Square-planar complexes structural variation

Lippard characterized the structural effect of variation in the alkyl bridge length in tropocoro-nand complexes of zinc (80). Six complexes of ligands with bridge lengths from n = m = 3 to n = m = 6 were structurally characterized.702 The complexes had distorted tetrahedral-square planar geometries, with the exception of the m = n = 3 adduct which was five-coordinate with an additional pyridine ligand. 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

In Table II the reduction potentials of a variety of nickel(III) complexes are reported. The values have been corrected as well as is possible for standard conditions and it is revealing that the potentials, particularly of complexes which have a strong square-planar ligand, show marked variations depending on the donor ability of the solvent and coordinating anions. This has itself evolved into a structural probe (108, 117) but clearly it makes comparisons difficult and overinterpretation somewhat dangerous. 

Although square-planar configuration is customarily considered classical for v/c-dioximate of nickel(II), attempts have been made repeatedly over the years for preparing the above complexes in other configurations also. By employing weakly polar solvents and some other variations, success has been claimed in the preparation of mono(dioxime) complexes of nickel(II).42,43 The dichloro-bis(l,2-cyclohexanedione dioximato)nickel(II) has been shown to have an octahedral vie structure.44 Examples of tris(dioxime) complexes of transition metals in general45"18 and of bivalent atoms40,47 in particular are rare and structural details of only a tris(dioxime) complex of cobalt(III) are known.48 In a more recent publication,49 the crystal structure of tris(l,2-cyclohexanedione dioximo)nickel(II) sulfate dihydrate has been elucidated. 

Metal chelates are preser t in the complexes of Schiff bases of amino-(X = NR2), hydroxy- (X = O), and mercapto- (X = S) derivatives of monoheteroaromatic five-membered systems, azoles, and azines. A few publications on the complexes of azomethines of the monoheteroaromatic five-membered systems have appeared only recently. The X-ray structural study of copper(II) bis(2-N-n-octyliminomethyl)benzo[ 7]thio-phene-3-olate) 199 (04ZNK1696) is interesting in the sense that its square-planar structure is complemented by an extended octahedral one due to the intermolecular contacts of the thiophene sulfur with the copper site. Among the azole complexes, azomethine derivatives of p3Tazole (Equation (32)) prevail (05RCR193). There are several types of coordination units (N,N-, N,0-, N,S-, N,Se-) created by variation of the donor sites X in position 5 of the pyrazole ring. When X = O, S, Se, tautomer b is realized, whereas when X = NR, tautomer a predominates (Equation (32)). However, irrespective of the type of tautomer, in the chelates the coordination units have practically equalized bonds. 

The reaction of superoxotitanium(IV) with a number of substrates has been monitored by stopped-flow techniques/ In 1 M perchloric acid, the oxidation of iodide and bromide proceeded with second-order ratde constants of 1.1 x 10 M s and 2M s respectively. It is proposed that the reduction of superoxotitanium(IV) proceeds by a one-electron mechanism. Based on proton dependences, the species TiO " is more reactive than the protonated form Ti02(0H)2. The chromium chelate, bis(2-ethyl-2-hydroxybutyrato)oxochro-mate(V), is reduced by iodide, generating a Cr(IV) intermediate. The reaction is considered to proceed through formation of an iodine atom (T) for which both Cr(V) and Cr(IV) compete. In aqueous solution, [Co(EDTA)] forms a tight ion pair with I . Upon irradiation of this ion pair at 313 nm, reduction of [Co(EDTA)] to [Co(EDTA)] occurs with oxidation of 1 to IJ. The results may be interpreted on the basis of a mechanism in which [Co(EDTA)] and V are the primary photoproducts where the latter subsequently disproportionate to I3 and 1 . The kinetics and mechanism of the oxidation of 1 by a number of tetraaza macrocyclic complexes of Ni(III) have been reported. Variations in rate constants and reaction pathways are attributable to structural differences in the macrocyclic ligands. Of interest is the fact that with some of the Ni(III) complexes, spectrophotometric evidence has been obtained for an inner-sphere process with characterization of the transient [Ni(III) L(I)] intermediates. Iodide has also been used as a reductant for a nickel(III) complex of R-2-methyl-1,4,7-triazacylononane. In contrast to the square-planar macrocycles, the octahedral... 

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