Applications to Coordination Compounds

We have studied some coordination compounds involving main-group Lewis acids (51a). In collaboration with Steve Hartman (51 b) we 

Chan and Cook (53) studied the factors affecting the mass spectral sensitivity for ions sampled by field evaporation from a liquid matrix. These results, including those on metal complexes, show some of the types of interactions expected for ions in solutions, and they obtain results consistent with FAB-MS. 

They also proposed the method as a means of measuring stability constants for the complexes. As well, they analyzed the thermodynamics of the desorption process and were able to calculate the localized temperature which must be created in a small volume around the 

Barber et al. (61) have used FAB to study the antibiotics bleomycin A2 and B2 and their metal complexes. In particular, the ferrous sulfate complex of bleomycin A2 shows a pseudomolecular ion at mlz 1566, corresponding to the salt (A2 - H+ + FeS04) since the amide hydrogen of the histidine residue is lost on complexation. An ion at mlz 472 corresponds to the ferrous ion complexed to the surrounding ligands but lacking the disaccharide groups and the peptide chain. 

Another biomedical area of interest in metal complexes concerns various anticancer drugs. Puzo et al. (117) and Theodoropoulos (63) have used FAB to study cisplatin [i.e., cis-Pt(NH3)2Cl2] and its complexes and related compounds. For the 1 2 complex with guanosine, peaks due to the complex less a Cl, or less Cl and HC1, along with their corresponding glycerol adducts were observed. Less clear results were observed for other amino acid complexes (63). Cohen et al. (62) and Costello et al. (64) have used FAB to study the complexes of techni-tium(III) and technitium(I) and -(V), respectively, these compounds 

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Applications to Coordination Compounds of Heavy-Metal Atoms... 

Fast-atom bombardment spectroscopy, 28 1-22 applications to coordination compounds, 28 13-17... 

MMGK (Molecular mechanics with Gillespie-Kepert terms) [193] is designed for application to coordination compounds. It is based on CHARMM, but an additional term describing repulsion of some effective interaction centers placed on the coordination bonds is added. 

Application of coordination compounds in medicine, materials chemistry, and as catalysts are mentioned and are cross-referenced to a fuller discussion in Volume 9. Comment is made on application of complexes in nanotechnology, and on the molecular modeling of complexes. The material cannot be totally comprehensive because of space limitations, but is selected in such a way to give the most effective review of discoveries and new interpretations. 

Some of the important types of coordination compounds occur in biological systems (for example, heme and chlorophyll). There are also significant applications of coordination compounds that involve their use as catalysts. The formation of coordination compounds provides the basis for several techniques in analytical chemistry. Because of the relevance of this area, an understanding of the basic theories and principles of coordination chemistry is essential for work in many related fields of chemistry. In the next few chapters, an introduction will be given to the basic principles of the chemistry of coordination compounds. 

The applications of coordination compounds in catalysis that have been shown are by no means the only important cases. In fact, there are numerous reactions in which homogeneous catalysis forms the basis for a great deal of chemistry. From the examples shown, it should be apparent that this is a vast and rapidly developing field. It is also one that is important from an economic standpoint. Although the basic principles have been described in this chapter, the literature related to catalysis is extensive. For further details and more comprehensive reviews of the literature, consult the references listed. 

The coverage has been limited to the applications of coordination compounds or of the coordination chemistry of relatively soluble ligands and metal species. Numerous chemical agents in photographic systems function by means of adsorption on silver halide grains and silver metal surfaces. Such chemical interactions lie outside the confines of coordination chemistry defined for this work and have not been discussed. 

The pioneering applications of molecular mechanics to coordination compounds were conformational analyses127,281. Recent applications involving the computation of conformer equilibria discussed in this chapter are studies of solution structure refinements126,29 1, racemate separations131 3il and the evaluation of reaction pathways11 1,34,3S1. The importance of conformer equilibria in the areas of electron transfer rates and redox potentials is discussed in Chapter 10, and many examples discussed in the other chapters of Part II indicate how important the prediction of conformational equilibria is in various areas of coordination chemistry. 

