Stabilization of unstable d-metal oxidation states

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 primary characteristic of d-transition metals is their ability to assume several oxidation states with different stabilities. Of special interest is the stabilization of unstable oxidation states of transition metals, which is of great significance in explaining the essence of "strengthening of the main valence by means of saturation of secondary valence forces" (coordination number) (7). 

The relative stabilities of the dioxides, sesquioxides and monoxides for first period transition metals are given in Figure 7.11(c). The stability of the higher oxidation state oxides decreases across the period. As we will discuss later, higher oxidation states can be stabilized in a ternary oxide if the second metal is a basic oxide like an alkaline earth metal. The lines in Figure 7.11(c) can in such cases be used to estimate enthalpies of formation for unstable oxidation states in order to determine the enthalpy stabilization in the acid-base reactions see below. Finally, it should be noted that the relative stability of the oxides in the higher oxidation states increases from the 3d via 4d to the 5d elements, as illustrated for the Cr, Mo and W oxides in Figure 7.11(d). 

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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