Synthesis of high-nuclearity carbonyl clusters

The logical synthesis of HNCC presents a major problem. The transition metals exhibit a complex array of bonding modes and a far more extensive range of coordination numbers than their main group counterparts, which have dependable valences and, in the main, strongly directional bonds. This, combined with the relative similarities of M—M, M—CO, and M—H bond energies (454), makes the designed synthesis of HNCC difficult. 

Synthetic methods used for the preparation of HNCC may be classified in to two broad categories, depending on whether or not they involve the use of redox conditions they are discussed below accordingly. 

Pyrolysis reactions of mononuclear carbonyls and low-nuclearity cluster compounds have been used extensively in the syntheses of HNCC of osmium (54, 72,80,95,108), ruthenium (18,20,29), and, more recently, rhenium (2-4). The reactions have been carried out either in inert solvents or, to facilitate the ejection of CO or other volatile ligands, in the solid state under vacuum. Condensation processes under pyrolytic conditions are rarely specific and, as such, lead to the formation of a wide range of products. In order to obtain optimum yields of a particular HNCC, the reaction conditions must be carefully screened. Solution reactions offer advantages such as the ability to monitor the progress of the reaction using IR spectroscopy. As they often give 

The HNCC of rhenium, [Re6C(CO)igH2] , [Re,C(CO)2i] -, and [RegC(CO)24] , have been produced in high yields from the solution thermolysis of [Re(CO)4H2] 2-4). In contrast, vacuum pyrolysis of Re2(CO)io (182-300°C) has been shown to yield only rhenium metal 41), unless pyrolyzed in the presence of metallic indium, in which case Re4(CO)i2 InRe(CO)5 4 was obtained in good yield (279). 

Sequential build-up of clusters, utilizing as coupling agents ligands that contain uncoordinated pairs of electrons, has been widely used by Vahren-kamp s group (450) and others in the synthesis of trinuclear and tetranuclear systems. This approach has been extended by Adams et al. to the synthesis of 

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In the synthesis of high nuclearity carbonyl clusters, small clusters are often used as starting materials ... 

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