The exponential in Eq. 2.14 represents the average over the system described by the hamiltonian Hx, and the corresponding series of conformers and configurational isomers is usually created by molecular dynamics or Monte Carlo methods. When the two systems X and Y are very similar, the exponential term vanishes, leading to a very slow convergence of the average in Eq. 2.14. A number of techniques have been described to overcome this problem 43 441. One of the few applications of this method to coordination compounds is the investigation of O2 and CO affinities to iron porphyrins[45]. 

Each donor atom must be assigned a priority number based on the rules developed by Cahn, Ingold and Prelog (the CIP rules).13 These priority numbers are then used to form the configuration index for the compound. The application of the CIP rules to coordination compounds is discussed in detail in Section IR-9.3.5 but, in general, donor atoms that have a higher atomic number have higher priority than those that have a lower atomic number. 

The chemistries of scandium and yttrium are often reported together (although yttrium is also often grouped with the lanthanides) so in this section they will be discussed together. Reference, where appropriate, will be made to the first volume in this series, Comprehensive Coordination Chemistry The Synthesis, Reactions, Properties and Applications of Coordination Compounds (CCC, 1987). Volume 3 of CCC (1987) contained a chapter entitled Scandium, Yttrium and the Lanthanides and this chapter is designed to follow part of that chapter. The articles reviewed will cover the period 1982-2001 although earlier work may be cited. Volume 2 of CCC (1987), devoted to ligands, will also be referred to in this chapter. 

The radius ratio rule is only applicable to ionic compounds. In silicate minerals, however, it is the bonds between oxygen and silicon and between oxygen and aluminium (Al) which are structurally important. These bonds are almost equally ionic and covalent in character and the radius ratio rule predicts the coordination of these ions adequately. 

The last example to be mentioned deals with the application of coordination compounds attached to polymers and their use as immobilized catalysts. This technique has been used for a long time in organometallic catalysis. Similar reactions with biomimetic catalysts, as with Cu(II) oxidases, are less well known, and a review for polymeric copper imidazole complexes used in oxidative phenol coupling is available. 

Crystal-field theory can be used to explain many observations in addition to those we have discussed. The theory is based on electrostatic interactions between ions and atoms, which essentially means ionic bonds. Many hnes of evidence show, however, that the bonding in complexes must have some covalent character. Therefore, molecular-orbital theory c s > (Sections 9.7 and 9.8) can also be used to describe the bonding in complexes, although the application of molecular-orbital theory to coordination compounds is beyond the scope of our discussion. Crystal-field theory, although not entirely accurate in all details, provides an adequate and useful first description of the electronic structure of complexes. 

Another method, which allows the structural characterization and elucidation of the reactivity of transient species using infrared spectroscopy, is to observe them in real time, using fast time-resolved infrared (TRIR) spectroscopy. In this section we shall focus on the application of fast (submillisecond) and ultrafast (subnanosecond) TRIR spectroscopy to coordination compounds, and describe experiments that cannot be performed using conventional infrared spectrometers. 

In one of the first important applications of transient RR spectroscopy to coordination compounds, Ballinger, Woodruff, and co-workers, used 10 ns, 355 nm pulses from a Nd YAG laser to study the MLCT state of Ru(bpy)3 +, which has a lifetime (at room temperature in aqueous solution) of 600ns. The ground state of the complex exhibits relatively weak absorption near 350 mn. However, the high photon fluxes available are sufficient to saturate the irradiated volume in excited state within a few picoseconds, so that scattering occurs from the MLCT excited state. Fortuitously, in this case and many others, the MLCT state possesses a ligand centered tt-tt absorption band near 350 nm, so that the 355 nm laser wavelength is in resonance with the MLCT excited-state species. 

In summary, TDDFT enables reliable theoretical predictions of optical spectra, and many other properties, of large coordination compounds. In view of algorithmic and hardware improvements, and ongoing research to improve the xc functionals, the future prospects for TDDFT and its application to coordination chemistry seem bright. 

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