Osmium dusters

Scheme 1. Best syntheses of various osmium dusters on the surface of...

As you become more familiar with transition metal dusters (no nonmetals in the framework) you will come to associate doso structures with numbers of electrons. A trimer will have 48 electrons, a tetrahedron will have 60 electrons, a trigonal bipyramid will have 72 electrons, and an octahedron will have 86. Some care is required, however, as can be illustrated with Os,H2(CO),0. An electron count gives us 46 electrons rather than 48. If, however, we allow for one Os—Os double bond, the electron count is as expected. In accord with this expectation, one osmium-osmium bond is found to be shorter than the other two and the complex shows the reactivity expected for an unsaturated complex. 

The oxo complexes of osmium dominate the chemistry of the element, the duster carbonyls notwithstanding, and the tetroxide is the single most important compound of the element. 

FIb.1. ORTEP drawing (50% ellipsoids) of the one-to-one CM-osmium tetroxide adduct Cgo(( 04)(4-tM-butylpyridine)2 showing the relationship of the m unit with the carbon duster. 

The other vibrational spectroscopies, although less easily applied, may provide complementary structural information. Raman spectroscopy has been used to detect metal-metal bonds in metal oxide supported osmium [86] and iridium [87] clusters. This method might be expected to find application in the study of zeolite supported metal carbonyl dusters, but it is still far from routine since samples are subject to destruction by laser beams, and fluorescence often prevents measurement of useful spectra. 

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