Transition metal compounds relative stability

The thermodynamic stability of coordination compounds is relatively easy to determine, and provides us with a valuable pool of data from which we may assess the importance of ligand-field and other effects upon the overall properties of transition-metal compounds. The bulk of this chapter will be concerned with the thermodynamic stability of transition-metal compounds, but we will briefly consider kinetic factors at the close. 

The discovery of hexamethyltungsten by Shortland and Wilkinson (259, 260) has prompted an interest in the synthesis and physical properties of alkylated transition metal compounds. The relatively high thermal stability of these derivatives is plausibly ascribed to the lack of a low-energy decomposition pathway. This is particularly true in the case of ligands such as Me3SiCH2 — and Me3 CCH2 for which /3-hydrogen transfer is precluded (43, 282). 

This article is confined to organo-transition-metal compounds having chiral metal atoms whose optical activity has been demonstrated. Only those compounds are discussed in detail for which there is a choice with respect to the metal configuration and for which a separation or at least an enrichment of isomers with opposite metal configuration has been achieved. After the treatment of such topics as optical resolution, optical purity, optical stability, optical induction, stereochemistry of reactions, relative and absolute configurations, Table I (Section XVII) collects the information available for the compounds under consideration. 

The distribution of electrons over d orbitals in complexes of different geometry controls the most stable geometric configuration. This principle can also be used to understand the chemical bonding of transition metal compounds and their surfaces. We will illustrate this first by analyzing the relative stability of octahedral NiO versus trigonal prismatic M0S2. 

Since the number of electrons required to achieve Nirvana is so large for transition metal species, the stability differences of other electron counts are not too forbidding. Consequently, the 18e rule is softer than the octet rule, and we may expect to find relatively persistent radical complexes with 17 electrons, and complexes with 16e or even less. These cases are in fact extremely interesting because the electron-deficient complexes can serve as catalysts that activate other molecules. Despite all these qualifications, the Law of Nirvana for transition metal compounds is a very useful guide for constructing transition metal complexes and for considering their reactivity (propensity to react) and properties. Let us see how the rule is applied along with the click bond method. 

Comment on Relative stability of the AI12W structure in Al-transition-metal compounds . Phys. Rev. B45 (1992) 7509-7510. 

Ions with a half-filled 3d sub-level (3d ) or a filled 3d sub-level (3d ) are usually relatively stable, but a number of factors are involved in determining the stability of transition metal compounds in the solid state (including lattice enthalpies). 

The second part of the present volume complements and extends the collection of electron paramagnetic resonance (EPR) data for coordination and organometallic transition metal compounds which appeared in the volumes II/2, II/8 and II/IO of the Landolt-Bornstein, New Series [26,27,28]. The literature covered here adds directly onto that included in the previous volume [28] and extends until the end of 1972. It is interesting to realize that the amount of EPR data compiled in this volume is almost equal to, and in fact by 2 % higher than that in volume II/IO which covers the years 1969 and 1970. On the other hand, it had been noted that the number of data in the volume II/IO had increased by 50 % over that in volume II/8 (covering the years 1964 to 1968). This result indicates that the amount of published data per year has now stabilized on a relatively high level. 

For the transition metals it is often impossible to reach a noble gas structure except in covalent compounds (see effective atomic number rule) and it is found that relative stability is given by having the sub-shells (d or f) filled, half-filled or empty. 

In the introductory chapter we stated that the formation of chemical compounds with the metal ion in a variety of formal oxidation states is a characteristic of transition metals. We also saw in Chapter 8 how we may quantify the thermodynamic stability of a coordination compound in terms of the stability constant K. It is convenient to be able to assess the relative ease by which a metal is transformed from one oxidation state to another, and you will recall that the standard electrode potential, E , is a convenient measure of this. Remember that the standard free energy change for a reaction, AG , is related both to the equilibrium constant (Eq. 9.1)... 

Transition metal alkyls are often relatively unstable earlier views had attributed this either to an inherently weak M—C bond and/or to the ready homolysis of this bond to produce free radicals. Furthermore, the presence of stabilizing ir-acceptor ligands such as Cp , CO, or RjP was regarded as almost obligatory. However, (1) the M—C bond is not particularly weak compared say to the M—N bond, and (2) the presence of the new type of ligand on the metal could make the complex kinetically stable thus, even isoleptic complexes, i.e., compounds of the form MR , might be accessible 78, 239). These predictions have largely been borne out (see Table VII). 

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