RHODIUM CARBONYLS
Rhodium carbonyls present significant interest for their use as rhodium CVD precursors because of their volatility.
Main 2 methods for the synthesis of rhodium carbonyls are following:
Direct reaction of rhodium metal with carbon monooxide:
4 Rh + 16 CO → 2 Rh2(CO)8 (Matrix) → Rh4(CO)12 + 4 CO
Reduction of rhodim chloride by excess of CO (which acts as reducing agent) at high pressure:
6 RhCl3 + CO(exc.) → Rh6(CO)16 + m CO2 + …(CO as reducing agent; 60 °C, 40 bar)
Dirhodium octacarbonyl [Rh2(CO)8]
The synthesis of authentic dirhodium octacarbonyl [Rh2(CO)8] compound was reported to be succeeded only with difficulty at low temperature conditions; [Rh2(CO)8] easily disporportionates to Rh4(CO)12 and 4CO at temperature as low as -48°C.[i]
Dirhodium octacarbonyl [Rh2(CO)8] was reported to be tested as precursor for the laser-induced CVD growth of rhodium thin films, however, keeping in mind its ease of disproportionation most probably other rhodium carbonyl was actually used.[ii]
[i] L. A. Hanlan, G.A. Ozin, J. Am. Chem. Soc., 1974, 96 (20), pp 6324–6329, DOI: 10.1021/ja00827a013, http://pubs.acs.org/doi/abs/10.1021/ja00827a013?journalCode=jacsat “Synthesis using transition metal diatomic molecules. Dirhodium octacarbonyl and diiridium octacarbonyl”
[ii] K. L. Kompa, Angew. Chem. Int. Ed. Engl. 27, 1314 (1988) (Angewandte Chemie International Edition in English, Vol 27, Iss 10, p 1314–1325, 1988 , Laser Photochemistry at Surfaces—Laser-Induced Chemical Vapor Deposition and Related Phenomena »
Tetrarhodium dodecacarbonyl [Rh4(CO)12]
Synthesis, structure
Rh4(CO)12 was synthesized in moderate yield (60 %) by reduction of Rh2(CO)4Cl2 with CO at atmospheric pressure and room temperature in the presence of H2O. (When organic solvents or just minor quantities of water wer used, Rh6(CO)16 was obtained (yield 30%).)
When performing the same reaction under alkaline conditions, Rh4(CO)12 was obtained using hexane containing suspended NaHCO3 (82%), and Rh6(CO)16 in H2O-alcohol in the presence of LiOAc (77%). High yield, simplicity and easy availability of Rh2(CO)4Cl2 make last approach most practical.
The physical and chemical properties of the cluster
carbonyl Rh4(CO)12 and comparison with Rh6(CO)16 were discussed.[i]
[i] P. Chini, S. Martinengo, Inorg. Chim. Acta, Vol. 3, 1969, p. 315–318, « Synthesis of rhodium carbonyl compounds at atmospheric pressure. III. Synthesis of Rh4(CO)12 and of Rh6(CO)16 », http://www.sciencedirect.com/science/article/pii/S0020169300925027
Hexarhodium hexadecacarbonyl [Rh6(CO)16]
Synthesis, structure of [Rh6(CO)16]
Fig. Molecular structure of Rh6(CO)16
Hexarhodium hexadecacarbonyl [Rh6(CO)16] has been synthesized as air-stable black crystals [Rh6(CO)16] , characterised X-Ray crystallographically: it crystallizes
in monoclinic unit cell of symmetry I2/a with dimensions a= 17.00, b=9,78A, c=17.53, beta = 121°45’. It was shown to be identical with previously reported Rh4(CO)11, which actually has a hexameric structure Rh6(CO)16] (first known
hexanuclear metal carbonyl). It’ also first example of polynuclear carbonyl with CO grup bonded to 3 metal atoms. [Rh6(CO)16] has density 2,87 g/cm3 by the flotation method, calculated density 2,864 (based on [Rh6(CO)16] units per . [i]
The structure and bonding in Rh6(CO6) was analysed by calculations (sum-over-states density-functional perturbation theory (SOS-DFPT). The electronic structures of the clusters was discussed by using plots of electron localization functions (ELF). [ii]
[i] E.R. Corey, L.F. Dahl, W. Beck, J. Am. Chem. Soc., 1963, 85 (8), pp 1202–1203, DOI: 10.1021/ja00891a040; http://pubs.acs.org/doi/abs/10.1021/ja00891a040, « Rh6(CO)16 and Its Identity with Previously Reported Rh4(CO)11 »
[ii] M. Kaupp, Chem. Ber., 1996, Vol.129, Iss.5, p.527–533, « Analysis of 13C and 17O Chemical Shift Tensors and an ELF View of Bonding in Fe2(CO)9 and Rh6(CO)16 », http://onlinelibrary.wiley.com/doi/10.1002/cber.19961290509/abstract
Vapor pressure of Rh6(CO)6
Vapor pressure of solid Rh6(CO)16 (and for comparison for Os3(CO)12, Ru3(CO)12) was determined by the gravimetric torsion effusion method. The vaporization study of Rh6(CO)16 showed that it decomposed to Rh metal at 325 - 365 K (52-92°C). The measured molecular weight of the effusing gas from the solid Rh6(CO)16 was 27.75 g/mol, close to CO (compared to the values expected in absence of decomposition of 1065.56 g/mol). The total vapor pressures, the average molecular weights of the effusing gases, equilibrium constants for the vaporization reactions, their enthalpies, entropies, and Gibbs energies were determined.[i]
[i] D. Chandra, M. L. Garner, K. H. Lau, Journal of Phase Equilibria, 1999, Volume 20, Issue 6, pp 565-572, http://link.springer.com/article/10.1361/105497199770340554
Rh6(CO)6 for MOCVD
[i] TJ Lee, BC Gates - Catalysis Letters, 1991 , Rhodium supported in basic zeolite Y: A selective and stable catalyst for CO hydrogenation
[ii] D Chandra, H Mandalia, ML Garner, MK Blakely, Advances in X-Ray…, 1995, Line Profile Analyses of Rhodium Metal Obtained by Decomposition of Rhodium Carbonyl
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