Recent Research Developments

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October 15, 2003
    Metal Carbonyl Anions: from [Fe(CO)4]2- to [Hf(CO)6]2- and Beyond
    John E. Ellis

    Metal carbonyl anions have a rich history in chemistry and are important precursors to a tremendous variety of organometallic, inorganic, and organic species. They are also of considerable interest as the first compounds to contain transition metals in formally negative oxidation states and exhibit reactivity patterns that bear striking similarities to those of halides, chalcogenides and pnictides.  For example, metal carbonyl monoanions, such as [Co(CO)4]-, [Mn(CO)5]-, and [V(CO)6]-, etc. are now often considered to be transition metal analogs of classic pseudohalides, e.g., azide, cyanide, and thiocyanate ions.  Since the isolation of the first example in 1931, [Fe(CO)4]2-, related metal carbonyl anions have been reported for nearly all transition metals, and the most recent entry, [Hf(CO)6]2-, was reported in 1990 and first discovered in our laboratory.  Hexacarbonylhafnate(2-), which was structurally characterized by Professor Doyle Britton of this Department, see the figure below, remains the only example of a substance containing the element hafnium in a formally negative oxidation state.  Related metal carbonyl tri- and tetra-anions have also been prepared in this laboratory and contain transition metals in their lowest known oxidation states of -3 and -4, respectively.  Whereas the position of a transition metal in the periodic table places definite restrictions on its maximum possible oxidation state, for example, manganese "can't go higher than +7", as it is in the permanganate ion, there appear to be no apriori restrictions on corresponding minimum possible oxidation states.  So, in principle, compounds containing transition metals in oxidation states lower than -4 may exist, provided the coordinated ligands are resistant towards reduction. Whether carbon monoxide will prove to be a suitable ligand in this endeavor is among the topics discussed in a recent review by this author, published in the journal "Organometallics" (2003), vol.22, pp.3322-3338.  It presents a chronological survey of the syntheses and characterizations of homoleptic mononuclear metal carbonyl anions and considers prospects for the syntheses of new examples of this intriguing class of compounds for the very early and late transition metals, as well as for the lanthanides and actinides, for which carbonyl anions of any type are presently unknown.  Below is shown the cover of the issue of Organometallics in which this article appeared.  The cover molecule illustration represents hexacarbonylhafnate(2-), see above, and was kindly provided by Professor Arnold Rheingold of the University of California, San Diego.

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