Trivalent cation transition metal complexes

Crystal field spectral measurements of transition metal ions doped in a variety of silicate glass compositions (e.g., Fox et al., 1982 Nelson et al., 1983 Nelson and White, 1986 Calas and Petiau, 1983 Keppler, 1992) have produced estimates of the crystal field splitting and stabilization energy parameters for several of the transition metal ions, examples of which are summarized in table 8.1. Comparisons with CFSE data for each transition metal ion in octahedral sites in periclase, MgO (divalent cations) and corundum, A1203 (trivalent cations) and hydrated complexes show that CFSE differences between crystal and glass (e.g., basaltic melt) structures,... 

The relative ability of the transition metal ions to form complex ions is Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+ for the divalent cations and Cr3+ — Mn3+ > Fe3+ < Co3+ for the trivalent cations. The strongest complexing divalent cation is Cu(II). Fe(III) is the weakest complexing trivalent transition metal ion, but is stronger than other trivalent cations such as Al3+ and the lanthanides. The heats of hydration (Table 3.2), strengths of EDTA complexes (Table 3.5), and solubility products of metal hydroxyoxides (Table 3.3) also follow this general order, with water, EDTA, and OH" as the respective ligands. Stability constants less than I09 indicate the weaker ion-ion interaction of ion pairs. 

The guest cations hitherto examined cover broadly uni- to trivalent and inorganic to organic ions that include alkali, alkaline earth, heavy and transition metal ions, as well as (ar)alkyl ammonium and diazonium ions. As to the complex stoichiometry between cation and ligand, both 1 1 stoichiometric and 1 2 sandwich complexes are analyzed. The solvent systems employed also vary widely from protic and aprotic homogeneous phase to binary-phase solvent extraction. 

A large number of bivalent and trivalent cations were utilized as catalysts in this reaction. The effectiveness of the cation as a catalyst strictly paralleled its power of forming a complex with the malonate ion, a model for the transition state of the reaction. This parallelism strikingly shows that the ability of a metal ion to catalyze this process depends on its ability to complex with the transition state. 

Most zeolites have an intrinsic ability to exchange cations [1], This exchange ability is a result of isomorphous substitution of a cation of trivalent (mostly Al) or lower charges for Si as a tetravalent framework cation. As a consequence of this substitution, a net negative charge develops on the framework of the zeolite, which is to be neutralized by cations present within the channels or cages that constitute the microporous part of the crystalline zeolite. These cations may be any of the metals, metal complexes or alkylammonium cations. If these cations are transition metals with redox properties they can act as active sites for oxidation reactions. 

The cadmium(II) complex corresponding to 9 (M = Cd n = 2) was the first texaphyrin made [6], This aromatic expanded porphyrin was found to differ substantially from various porphyrin complexes and it was noted that its spectral and photophysical properties were such that it might prove useful as a PDT agent. However, it was also appreciated that the poor aqueous solubility and inherent toxicity of this particular metal complex would likely preclude its use in vivo [29-31], Nonetheless, the coordination chemistry of texaphyrins such as 9 was soon generalized to allow for the coordination of late first row transition metal (Mn(II), Co(II), Ni(II), Zn (II), Fe(III)) and trivalent lanthanide cations [26], This, in turn, opened up several possibilities for rational drag development. For instance, the Mn(II) texaphyrin complex was found to act as a peroxynitrite decomposition catalyst [32] and is being studied currently for possible use in treating amyotrophic lateral sclerosis. This work, which is outside the scope of this review, has recently been summarized by Crow [33],... 

Apart from some Weakley-type complexes (Table 22), there has been little systematic investigation of lanthanide-molybdate complexes. Multinuclear NMR studies of solutions of the well-established dodecamolybdates [MIVMo12042]8- (M = Ce, U), with a range of di- and trivalent transition-metal and lanthanide cations, show the formation of complexes incorporating two metal cations. In most cases these complexes are labile on the NMR timescale.368-370... 

A CZE method was used to separate EDTA complexes of selected trivalent and divalent transition-metal ions. By adding a surfactant to the separation buffer, an improvement in peak shape and short migration times was obtained (341 and Fig. 8). The ions characterized by CIA consist of 147 ionic species, including inorganic anions, inorganic cations, and organic anions (329). 

Lewis acids act as electron pair acceptors. The proton is an important special case, but many other compounds catalyze organic reactions by acting as electron pair acceptors. The most important Lewis acids in organic reactions are metal cations and covalent compounds of metals. Metal cations that function as Lewis acids include the alkali metal monocations Li+, Na+, K+, di- and trivalent ions such as Mg +, Ca, Zn +, Sc, and Bi + transition metal cations and complexes and lanthanide cations, such as Ce + and Yb. Neutral electrophilic covalent molecules can also act as Lewis acids. The most commonly employed of the covalent compounds include boron trifluoride, aluminum trichloride, titanium tetrachloride, and tin(IV)tetrachloride. Various other derivatives of boron, aluminum, titanium, and tin also are Lewis acid catalysts. 

Complicating the development of ISEs for higher actinide ions is their inherent radioactivity. They also have chemistry tiiat often differs from that of the uranyl cation. Actinides from americium to lawrencium display solution-phase chemical features that resemble those of the trivalent lanthanides. Conversely, in certain oxidation states, the early actinides (thorium through neptunium) often mimic transition metals. Also, as mentioned above, many of the actinides can exist in a large number of oxidation states. For instance, in the case of plutonium, four oxidation states can exist simultaneously in aqueous solution. Finally, as true for the lanthanides, complex salts with hydroxide, halogens, perchlorates, sulfates, carbonates, and phosphates are well known for most of the actinides. 

Group 3 contains divalent transition metal ions with a d" electronic configuration, 1 q 9, divalent d ° cations, and large d trivalent metal ions. Examples include NP, Cu, Zn, Cd, and Tl. They prefer complexes having a MNg coordination sphere. 

Group 4 is made up of small trivalent d metal ions and some of the trivalent transition metal ions with d electronic configurations, 1 q 9. Examples are Cr, Fe, and Ga. These cations share properties with cations from both Groups 2 and 3 and prefer complexes with a MN3O3 coordination sphere. 

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