Isomerization octahedral forms

Black and McLean pointed out that a hexadentate ligand can yield either of two isomeric octahedral complexes of meso or racemic stereochemistry (Fig. 9) [52, 53]. These differ primarily in that the former consists entirely of facially coordinating tridentate loops the latter comprises two meridional loops (i.e., those in which three adjacent donor atoms, as well as the metal ion, lie in a plane). In all encapsulated complexes reported to date, 18S6 coordinates in the centrosymmetric meso form. This behavior contrasts with the racemic coordination found for corresponding hexaaza macro-cycles [144-147]. 

Violet, easily hydrolysed, PdFp is produced when Pd [Pd F(sl s refluxed with SCF4 and is notable as one of the very few paramagnetic compounds of Pd. The paramagnetism arises from the configuration of Pd which is consequent on its octahedral coordination in the rutile-type structure (p. 961). The dichlorides of both Pd and Pt are obtained from the elements and exist in two isomeric forms which form i.s produced depends on the exact experimenial conditions used. The more usual a form of PdCb is a red material with... 

This compound has an octahedral coordination sphere slightly distorted towards a trigonal prism. Oxidation by air leads to Ru2(S2CNEt2)5 which exists in two isomeric forms (Figure 1.52) [138]. 

These results are consistent with active sites consisting of highly distorted octahedral WOx clusters on Zr02. Acid sites formed by these octahedral WO surface species are more effective isomerization sites than previously reported tetrahedral WOx species on AI2O3 [17], possibly because of the ability of WOx clusters to form metastable proton-containing complexes during catalytic isomerization reactions. 

The structure of [IrCl2(edmp)2]PF6, (193), edmp = (2-aminoethyl)dimethyl phosphine, confirms that the coordination geometry around the Ir center is approximately octahedral. If the edmp ligand is replaced by edpp, edpp = (2-aminoethyl)diphenylphosphine, then the isomeric form (194) may be isolated and structurally characterized. The X-ray crystal structure of fac-[Ir(edmp)3]Cl3 5H20 has also been reported.338... 

The interaction of thiosemicarbazide with Ni11 has been studied in solution and in the solid state, and the properties and stereochemistry of two isomeric forms, a and (3, of [Ni(432)3]I2 have been reported. The crystal structure of the /3-form reveals that Ni is octahedrally coordinated by three molecules of (432) through their S and hydrazine N atoms.1220... 

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

Fig. 4. The Ni 2p regions of the X-ray photoelectron spectra of the isomeric forms of the isomeric forms of bis(N-methylsalicylaldi-mine)nickel (II). A, square planar isomer B, octahedral polymer. Reproduced with permission from Ref.3s)...

This is by far the most studied of the isomerisms, with those of square planar and octahedral complexes receiving the major attention. As we shall see, developments in the preparation and separation of isomeric forms and the clever use of nmr techniques have played a major part in the increased understanding of the field. 

Although outside the main thrust of the book, attention should be drawn to the important use that octahedral complexes are receiving as molecular probes. Metal complexes can sometimes recognize specific sites on biological material. The array of isomeric forms possible with some metal complexes (previous section) adds a dimension to their use as probes. 

It is found on analysis of the problem that when only two d orbitals are available for combination with the s and p orbitals six equivalent bond orbital- of strength 2.923 (nearly as great as the maximum 3 for the best spd hybrid) can be formed, and that these six orbitals have their bond directions toward the corners of a regular octahedron. We accordingly conclude that complexes such as [CoCNHs ], [PdClc]—, and [PtCle] — should be octahedral in configuration. This conclusion is of course identical with the postulate made by Werner to account for isomerism in complexes with different substituent groups,1 and verified also by the x-ray examination of Co(NH8)el3, (NHOaPdCU, (NH4)2 PtCl , and other crystals (see Fig. 5-1). 

There is only one form of a monosubstituted octahedral complex MAiB. A disubstituted complex MA4B2 can exist in two isomeric forms, cis and trans ... 

This is by for the most common coordination number. With certain ions si -coordinate complexes are predominant. For example. chromhim(lll) and cobah(MI) are almost exclusively octahedral in their complexes. It was this large series of octahedral Cr(III) and Co(III) complexes which led Werner to formulate his theories of coordination chemistry and which, with square planar plalinum(II) complexes, formed the basis for almost all of the classic work on complex compounds. Before discussing the various isomeric possibilities for octahedral complexes, it is convenient to dispose of the few nonoctahedral geometries. 

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