Osmium carbonyl clusters substitution

Substituted carbonyl clusters react with halogens in much the same way as their parent carbonyls. The final products of the reaction of halogens with Ru3(CO)9(PPh3)3 are the dimers [Ph3PRu(CO)2X2]2, as detected by Piacenti and co-workers (325, 326), but the initial products are the mononuclear Ph3PRu(CO)3X2 which dimerize with loss of CO just as the Ru(CO)4X2 compounds did (212). No trinuclear products were detected. The corresponding trisubstituted osmium carbonyl cluster... 

Ruthenium and Osmium. The general methods by which osmium carbonyl clusters may be activated have been outlined. Routes to reactive substitution products of... 

In transition metal-main group element clusters, there is the possibility of ligand substitution at either type of element center. Displacement of an exo-cluster ligand on one of the metal centers (equation 10) is to be expected (see Mechanisms of Reaction of Organometallic Complexes) However, displacement at a main group cluster site has also been observed (equation 11 ). Indeed, phosphine substitution takes place exclusively at the boron atom, and the osmium-substituted BCO complex can only be prepared by synthesizing it from the phosphine-substituted osmium carbonyl starting material. 

The substituted carbonyl cluster Ru3(CO)g(PPh3)3 reacts at 150°C and 150 atm CO pressure to form Ru(CO)4PPh3 exclusively, whereas the analogous osmium cluster produces Os3(CO)io and free PhgP (326, 327). 

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

Examples of the first type of these reactions are found in several systems but the reaction mechanisms may follow different pathways. One route is substitution of one of the carbonyl ligands as has been observed in some osmium and ruthenium complexes (458, 459). The other mechanism involves metal-metal bond fission (414, 428), and in some cases this means the formation of cluster compounds with a smaller number of metal atoms [Eq. (20)] (460). 

Reactions of ruthenium and osmium cluster carbonyls with heteroatom-substituted and functionalized alkynes (metallocycles) 00IZV1. 

A series of carbonyl-substituted osmium clusters of general formula Os6(CO)2i-. Lv (L = P(OMe)3, X = 1-6 L = MeCN, x = 1-3) is also structurally related to [Fe3Pt3(CO)i5] and features six extra cluster-valence electrons. As predicted theoretically,these raft-like osmium clusters have a similar capacity to add two more electrons reversibly. The electro-generated anions are stable on the cyclovoltammetry time-scale. 

The most extensive studies of the chemistiy of cluster complexes have been associated with the trinuclear cluster unit, as may be anticipated. A wide range of substitution reactions has been demonstrated for both Ru3(CO)i2 and Os3(CO)i2, with the full range of ligands normally employed in the study of metal carbonyl chemistry. In genera 1, the trinuclear osmium cluster is more readily maintained, ruthenium often giving rise to cluster breakdown, yielding mononuclear and binu-clear adducts. This reflects the increased bond enei of the metal-metal bond on descending the triad (see Table X later in this section). 

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