The solid-state structure of the cationic MeO-Biphep Rh(I) compd. [Rh((S)-MeO-Biphep)(P{OMe}3)2]BF4 (3) was detd. 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) .ANG. from the P1-Rh-P1' plane. This distortion resembles that found for PdBr(p-NCC6H4)((S)-MeO-Biphep) (1). D. functional calcns. on systematically varied models of 1 reveal three major components to be responsible for the obsd. distortion from square-planar geometry: (i) attractive arom. ring p-p interactions, (ii) electronic stabilization of coplanar arom. rings in pseudo-trans positions, and (iii) P-Ph and MeO-Biphep-Ph intraligand repulsive steric interactions. Addnl. DFT studies on the p-tolyl-Binap analog of 1, PdBr(p-NCC6H4)((R)-p-Tol-Binap) (2), explain the source of the extremely long Pd-P2 bond distance, 2.437(1) .ANG., in 2. Despite the structural similarity between 1 and 2, the calcns. rationalize the observation of a pronounced distortion from a square-planar geometry in the former that is essentially absent in the latter. [on SciFinder (R)]