Coordination compounds characteristic features

It is apparent, from the above short survey, that kinetic studies have been restricted to the decomposition of a relatively few coordination compounds and some are largely qualitative or semi-quantitative in character. Estimations of thermal stabilities, or sometimes the relative stabilities within sequences of related salts, are often made for consideration within a wider context of the structures and/or properties of coordination compounds. However, it cannot be expected that the uncritical acceptance of such parameters as the decomposition temperature, the activation energy, and/or the reaction enthalpy will necessarily give information of fundamental significance. There is always uncertainty in the reliability of kinetic information obtained from non-isothermal measurements. Concepts derived from studies of homogeneous reactions of coordination compounds have often been transferred, sometimes without examination of possible implications, to the interpretation of heterogeneous behaviour. Important characteristic features of heterogeneous rate processes, such as the influence of defects and other types of imperfection, have not been accorded sufficient attention. 

Also mercury(II) selenite is polymorphic and two new modifications, p-HgSe03 and y-HgSe03, have been recently described.134 They show the mercury atoms in sevenfold coordination of oxygen atoms, and the [I Ig07] polyhedra are linked by the pyramidal selenite ions. A remarkable and unusual mercury selenite is the mixed valent compound (HgSe03)3E[gSe.134 As a characteristic feature, the crystal structure contains [Hg3Se] pyramids with distances Hg Se of 2.489 A. 

Molecular diatomic sulfur forms part of complex compounds for which the localization modes of a coordination bond are a characteristic feature 50-52 (E = S) [11,13,71,78,81], and in fact the same as for those formed by oxygen. 

On the basis of 535 have been synthesized, and studied in detail, coordination compounds with practically all metals [931-933]. Two groups of complex compounds with O-coordinated (535) and S-coordinated (536) ligands were isolated. A detailed summary of complexes containing sulfoxides in different coordination modes, and the results of their x-ray diffraction studies including the evaluation of structural parameters for M — O and M — S bonds, have been reviewed [933]. It is emphasized [112] that the O-coordination 535 is formed with hard and intermediate metals, while the S-coordination 536 is a characteristic feature for soft Pearson acids (see Sec. 1.2.2). For complexes obtained on the basis of organoseleno-oxides 535 (E = Se) and organotelluro-oxides 535 (E = Te), the hard-hard or soft-soft interactions could also be expected [112]. However, direct proofs of such coordination modes for complexes of type 534 (E = Se, Te), described elsewhere [112], are absent in the available literature. 

CHARACTERISTIC FEATURES OF THE ELECTRONIC STRUCTURE OF COORDINATION COMPOUNDS... 

Reactions of coordination compounds share some characteristics with reactions of other molecules, both organic and inorganic, so an understanding of coordination compound reactions can draw on some familiar concepts. However, the chemistry of coordination compounds has some additional features because the molecules have more complex geometries and more possibilities for rearrangement, the metal atoms exhibit more variability in their reactions, and different factors influence the course of reactions. 

In compound 76 and in dimesitylmanganese, which crystallizes as the trimer [Mn(mesityl)2]3 (77) " the degree of association is limited by the bulk of the substituents. All of these systems show the characteristic features of 3c-2c Mn-C-Mn bridge bonding greater Mn-C interatomic distances to the bridging (hypercoordinated) carbon atoms than to their terminal counterparts sensitivity of the metal-carbon distance to the metal coordination number and acute Mn-C-Mn bond angles at the hypercoordinated carbon atoms. 

This chapter is concerned with a quantum chemical description of sulphonic acids and their derivatives. The characteristic feature of this class of compounds is the four coordination of the central sulphur atom, including the sulphonyl fragment > S02, where one of the other coordinating ligands is usually an acidic (—OH) or acid derivative group. We have taken the definition of acid derivatives here in a broad sense. Therefore, some of the material in this chapter is relevant to, and refers to, the previous volume in this series on sulphones1. 

---