Stabilization of unstable d-metal oxidation states by complex formation

Stabilization of Unstable d-Metal Oxidation States by Complex Formation... 

The stabilization of unstable d-metal oxidation states by complex formation has been studied for many years as one of the important problems of coordination chemistry. Alfred Werner paid attention to this, writing, "as a very peculiar phenomenon of the strengthening of primary valence by means of secondary valence forces, saturation has been often observed. The essence of this phenomenon has not been clear until now" (7). He then gave some examples of stabilization by formation of oxide and chloride complexes in the cases of Fe(VI), Mn(III), and Pb(IV). He pointed out that very unstable C0X3 salts can be stabilized by the coordination of ammonia molecules. Similarly, silver(II) compounds may be isolated only as the tetrakis(pyridine) adduct [Ag(py)4]S20g (7). 

Yatsimirskii, K.B. 1994. Stabilization of unstable d-metal oxidation states by complex formation, in Coordination Chemistry, ACS Symposium Series, 565 207-212. 

The most stable oxidation state for all lanthanide elements is the +3 state. This primarily arises as a result of the lack of covalent overlap, which stabilizes low and high oxidation states in the d-block metals by the formation of Ji bonds. While some zero-valent complexes are known, only the +2 and -1-4 oxidation states have an extensive chemistry and even this is restricted to a few of the elements. The reasons for the existence of compounds in the -1-4 and -j-2 oxidations states can be found in an analysis of the thermodynamics of their formation and decomposition reactions. For example, while the formation of all LnF4 and LnX2 is favorable with respect to the elements, there are favorable decomposition routes to Ln for the majority of them. As a result, relatively few are known as stable compounds. Thus L11X4 decomposition to L11X3 and X2 is generally favorable, while most UnX2 are unstable with respect to disproportionation to LnXs and Ln. 

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