Osmium coordination numbers

The distinction between the first member of the group and the two heavier members, which was seen to be so sharp in the early groups of transition metals, is much less obvious here. The only unsubstituted, discrete oxoanions of the heavier pair of metals are the tetrahedral [Ru 04] and [Ru 04]. This behaviour is akin to that of iron or, even more, to that of manganese, whereas in the osmium analogues the metal always increases its coordination number by the attachment of extra OH ions. If RUO4 is dissolved in cold dilute KOH, or aqueous K2RUO4 is oxidized by chlorine, virtually black crystals of K[Ru 04] ( permthenate ) are deposited. These are unstable unless dried and are reduced by water, especially if alkaline, to the orange... 

We have previously seen examples of the carbon-like formulas of mononuclear and dinuclear osmium compounds, namely the methane-like tetrahydride (4.50c), ethylene-like H20s=CH2 (4.51c) and H2Os = OsH2 (Table 4.15), acetylenelike HOs = CH (4.54c) and HOs = OsH (Table 4.15), allene-like H2C = Os = CH2 (4.55a), and so forth. While the coordination numbers and Lewis-like formulas are formally analogous, the actual structures of Os and C species may be quite similar (e.g., the Td structures of OsfL and CH4) or dissimilar (e.g the strongly bent Cs structure of H20s = CH2 [Fig. 4.13(c)] versus the planar D2h structure of H2C = CH2). 

Numerous phosphine and arsine complexes have been synthesized and characterized predominately with osmium in the +2, + 3 or +4 oxidation states. Examples include [OsCl2(dppm)2] [108341-10-2], [OsC13(P(CH3)2(C6H5)3] [20500-70-3], [0s2Cl6(dppm)2(0)] [87883-12-3], and [Os(AsC2Hb(C6Hb)2)4H2] [27498-19-7]. An example of an unusually low oxidation state is the Os(—2) complex K2[Os(PF3)4] [26876-74-4]. High coordination numbers and formal oxidation states are found in the phosphine hydrides, eg, [Os(P(CH3)(C(5HB)2)H6] [25895-55-0] and... 

IR and EXAFS data representing osmium carbonyls on y-Al203 led to the identification of osmium di- and tricarbonyls and of the conversion of the latter into the former. The loss of one CO ligand from the tricarbonyl complex (indicated by infrared spectroscopy and confirmed approximately by EXAFS spectroscopy) was accompanied by an increase in the Os-O coordination number from2.9 to 3.9 (or nearly from 3 to 4) (Deutsch, Chang, and Gates, 1993), as follows ... 

Table 2 Unusual Coordination Numbers for Osmium Complexes...

The complex Os(SCl0Hl3)4(CNMe) is made from OsCl3, the lithium salt of 2,3,5,6-tetramethyl-benzenethiolate and 2,3,5,6-tetramethylphenyl sulfide it is green-yellow. The X-ray crystal structure of the ruthenium salt shows it to be trigonal bipyramidal, with the acetonitrile in the axial position. The osmium complex is isomorphous it would seem therefore to be the only example so far reported of a trigonal bipyramidal osmium(IV) complex. In an attempt to make a complex of lower coordination number 2,4,6-triisopropylbenzenethiolate (SC15H23) was used but the complex was still pentacoordinate, i.e. Os(S15H23)4(MeCN).719... 

With the metals of the first transition series, the maximum coordination number of higher oxidation states is six, and this is so firmly fixed that in their fluoride complexes the oxidation state of the metal in question can be fixed by controlling the mol fraction of alkali metal present 26). Thus, the fluorination of a vanadium salt in the presence of a one, two, or three mol ratio of potassium ion, yields KV F6, KgWVFe, or KsV Fe. The same tendency is shown, but to a lesser degree, by metals of the second transition series, as exemplified by KRuFe and KgRuFe. For an unusual example in the third series, note that OsFe is known, OsFt is not stable, but heptavalent osmium is found as six-coordinated OsOFs (27). 

Certain neutral complexes of bivalent ruthenium (1) and osmium (1,2), and tervalent and univalent iridium (3,4) have been discovered to activate molecular hydrogen in solution at normal conditions. These reactions are of considerable interest because they permit comparison of hydrogen-activating characteristics of metal complexes with different coordination numbers and formal oxidation states, two properties which are believed to represent important factors in determining catalytic activation of hydrogen. 

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