Stabilization of unstable d-metal

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 tetra-cA-cycIononatetracne 241 is unstable and easily rearranges at 23 °C (t /2 50 min) to the isomeric d.v-8,9-dihydroindcne 242 (equation 77)89. It is interesting, however, that the iron(III) tricarbonyl complex of tetraene 241 is stable for many days at room temperature and isomerizes to the Fe-complex of 242 only upon heating in octane at 101 °C89. The principle of stabilization of the reactive multiple bonds with metal carbonyl complexes is well-known in modem organic synthesis (e.g. see the acylation of enynes90). 

There are ammoniates of PtCl2, of halides of other platinum metals and of cobalt and nickel, too, some of which have been mentioned before in, Section 50. The cobalt complexes clearly show the importance of the completed d shells for the stability of the complex. Non complex compounds of trivalent cobalt are very unstable. Solutions of divalent cobalt in ammonia, however, are readily oxidized by air, because the NH3 complex of trivalent cobalt Co(NH3)6 3+ClT has eighteen electrons used in bond formation, whereas the ion Co(NH3) + would have nineteen electrons. 

The occurrence of stable neutral. binary carbonyls is restricted to the central area of the d block (Table 19.3), where there are low-lying vacant metal orbitals to accept o-donated lone-pairs and also filled d orbitals for jr back donation. Outside this area carbonyls are either very unstable (e.g, Cu, Ag, p. 1199), or anionic, or require additional ligands besides CO for stabilization. As with boranes and carboranes (p, 181), CO can be replaced by isoelectronic equivalents such as 2e , H , 2H or L. Mean bond dissociation energies Z>(M-CO)/kJ mol increase in the sequence Cr(CO)fi 109, MoCCO>6 151. W(CO)6 176, and in the sequence Mn2(CO)so lOO, Fe(CO)s 121, Co2(CO)g 138. NiCCO)4 147. 

In contrast with the stability of the alkyl-metalloporphyrins discussed above, pulse radiolytic studies on nickel and manganese porphyrins " indicated that reactions of alkyl radicals with these porphyrins yield very unstable species. Both Ni P and Ni P react very rapidly wi4 alkyl radicals to form Ni-C bonds. The only Ni-C bond that was found to be stable was that of CFjNi P. Other RNi P decayed with half lives of the order of seconds to yield Ni P. RNi P decayed even more rapidly, within milliseconds, also forming Ni P. It was suggested that the reaction between R- and Ni P is an equilibrium reaction forming RNi" P and that the decay of this species is through the dimerization of R- + R -. The reaction of alkyl radicals with Mn P is also rapid and probably occurs via addition to the metal, but the adduct immediately decomposes to yield Mn" P. These wide variations in the stability of the metal-carbon bonds in the various alkyl-metalloporphyrins have been rationalized in terms of the radius of the metal ion relative to the size of the porphyrin cavity and in terms of the number of d electrons in the metal center. "... 

With the exception of polynuclear boron hydrides, cluster chemistry has developed mainly over the past 40 years. Prior to this a number of compormds that would now be classified as clusters had been but their nature was not recognized. It is perhaps interesting to ask the question of why cluster compounds, that is molecules containing metal-metal bonds, were considered prior to 1940 to be a relatively uncommon feature in inorganic chemistry The problem was in part associated with the dominance of organic chemistry coupled with the difficulties in the rapid determination of the structure of compounds of this type. The determination of the X-ray structure of relatively simple compounds could take months or years to accomplish. As stated above, for the s and p block elements the compounds were also generally found to be unstable to air and moisture, whilst for the d and f block the compounds were often very complex in structure and the stoichiometry difficult to assess. This led to a general lack of appreciation of the occurence and stability of metal-metal or element-element bonds. 

FIGURE 1.7 The effect of turning on the ir interaction between a ir-acceptor ligand and the metal. The unoccupied, and relatively unstable ir orbitals of the ligand are shown on the right. Their effect is to stabilize the filled d, orbitals of the complex and so increase A. In W(CO)6, the lowest three orbitals are filled. 

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