ELECTRONIC-STRUCTURE OF RH-2(MU-CO)(CO)(2)(H2PCH2PH2)(2) - AN EXAMPLEOF A NON-A-FRAME STRUCTURE

Calculations based on density functional theory (DFT) and Hartree-Fock configuration interaction (HF-CI) methodology have been carried out t o investigate the rhodium-rhodium coupling in Rh-2(CO)(2)(dppm)(2), 1 (dppm = Ph(2)PCH(2)PPh(2)) and in Rh-2(mu-CO)(CO)(2)(dppm)(2), 2. DFT geometries, obtained with the Dgauss program, are in good agreement wi th those determined from X-ray, but HF geometries, calculated using th e same basis sets, yield bond distances systematically too long. Calcu lations indicate that the rhodium atoms in 1 are linked by a single bo nd. The insertion of a semibridging carbonyl between the two metal ato ms leads to a shortening of the rhodium-rhodium distance and also to a noticeable weakening of the metal-metal interaction. Both effects, an d also the stabilization of the HOMO of 2, are related to an observed change from square planar to tetrahedral of the ligand environment of the Rh atom proximal to the inserted CO. Both MO analysis and bond cha racterization from the topology of the charge density confirm the exis tence of a bonding interaction between the semibridging carbonyl and t he distal rhodium atom. The electronic structures of the dicationic co mplex [Rh-2(CO)(3)(dppm)(2)](2+) and of the A-frame-like, isoelectroni c system Rh2Br2(mu-CO) (dppm)(2) are also discussed. The electron defo rmation density is derived from 2 by means of several methodological a pproaches, namely, HF, HF-CI, DFT, and DFT + gradient corrections. The HF deformation density obtained in the plane containing the metals an d the three CO ligands is discussed, as well as the ''correlation dens ity'' obtained from the difference maps DFT - HF and Cl - HF.