Complexes of other geometries

As the ligand-field splitting of the d orbitals in lower symmetry complexes lead to lower degeneracies and so a need for more parameters to describe them (see Table 6.6), so the d-d transitions of complexes of other than octahedral and tetrahedral geometries can only be discussed by introducing these parameters. For square planar complexes, for instance, two additional parameters must be introduced. 

18 The spectrum of [Co(NH3)4(ox)] (second down, indicated by vertical bars) to a first rather good approximation is a weighted interpolation of those of [Co(NH3)e] and [Co(ox)3] . This is an example of an application of the rule of average environment. 

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Square Planar MO diagrams for complexes of other geometries. The metal atom or ion in each case... 

Indirect substitution of the type indicated in (4.1) and (4.2) appears to be the method much preferred by octahedral complexes, while direct substitution is more relevant with square-planar complexes. This situation could perhaps be predicted in view of the more crowded conditions with octahedral than with planar complexes. For other geometries both routes are used. 

Special attention should be paid to spherical geometry, since the mathematical treatment of spherical microelectrodes is the simplest and exemplifies very well the attainment of the steady state observed at microelectrodes of more complex shapes. Indeed, spherical or hemispherical microelectrodes, although difficult to manufacture, are the paragon of mathematical model for diffusion at microelectrodes, to the point that the behavior of other geometries is always compared against them. 

The effects of the mixed supersonic expansion of CDMA with various solvent molecules (such as cyclohexane, carbon tetrachloride, acetone, acetonitrile, methanol, dichloromethane and chloroform) on the emission spectra have been investigated by Phillips and co-workers [82d[. The cluster size distribution was varied by changing the nozzle temperature and the partial pressure of the solvent. Two emission components were observed in each case. The long-wave emission was attributed to dimers (which can be isolated or solvated) and to monomer complexed with chloroform or dichloromethane (of unknown stoichiometry). On the other hand, it has been reported by Bernstein and co-workers [84] that CDMA forms with acetonitrile two kinds of 1 1 complexes of different geometry. The first cluster has a structured excitation spectrum, similar to that of the bare molecule, but blue shifted by about 252 cm . The second exhibits a broad excitation spectrum with some resolvable features between 31400 and 31 600 cm (Table 2). The complexes show different fluorescence spectra excitation into the broad absorption leads to the red-shifted emission with respect to that of the monomer (Figure 8) and of the blue ... 

Ruthenium(III) and osmium(III) complexes are all octahedral and low-spin with 1 unpaired electron. Iron(III) complexes, on the other hand, may be high or low spin, and even though an octahedral stereochemistry is the most common, a number of other geometries are also found. In other respects, however there is a gradation down the triad, with Ru occupying an intermediate position between Fe and Os . For iron the oxidation state +3 is one of its two most common and for it there is an extensive, simple, cationic chemistry (though the aquo... 

Using the same principles developed for octahedral complexes, we will now consider complexes with other geometries. For example. Fig. 21.26 shows a tetrahedral arrangement of point charges in relation to the 3<7 orbitals of a metal ion. There are two important facts to note ... 

For complex systems that are difficult or impossible to crystallize (e.g., biological or environmental samples), other diffraction techniques such as EXAFS spectroscopy can often provide details about the Pb coordination environment. The information about a metal complex that is available from EXAFS spectroscopy includes the coordination number of the metal ion and bond distances. Bond angles and geometry are difficult to determine directly but can be inferred from careful comparisons to model complexes of known geometry (248, 249). The identity of coordinated atoms can also be determined from the EXAFS spectrum, although it is often difficult to differentiate between atoms of similar atomic number (e.g., N vs O) (250). Only a few Pb(II) systems have been examined by EXAFS, but the following studies provide excellent examples of the kinds of questions EXAFS is uniquely suited to answer (157, 158, 162, 251-259). 

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