Transition metal clusters disproportionation

Reviews have appeared of photochemical disproportionation of metal-metal bonded carbonyl dimersphotofragmentation of transition metal cluster carbonyls in the gas phase, and f-element photochemistry. ... 

Reduction of (5) with alkali metal forms the green cluster anion Co6(CO)i5 , which can be further reduced to the red dianion Co6(CO)i4 , and isolated as a potassium salt. The latter cluster anion is an early example of the expanding family of highly reduced transition metal carbonyls, for example Na3Co(CO)3, where up to two CO groups are replaced by two electrons each. Co6(CO)i5 is also the main product from valence disproportionation of Co2(CO)s in ethanol. Oxidation (or acidification) of the cluster anions gives Co6(CO)ie. 

It is sometimes very difficult to obtain stable, homogeneous solutions of inorganic compounds in molten salt. This is particularly so for the compounds of the transition metals which characteristically exhibit a range of oxidation states for each element. Tantalum, which can exist in the oxidation states V, IV, II, I, —I, may form cluster compounds in the lower states these are capable of disproportionating, whereas the higher oxidation state compounds are rather volatile at the operating temperatures of many molten salts. Some further details of these phenomena are also given in Section 4. 

The ionic model is of limited applicability for the heavier transition series (4d and 5d). Halides and oxides in the lower oxidation states tend to disproportionate, chiefly because of the very high atomisation enthalpies of the elemental substances. Many of the lower halides turn out to be cluster compounds, containing metal-metal bonds (see Section 8.5). However, the ionic model does help to rationalise the tendency for high oxidation states to dominate in the 4d and 5d series. As an example, we look at the fluorides MF3 and MF4 of the triad Ti, Zr and Hf. As might be expected, the reaction between fluorine gas and the elemental substances leads to the formation of the tetrafluorides MF4. We now investigate the stabilities of the trifluorides MF3 with respect to the disproportionation ... 

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