Trigonal bipyramidal complexes reactivity

Owing to the reduced reactivity of the trialkylsilylarsane compared with the isostructural phosphane, complexes with a higher arsanide content are isolated, where the trigonal bipyramids are interconnected via common corners, edges and faces (62, 66 and 68). 

This oxidation state is rare for nonorganometallic complexes, being found only in the trigonal-bipyramidal [Os(PR3)5] complexes and in reactive intermediates such as [Os(PR3)4] and [Os(PR3)3] (15). 

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. 

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