Identification of rhodium-rhenium nonacarbonyl RhRe(CO) 9. Spectroscopic and thermodynamic aspects

Abstract

The hydrido metal carbonyl HRe(CO) 5 reacts rapidly with the cluster Rh 4(CO) 12 at room temperature in n-hexane solvent under CO and H 2 to give the coordinately saturated dinuclear carbonyl complex RhRe(CO) 9. At room temperature and low partial pressures of CO and H 2 (P i < 2.2 MPa), an observable equilibrium is established between the reactants: 4HRe(CO) 5 + Rh 4(CO) 12 + 4CO → 4RhRe(CO) 9 + 2H 2. This observation implies the facile activation of molecular hydrogen on RhRe(CO) 9 at mild conditions and in the presence of CO. A pure component spectrum of RhRe(CO) 9 was obtained by BTEM analysis from the in situ FTIR spectroscopic measurements of the equilibrated solutions. The new species has absorbance maxima at 1985.6(s), 2012.2(w), 2026.6(vs, br), 2075(s), and 2127.2(w) cm -1, indicative of local trigonal bipyramidal geometry on the -Rh(CO) 4 moiety and square bipyramidal geometry on the -Re(CO) 5 moiety. Equilibrium measurements on the temperature interval T =289.7-308.2 K and the partial pressure intervals 0.2 MPa < P CO < 2.2 MPa and 0.05 MPa < P H2 < 2.0 MPa were performed, providing an enthalpy of reaction Δ rH = -116 ± 29 kJ/mol and entropy of reaction Δ rS = -312 ± 99 J/(mol K). Attempts to isolate RhRe(CO) 9 by crystallization at -78°C were unsuccessful.