Transition metal ions dissociation

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

There are three types of electron transfers, firstly the generation of an electron electrochemically, by y-irradiation, or by photolytic dissociation, secondly the transfer of an electron from an inorganic or organic compound, referred to as a nucleophilic homolytic leaving group (Zollinger, 1973 a), and thirdly a transfer from a transition metal or transition metal ion complex. In this section we will discuss the fundamental aspects of these three types. In the following sections and in Chapter 10, specific examples and synthetic applications will be summarized. 

Atomic metal ion-hydrocarbon reactions bond dissociation energies for fragments, 15,16t endothermic reactions, 13,15,17f Atomic transition metal ion reactions development of approach for real-time measurements of dissociation kinetics, 39 ion beam apparatus, 12,14f studies of... 

These and many similar examples resulted in a highly successful general picture of transition-metal ions M coordinated by closed-shell ligands L (anionic or neutral) to form complex cluster ions [ML ]9 in solution. The characteristic coordination shell of each M corresponds to a specific number of sites, with idealized geometry that dictates the possible number of distinct [M(Li) (L2)m. .. ]q structural isomers. Each cluster ion is subject to equilibria with other cluster ions or dissociated ligands in solution,... 

By single electron transfer from an electron donor, e.g. a transition metal ion, a trivalent phosphorous derivative or a base, followed by dissociation of the intermediate diazenyl radical in an aryl radical and dinitrogen. The aryl radical reacts with the solvent or with added reagents in various ways, as shown by the relatively large number of classical named reactions (Sandmeyer, Pschorr, Gomberg-Bachmann, Meerwein reactions). 

A better insight into the mechanistics of water exchange on the 2nd and 3rd row d-transition metal ions came from quantum chemical calculations 87). For the rhodium(III) ion two transition states have been obtained, one for the dissociative and one for the interchange mechanisms, whereas for the... 

The relationship (Eq. (5)) has been obtained on the basis of ideal h parameters calculated from Eq. (4). In order to check that the relationship is a real one in terms of experimentally determined spectral data, a similar plot to Fig. 2 was made but using h values obtained, through Eq. (3), from experimental Sq - frequencies for Pb . (Excluded from the plot is the datum referring to 100% phosphoric acid because it has previously been apparent that whereas transition metal ions undergo solvolytic dissociation (see above), the Sq Pj frequency corresponds to undissociated H3PO4 (5).) Comparison of the experimental plot (Fig. 3) with Fig. 2 confirms that the relationship of Eq. (5) has an experimental as well as a theoretical basis. 

The trans cis isomerization of [Cr(C204)(H20)2] is also catalyzed by metal ions and a detailed study of catalysis by Mg11 has been published.540 Dissociation of oxalato complexes of Crin of the type [Cr(ox)n(OH2)6 2n](3 2n)+ (n = l, 3) is also promoted by a series of transition metal ions.541... 

These unique eight-coordinate complexes of the first-row transition metal ions maintain the four-membered chelate ring even in solution, where they dissociate into the tris-complex species.94 97... 

However, it should be noted that scheme (6.2), which in general correctly reflects the homogeneous dissociation of H202 in the liquid phase in the presence of transition metal ions, for instance, Fe2+ or Fe3+, must be complemented by the stages, suggested later on Ref. [25] ... 

In addition to UV/visible flash photolysis and TRIR spectroscopy, other techniques have been used for the detection of transition metal-noble gas interactions in the gas phase. The interaction of noble gases with transition metal ions has been studied in detail. A series of cationic dimeric species, ML" " (M = V, Cr, Fe, Co, Ni L = Ar, Kr, or Xe), have been detected by mass-spectroscopic methods (55-58). It should be noted that noble gas cations L+ are isoelectronic with halogen atoms, therefore, this series of complexes is not entirely unexpected. The bond dissociation energies of these unstable complexes (Table IV) were determined either from the observed diabatic dissociation thresholds obtained from their visible photodissociation spectra or from the threshold energy for collision-induced dissociation. The bond energies are found to increase linearly with the polarizability of the noble gas. 

Many reactions of organic chemistry are catalyzed by complexes of transition-metal ions, most notably those of Group VIII. Here, die catalyst is a system of different complexes that are linked by ligand-exchange and ligand-dissociation equilibria and differ in their catalytic properties, occasionally with rather counterintuitive results such as a decrease in rate with increase in pressure in a reaction in which gas is consumed. 

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