Shapes of transition metal complexes

Although the ligand field theory can be used to rationalize the geometry of some transition metal molecules and complex ions, the study of the shapes of transition metal molecules in terms of the electron density distribution is still the subject of research and it has not reached a sufficient stage of development to enable us to discuss it in this book. 

The imprinting of polymer supports is an exciting development in the immobilization of transition metal complexes. The process involves the copolymerization of an inorganic or an organic template into a crosslinked polymer network. In a subsequent step, the template is chemically removed leaving an imprint of molecular dimensions in the resin. Ideally, the imprint retains chemical information related to the size and shape of the template. This approach has been used to prepare chiral imprints in otherwise achiral polymer networks. The method is outlined in Scheme... 

The angular overlap model, which has been of use in understanding the electronic structure and spectra of transition metal complexes is used to look at the factors which influence the shapes and relative bond strengths in main group systems AB ( = 2-7). Whilst the method is of some interest in itself, the main value of this paper is to show how several molecular orbital effects (Ugand-central atom p orbital bond energy, central atom s orbital involvement, and non-bonded interactions) contribute to determine the overall geometry. 

The d orbitals of these species resemble the orbitals of certain organic molecules or fragments, both in their shape and their electronic occupation. It is therefore important to be familiar with them if one wishes to establish a link between the electronic structures of transition metal complexes and of organic molecules (the tsolobal analogy, see Chapter 5). 

FIGURE 9.6 Common geometries and 3D shapes of transition metal (TM) complexes as a function of the number of bond pairs around the TM. This number is also called the coordination niunber (cn), which is shown within parentheses near each complex. 

Based on valence bond reasoning with nonorthogonal atomic-like orbitals, Kahn and Briat derived an elegant model that is capable of explaining and predicting magnetic behavior of transition metal complexes based on the shape of the localized magnetic... 

Valence bond (VB) theory can explain the shape and magnetic properties of transition metal complexes, but only at a simple level. It is an unrealistic model since it invokes the use of 4d orbitals in some complexes, such as [Fe(H20)6]. The 4d atomic orbitals are at a significantly higher energy than the 3d atomic orbitals. 

The d-d bands of transition metal complexes are weak, sometimes very weak. Some of the bands are sharp, some broad and some so broad that it is difficult to be certain that they exist. In the next section we shall consider the problem of intensities in this section, the problem of band shapes. The two are related. The very weak peaks are usually sharp. If they were not, they would escape detection and no doubt many broad, very weak, peaks do pass undetected. 

V S, C M Kelly and C R Landis 1991. SHAPES Empirical Force-Field - New Treatment of igular Potentials and Its Application to Square-Planar Transition-Metal Complexes. Journal of American Chemical Society 113 1-12. 

Similar effects have also been observed for solids and seem to become a very important source of information, because for a given ion the satellites are found to depend in intensity, position and shape on the type of ligand in transition metal complexes 24,13s,im, 174,175) Thus in many cases the study of shake-up peaks has provided additional evidence for a specific oxydation state 17> and will certainly increase our understanding of crystal and ligand field effects. 

The program and force field SHAPES, developed for transition metal complexes and tested for square planar geometries, uses a single Fourier term (Eq. 2.19), which is similar to the torsional angle term in many molecular mechanics programs (see Section 2.2.3 periodicity = m, phase shift = Fourier force constant kg is related to that of the harmonic potential, kg (Eq. 2.7) by Eq. 2.20. 

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