Binuclear homoleptic rhodium carbonyls: Structures, energetics, and vibrational spectra

The binuclear homoleptic rhodium carbonyls Rh(2)(CO)(n) (n = 8, 7, 6, 5) have been examined theoretically. Three energetically low-lying equilibrium structures of Rh(2)(CO)(8) were found, i.e., one doubly bridged C(2v) singlet structure and two unbridged singlet structures with D(3d) and D(2d) symmetry. The doubly bridged structure is the global minimum predicted to lie 3.4 kcal/mol below the D(2d) structure and 6.4 kcal/mol below the D(3d) structure. For Rh(2)(CO)(7) the global minimum is either a singlet C(2v) unbridged structure or a singlet doubly bridged C(s) structure within 1.8 kcal/mol depending upon the theoretical method. For Rh(2)(CO)(6), the global minimum is either a singlet doubly bridged D(2) structure or a singlet unbridged D(2d) structure depending upon the method. Triplet structures for Rh(2)(CO)(7) and Rh(2)(CO)(6) are predicted to be of high energies relative to the low energy singlet structures. For Rh(2)(CO)(5) the C(2v) singlet singly bridged structure lies below the C(2) or C(2v) triplet structures.