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

Abstract

The binuclear homoleptic rhodium carbonyls Rh₂(CO)_n (n = 8, 7, 6, 5) have been examined theoretically. Three energetically low-lying equilibrium structures of Rh₂(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 Rh₂(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 Rh₂(CO)₆, the global minimum is either a singlet doubly bridged D₂ structure or a singlet unbridged D_2d structure depending upon the method. Triplet structures for Rh₂(CO)₇ and Rh₂(CO)₆ are predicted to be of high energies relative to the low energy singlet structures. For Rh₂(CO)₅ the C_2v singlet singly bridged structure lies below the C₂ or C_2v triplet structures.