Osmium carbon ligands

Ruthenium and Osmium High Oxidation States 5.6.4.4.1 Carbon ligands... 

Carbon monoxide insertion into an osmium-carbon bond of a trinuclear cluster has been reported to give a cluster containing a t-ketene ligand ... 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

The co-condensation reactions described above have led to the formation of interesting new compounds and sometimes very unexpected products. The nature of the products formed for example in the osmium atom experiments indicate high degrees of specificity can be achieved. However, the detailed mechanisms of the co-condensation reactions are not known. It seems most likely that in all cases the initial products formed at the co-condensation temperature are simple ligand-addition products and that the insertion of the metal into the carbon-hydrogen bond occurs at some point during the warming up process. In support of this hypothesis we note the virtual absence of any... 

Cofacial ruthenium and osmium bisporphyrins proved to be moderate catalysts (6-9 turnover h 1) for the reduction of proton at mercury pool in THF.17,18 Two mechanisms of H2 evolution have been proposed involving a dihydride or a dihydrogen complex. A wide range of reduction potentials (from —0.63 V to —1.24 V vs. SCE) has been obtained by varying the central metal and the carbon-based axial ligand. However, those catalysts with less negative reduction potentials needed the use of strong acids to carry out the catalysis. These catalysts appeared handicapped by slow reaction kinetics. 

Initially, it was thought more likely that the electron poor metal atom would be involved in the electrophilic attack at the alkene and also the metal-carbon bond would bring the alkene closer to the chiral metal-ligand environment. This mechanism is analogous to alkene metathesis in which a metallacyclobutane is formed. Later work, though, has shown that for osmium the actual mechanism is the 3+2 addition. Molecular modelling lends support to the 3+2 mechanism, but also kinetic isotope effects support this (KIEs for 13C in substrate at high conversion). Oxetane formation should lead to a different KIE for the two alkene carbon atoms involved. Both experimentally and theoretically an equal KIE was found for both carbon atoms and thus it was concluded that an effectively symmetric addition, such as the 3+2 addition, is the actual mechanism [22] for osmium. 

Osmium(viii).—From studies of the extraction of OSO4 from alkaline aqueous solutions by carbon tetrachloride, it has been suggested that the anion [0s04(0H)] may exist in these media. Similar evidence was found for the presence of [OsOjN]" in aqueous ammonia solutions of 0s04. Examination of the i.r. spectra of a series of metal complexes of the ligand 8-amino-7-hydroxy-4-methylcoumarin, including [OsO LjjClj, indicate a correlation between certain vibrational bands, e.g. v(M—N), and the stability of the compounds. ... 

In the solid state, the equilibrium is in favor of the hydrido complex (III), and its crystal structure and that of the osmium(II) analog have been determined (38). Chatt also observed that, on heating the equilibrium mixture of (II) and (III), naphthalene was eliminated and the product Ru(dmpe)2 was also a tautomeric mixture. Here the tautomer-ism involves breaking and re-formation of carbon-hydrogen bonds in the methyl groups of the phosphine ligands (IV and V) ... 

Some interesting chemistry has appeared relating to the ability of the isocyanide ligand to stabilize unusual oxidation states. A series of palladium metal - metal bonded complexes has been synthesized by redox reactions involving two metal complexes in different formal oxidation states (33 -35). Similar ruthenium(I) and osmium(I) dimers have been prepared by an unusual homolytic fission of a ruthenium-carbon bond (36) or by singleelectron oxidation of Os(CNXylyl)5 (18). 

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