.2 degrees. The observed deviatio n of the halide from the C-2 axis is shown by IMOMM to be the result of a d irect interaction of the phenyl substituents of the axial ligands with the equatorial ligands and not to an electronic effect. The crystal structures for 2b and 2c have been determined.