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 C2v 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 C2v 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 D(2) 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 C2v singlet singly bridged structure lies below the C2 or C2v triplet structures.

Formation of supported rhodium clusters from mononuclear rhodium complexes controlled by the support and ligands on rhodium

“…5), indicating the formation of partially carbonylated dinuclear rhodium clusters. 16,31 Formation of these species did not alter the Rh-Rh coordination number indicated by EXAFS spectroscopy (Table 1), a result that demonstrates that the dirhodium clusters remained intact. 16 The n CO spectra are characterized by sharp, intense bands, with those at 1893 and 1853 cm À1 indicating bridging CO ligands and thus confirming the existence of Rh-Rh bonds.…”

Binuclear ruthenium carbonyl nitrosyls: Comparison with Fe2(NO)2(CO)n and Rh2(CO)n

“…However, the longer Ru-Ru bond of length 2.715 Å in 5S-2 is interpreted as a formal single bond giving one ruthenium atom the favored 18-electron configuration but the other ruthenium atom only a 16-electron configuration. The lowest energy structure of the isoelectronic Rh 2 (CO) 7 is quite different from either Ru 2 (NO) 2 (CO) 5 structure 5S-1 and 5S-2 since it is an unbridged structure consisting of an Rh(CO) 4 unit linked to an Rh(CO) 3 unit solely through a Rh-Rh bond [35]. In this Rh 2 (CO) 7 structure one rhodium atom has the favored 18-electron configuration but the other rhodium atom only a 16-electron configuration.…”

“…The Ru-Ru bond length of 2.645 Å in 4S-1 can be interpreted as a formal single bond to give each ruthenium atom a 16-electron configuration. The lowest energy structure of the isoelectronic Rh 2 (CO) 6 is very different from 5S-1 since it is an unbridged structure with a very short Rh"Rh distance of 2.663 Å interpreted as the formal triple bond required to give each rhodium atom the favored 18-electron configuration [35]. The similar metal-metal distances for the formal triple bond in this Rh 2 (CO) 6 structure and the formal single bond in the lowest energy Ru 2 (NO) 2 (CO) 4 structure 4S-1 can be related to the lack of bridges in the Rh 2 (CO) 6 structure but the presence of two bridges in 4S-1.…”

“…Comparison of the isoelectronic systems Ru 2 (NO) 2 (CO) n and Rh 2 (CO) n+2 Table 5 compares the lowest energy structures of the isoelectronic Ru 2 (NO) 2 (CO) n and Rh 2 (CO) n+2 systems (n = 6, 5, 4) using information from a previous DFT study [35] on the latter system for this comparison. For Rh 2 (CO) 8 one doubly CO bridged and two unbridged singlet structures are found within 30 kcal/mol (BP86) or 27 kcal/mol (MPW1PW91) of each other.…”

Development and testing of a compact basis set for use in effective core potential calculations on rhodium complexes

“…[28–31] Among the different ECP basis sets available in the literature, we mention here the Stuttgart RSC basis set,[32] which has been used to investigate the properties of rhodium compounds in Refs. [33–35], and the scalar‐relativistic Def2‐ n ZVP(PD) basis set,[36] n = S,T,Q (also known as SDD basis set), which has also been used in Refs. [37–41].…”