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Do all metals in metal oxo complexes (eg $\ce{Fe}$ in Ferrate, $\ce{Mo}$ in $\ce{MoO4}$,...) have positive oxidation states?

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Yes. Oxygen is much more electronegative than all of the transition metals, so it is given a formal -2 charge. Keep in mind, though, that these values are not intrinsic properties of these elements/molecules. Their use is intended to help us rationalize some of the chemistry and properties associated with these materials. As an interesting example, consider oxygen difluoride, $\ce{OF_2}$. Fluorine is the most electronegative element (that forms compounds with other elements) and is given a formal charge of -1. To balance the formal charges then, oxygen must have a +2 charge!

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  • $\begingroup$ And as a result, oxygen difluoride is seriously nasty stuff. Oxygen yearns to achieve that "natural" -2 ox state, and it will do so by oxidizing anything even slightly more willing to donate an electron. Transition metals are a gimme for a violent oxidation/halogenation reaction, and the substance is one of few strong enough to attach fluorine to xenon. $\endgroup$ – KeithS Sep 4 '13 at 2:54

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