Transition metal ions reaction variables

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

These compounds contain a developed system of conjugated double bonds imparting distinct semiconductor properties on them. Metal ions of variable valency can serve as the central ion M cobalt, nickel, iron, manganese, copper, and so on. In such systems, electron transitions can occur in the conjugated system of the ligands and in the electronic system of the central metal ion. These transitions are the basis for their catalytic activity toward various reactions. 

Zinc has a highly concentrated charge in comparison to its relatively small ionic radius (0.65 A) and binds modestly to anions such as carboxylates and phosphates. Its second characteristic is its high affinity for electrons, making it a strong Lewis acid, similar to copper and nickel. However, unlike the other two transition metal ions, it does not show variable valence, which might lead to it being preferred quite simply because it does not introduce the risk of free radical reactions. 

Copper(II) and zinc(II) are two of the more labile divalent metal ions and as a consequence the former is too labile for its water exchange rate to be determined by the NMR methods which utilize the paramagnetism of other divalent first-row transition metal ions, while the latter is diamagnetic and such NMR methods cannot be applied. However, it has been shown that water exchange rates and mechanisms can be deduced with reasonable reliability from simple ligand substitution studies, and this is one of the reasons for a recent variable-pressure spec-trophotometric SF study of the substitution of 2-chloro-l,10-phenanthroline on Cu(II) and Zn(II). The observed rate constants for the complexation reaction (kc) and the decomplexation reaction (k ) and their associated activation parameters for Cu(II) and Zn(II) are kc(298 K) = 1.1 x 10 and 1.1 x 10 dm mol" s", AH = 33.6 and 37.9 kJ mol", A5 = 3 and -2JK- mol", AV = 7.1 and 5.0 cm" mol", k 29S K) = 102 and 887 s", AH = 60.6 and 57.3 kJ mol", A5 = -3 and 4 J K" mol" and A V = 5.2 and 4.1 cm" mol". These data are consistent with the operation of an mechanism for the rate-determining first bond formation by 2-chloro-l,10-phenanthroline with the subsequent chelation step being faster [Eq. (18)]. For this mechanistic sequence (in which [M(H20)6 L-L] is an outer-sphere complex) it may be shown that the relationships in Eq. (19) apply. 

Metal ions of transition and other elements of variable valency, e.g. Ce, Co, Fe, V, Mn, etc., are known to oxidize polysaccharides rather selectively, producing macroradicals as intermediates which are capable of adding vinyl monomers and form graft copolymers. These initiators are redox systems which differ from those previously described by not producing free radicals of low molecular weight. Only macroradicals on the substrate are formed in the redox reaction. Some homopolymer may still be formed in the process, e.g. due to oxidation of monomer or other side reactions. ... 

When a d-metal atom loses electrons to form a cation, it first loses its outer s-electrons. However, most transition metals form ions with different oxidation states, because the d-electrons have similar energies and a variable number can also be lost when they form compounds. Iron, for instance, forms Fe2+ and Fe3+ copper forms Cuf and Cu2+. The reason for the difference between copper and potassium, which forms only K+, can be seen by comparing their second ionization energies, which are 1958 kj-mol 1 and 3051 kj-mol-1, respectively. To form Cu2+, an electron is removed from the d subshell of [Ar]3d10 but to form K2+, the electron would have to be removed from potassium s argonlike core. Because such huge amounts of energy are not readily available in chemical reactions, a potassium atom can lose only its 4s-electron. 

One-electron oxidation or reduction of saturated molecules frequently results in the generation of free radicals . The catalysis of certain free-radical reactions by ions or complexes of transition metals, such as (Tu, Co, and Mn, which exhibit variable oxidation states, is a consequence of this. Among such reactions are the autoxidations of hydrocarbons and other organic molecules (initially to hydroperoxides), which proceed by free-radical chain mechanisms in which the important propogation steps are ... 

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