Group 8 Iron, Ruthenium and Osmium

Catalysts Prepared from Metal Carbonyls of Group 8 Iron, Ruthenium and Osmium 323... 

In complexes [M(CO)4 (olefin)] and [Mcp(CO)2 (olefin)], the alkene molecules rotate as in the corresponding chromium and manganese group compounds. Iron, ruthenium, and osmium also form olefin coordination compounds possessing a and n bonds (Figure 6.8). 

Ruthenium is a rare, hard, silvery-white metallic element located in group 8, just above osmium and below iron, with which it shares some chemical and physical properties. Both ruthenium and osmium are heavier and harder than pure iron, making them more brittle and difficult to refine. Both ruthenium and osmium are less tractable and malleable... 

The elements of the second and third transition series also exhibit a variety of oxidation states. In general, the stability of the higher oxidation states increases down a periodic group. In group 8B, for example, the oxide of iron with the highest oxidation state is iron(III) oxide, Fe203. Ruthenium and osmium, though, form volatile tetrox-ides, RuC>4 and OsC>4, in which the metals have an oxidation state of +8. 

All three Group 8 metals form trinuclear clusters M3(CO)i2. However, while all carbonyl ligands in ruthenium and osmium dodecacarbonyl complexes coordinate to the metal center as terminal carbonyls, there are two bridged carbonyl groups in iron dodecacarbonyl. This may be due to the smaller van der Waals radius of the iron atom. In this section, Fe(CO)5, Fe2(CO)9and Na2[Fe(CO)4] are reviewed. 

Given the isoelectronic relationship between [CR] and [NO] and the ubiquity of this latter ligand in the coordination chemistry of later transition metals, the scarcity of mononuclear alkylidyne complexes of metals from groups 8-10 is surprising [1-4]. Isolated examples have been reported for iron [5], cobalt [6], ruthenium [4,7], osmium [4,8-9] and iridium [10]. Most of the examples known employ routes with extensive precedent in early transition metal systems, i.e., either electrophilic attack at the p-atom of a hetero carbonyl (CS [5], CTe [4], or C=CH2 [10]) or the Lewis-acid assisted abstraction of an alkoxide group from a carbene precursor [5] (Scheme 1). The one approach which is, too date, peculiar to group 8 metals involves reduction of a divalent dichlorocarbene complex by lithium aryls [4]. The limitation of this procedure to ruthenium and osmium is presumably not a feature of these metals but rather a result of the present lack of synthetic routes to suitable dihalocarbene precursor complexes of earlier metals. 

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