Electronically and sterically induced structural distortions in square-planar d(8) complexes

The solid-state structure of the cationic MeO-Biphep Rh(I) compound [Rh((S)-MeO-Biphep)-(P{OMe}(3))(2))]BF4 (3) has been determined by X-ray diffraction. The four P-donors deviate markedly from square-planar geometry, with the phosphite ligands P2 and P2' ca. +/-0.61(7) Angstrom from the P1-Rh-P1' plane. This distortion resembles that found for PdBr(p-NCC6H4)-((S)-MeO-Biphep) (1). Density functional calculations on a series of systematically varied models of 1 reveal three major components to be responsible for the observed distortion from square-planar geometry: (i) attractive aromatic ring pi-pi interactions, (ii) electronic stabilization of coplanar aromatic rings in pseudo-trans positions, and (iii) P-phenyl and MeO-Biphep-phenyl intraligand repulsive steric interactions. Additional DFT studies on the p-tolyl-Binap analogue of i, PdBr(p-NCC6H4)((R)-p-Tol-Binap) (2), explain the source of the extremely long Pd-PB bond distance, 2.437(1) Angstrom, in 2. Despite the structural similarity between 1 and 2, the calculations rationalize the observation of a pronounced distortion from a square-planar geometry in the former that is essentially absent in the latter.