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 M(VI) oxidation state is represented in the 4d series by the hexafluorides, MFg, of the elements Mo, Tc, Ru, and Rh. All are obtained by direct fluorination of the metal and are unstable powerfully oxidising species — once again the instability seems most marked at the end of the series. Unfortunately hardly any electronic spectral data exist. The first charge-transfer band of the d°MoF(s has been located at 54 kK. (42), and a study of the vibrational spectrum of RuF6 (43) revealed electronic bands at 1.95 and 1.4 kK., which are probably the F2, r5 Ti, and /13,... 

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). 

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

Answer (d) Element 114 falls below Pb in the Periodic Table. It should have the electron configuration [Rn] 5f " 6d °7s 7p. Chemically it should resemble lead. Its most stable oxidation state is likely to be 4-2, and compounds in the 4-4 state may be too unstable to exist at room temperature. It should form several compounds, including an oxide MO and several halides MX2 by analogy with lead, the chloride would be insoluble but a nitrate would be soluble. Element 114 could be a rather noble metal (though it has been suggested that the element might be a liquid or even a gas at room temperature ). 

About 400 compounds containing the T12-H2 ligand have been made that are stable. Almost all of them are octahedral with the metal in the d electron configuration, from Cr(0) to W(0) across the periodic table to Rh(lll) and Ir(III). There are a few dihydrogen complexes known with metals in other oxidation states. These are relatively rare. There are seven coordinate rhenium compounds like [Re(H2)(H)2(CO)(PMe2Ph)3]+ [8] and very unstable d systems like [Pt(ll2-H2)(PR3)2H]+ [9]. Neutral and cationic complexes are known but not anionic ones. Presumably anionic dihydrogen complexes are unstable because the metal s dTC electron richness promotes the oxidative cleavage of the H-H bond by d7t(M) —> o (H2) donation. 

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 d-metal complexes in unstable oxidation states may possess high or low redox potential values. These values depend on the ionization potentials (I) and on the difference between the solvation free energy for n+ and (n-l)+ charged ions (AG and AG .i, respectively) ... 

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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