FURTHER CONSIDERATIONS ON THE STRUCTURE AND BONDING IN EDGE-SHARING BIOCTAHEDRAL COMPLEXES

Extended Huckel MO calculations have been carried out on model compoun ds that mimic transition metal edge-sharing bioctahedral (ESBO) comple xes of stoichiometry M2X6L4 (X=anionic ligand with lone pairs for brid ge-bonding and for pi bonding, L = neutral 2-electron donor ligand), a nd for the corresponding mononuclear cis-MX4L2 system. The widening of the cis-L(eq)-M-L(eq) angle (eq = equatorial) is shown to cause the n arrowing of the opposite X-M-X angle which, for the ESBO complexes, di sfavors the formation of a strong metal-metal interaction. The calcula tions also address the importance of the M-X ir interactions in metal- metal bonded dimers. The X(ax) (ax = axial) ligands are shown to be be tter ir donors than the X(eq) ligands for the configurations d1-d1 thr ough d5-d5, the differential between the pi donating abilities in the two different positions being maximal for the d1-d1 configuration. Thi s effect is proposed to be responsible for the preference of d1-d1 sys tems for the ESBO structure having all L ligands in equatorial positio ns, whereas the metal-metal bonded ESBO compounds of all other electro nic configurations as well as all non-bonded ESBO complexes prefer the structure with two L(eq) on one metal and two L(ax) on the other one on steric grounds. The MO model presented here is also in excellent ag reement with the observed trends of M-Cl(ax), M-Cl(eq), and M-Cl(br) ( br = bridging) bond distances as a function of d(n)-d(n) configuration .