Other coordination centres

Many systems are known where centres of coordination polymerizations are generated. Since it would require a volume the size of the present one to discuss all of these, only a few examples can be presented here in order to illustrate the possibilities in this held. 

Monomers such as methyloxirane produce a chain the end of which cannot control the addition of further monomer. When the generation of a stereoregular, crystalline polymer is required, the mode of methyloxirane addition to the chain must be regulated by a catalyst. Hydrolysis of mixed Meerwein alkoxides [226] yields the compound 

The empty coordination sites on A1 and Zn are reversibly filled by electron pairs from alkoxide groups with intra- or intermolecular associate formation. The degree of association depends on the solvent, the type of metal M +, and the OR group in [(R0)4Al202M +] in benzene, n varies from 1 to 8. We assume that the monomers are inserted, with simultaneous ring opening, into the 

Copolymerization of C02 with oxiranes proceeds with high yield in the presence of catalysts produced by reaction of Et2Zn with polyphenols [229]. The prerequisite of high activity is the presence of an internal coordination bond and the existence of an empty coordination site on the Zn atom in the 

Active centres of coordination polymerizations can be formed by unusual combinations of elements. We have observed slow polymerizations of VC, MMA, S and AN yielding products of high melting point on complexes such 

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This section would not be complete without a brief mention of the use of molecular mechanics calculations in coordination chemistry, as these have been applied mainly to amine complexes 46,47 While most calculations relate to complexes of Co111, these are now being extended to complexes with other metal centres, e.g. Ni11.48... 

Chlorites have been studied spectroscopically mainly on account of Fe2+- Fe3+ IVCT bands near 14,300 cm-1 that contribute to their optical spectra (e.g., White and Keester, 1966 Faye, 1968b Smith and Strens, 1976 Smith, 1977). Two other bands centred near 11,500 cm-1 and 9,500 cm-1 provide estimates for the crystal field parameters of Fe2+ ions in chlorite of A0 = 11,200 cm-1 and CFSE = 4,300 cm-1. Crystal spectra of Cr3+-bearing chlorite, kammererite, yield absorption bands at 18,450 cm-1 and 25,000 cm-1, giving A0 = 18,450 cm-1 and a CFSE of 22,140 cm-1 for octahedrally coordinated Cr3+ ions surrounded by OH- ions in the brucite sheets. The spectra of other Cr3+-bearing clay silicates have been described (Calas et al., 1984), including clinochlore and stichtite. 

The polyhedral symbol must be assigned before any other spatial features can be considered. It consists of one or more capital italic letters derived from common geometric terms which denote the idealized geometry of the ligands around the coordination centre, and an arabic numeral that is the coordination number of the central atom. 

The other vibrational coordinates of X-H - Y are those related to the other two intermonomer vibrations, those related to internal vibrations in X-H and Y, and those of the centre of gravity of the whole system, which separates from all other coordinates. When this complex is isolated, these coordinates of the centre of gravity do not appear in the potential energy. They can consequently be discarded as they are independent of the other ones. The coordinates of internal vibrations are driven by force constants due to covalent bonds within molecules X-H and Y. They are, as seen in the following, much greater than the force constants due to H-bonds that drive the intermonomer vibrations. These much faster intramonomer vibrations consequently hardly mix with intermonomer vibrations, even if cross terms between these two kinds of coordinate appear in the potential energy they are well out of resonance, that is each of them displays vibration frequencies that are different, and the effect of these possible cross terms remains small in aU cases. We are then left with two kinds of normal modes of the complex those that are mainly composed... 

Every second layer is placed exactly above and below each other. The centres of the atoms in the third layer are sketched with small blue dots. The third layer is placed exactly above the first layer. Every atom touches twelve other atoms (six in the same layer, three in the layer below and three in the layer above). Thus the coordination number is 12. 

PCA has been successfully applied to the mapping of valence angle deformations at metals and other atomic centres. For example, Murray-Rust [65] has studied the deformations from ideal symmetry in PO4 tetrahedra, whilst Auf der Heyde and Burgi [7, 8, 9] have used PCA to study the Berry [66] pseudorotational interconversion of trigonal bipyramidal and square planar five-coordinate metal centres. The use of symmetry-adapted deformation coordinates (see Chapter 2) is now well established for this kind of work, and the chemical interpretation of results is covered in detail in Chapter 5. In our final example [67], we examine deformations at three-coordinate copper centres to show how a PCA based on the L-Cu-L valence angles leads naturally to an interpretation in terms of the relevant symmetry-adapted angular deformation coordinates. This is an example of the analogy between normal coordinate analysis and PCA, as noted by Murray-Rust [65]. 

The chromium(IIl) compound contains a central tridentate oxide ligand (Fig. 2), an arrangement which is typical of other oxo-centred derivatives of chromium(III), such as the carboxylato and sulfato complexes. In the diethylcarbamato derivative, the central oxide ligand deviates by only 0.07 A from the plane of the three chromium atoms. It should be noted that the chromium derivative still retains a chloride ligand in its coordination as the starting material is anhydrous CrCl3, this corresponds to incomplete removal of chloride from the kinetically inert 3d chromium(III) cation. 

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