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Terminal mono-oxo complexes of the late transition metal elements have long been considered too unstable to synthesize because of repulsion between the oxygen electrons and the mostly filled metal d orbitals. A platinum(IV)-oxo compound flanked by two polytungstate ligands, K^sub 7^Na^sub 9^[O=Pt(H^sub 2^O)L^sub 2^], L = [PW^sub 9^O^sub 34^]^sup 9-^, has now been prepared and isolated at room temperature as air-stable brown crystals. X-ray and neutron diffraction at 30 kelvin revealed a very short [1.720(18) angstrom] Pt-O bond and no evidence of a hydrogen atom at the terminal oxygen, ruling out a better precedented Pt-OH complex. Density functional theory and spectroscopic data account for the stability of the Pt(IV)-oxo unit by electron withdrawal into delocalized orbitals of the polytungstates.
Despite the high electronegativity of oxygen, the terminal oxo ligand is a strong π electron donor. Lone oxygen atoms therefore bind most stably to high valent early transition metals, such as Ti(IV), V(V), and their heavier congeners. In these complexes, electrons can delocalize from oxygen into the vacant d orbitals on the metal. Thus, d^sup 0^ to d^sup 2^ oxo compounds are ubiquitous, and the oxo ligand can be ancillary to reactions in the coordination sphere of the metal. Moving from left to right across the periodic table, the d orbitals fill with valence electrons, and oxo ligands are destabilized by repulsion (1-5). There are few stable d^sup 4^ metal oxo complexes, a hydrogen-bonded d^sup 5^ complex (6), and only a single reported d^sup 6^ oxo, the NaRe(O)(PhCCPh)^sub 2^ complex isolated by Mayer and co-workers (7). For iron and the later transition metals, even monomeric hydroxo compounds are rare.
At the same time, the instability of the late metal oxo linkage proves useful in catalysis. Iron-oxo intermediates are invoked in numerous enzymatic oxidations, both for heme-based oxidases (8-10) and non-heme oxygenases (11-17). Similarly, transient oxo intermediates are thought to play a major role in O2 activation at platinum surfaces, whether in automobile catalytic converters (18), fuel cells (19), or industrial catalysis (20). Efforts to model such intermediates have been hampered because, in the absence of an oxygen acceptor, O-coordination to these electron-rich metals leads to disproportionation or to cluster formation in order to diminish the electron repulsion. We reasoned that a stable platinum oxo complex...