Stability factors, transition metal complexes

All these arguments for the stability of the M—Rf complexes are speculative and they neglect considerations which may be significant, such as the relative free energies of the fluorocarbon and hydrocarbon radicals. It should be noted that generally very little is known about the importance of factors which affect the stability of transition metal complexes and that a very small change in bond energy may have a marked effect on thermal stability. 

Transition-metal complexes span an enormous range of stabilities. One of the principal aims of this chapter is to attempt to understand some of the factors which control these, and to determine the importance of ligand-field effects. Very extensive compilations of stability constants are available. 

It cannot be excluded that for first-row transition metal complexes, in addition to the above factors, the central atom spin state also plays a significant role in the stability of individual electronic and/or spin isomers, and in their ability to undergo photochemical interconversions. For some iron complexes such spin-state photoisomerizations are well-known [1, 121]. 

An important factor in the success of these reactions involves chelation-assistance by a heteroatom. Thus, the coordination of the heteroatom to the metal, brings the metal closer to the C-H bond and stabilizes the thermally unstable C-M-H species formed by the oxidative addition of a C-H bond to a low-valent transition metal complex. In addition, the use of the chelation-assistance leads to a high regioselectivity, which is an essential factor in organic synthesis. For reactions, a number of transition metal complexes - including ruthenium, rhodium, and iridium - are used as a catalyst, and ruthenium-catalyzed reactions will be described in this chapter [5]. 

Current interest in metal cluster compounds has arisen from the demonstration that metal-metal bonds play a key role in determining the chemistry of large classes of compounds, in particular, those with heavy metal atoms in low valent states. The occurrence of metal-metal bonding in transition metal complexes has been surveyed 21, 26, 59, 271, 275), and the criteria for metal-metal bonding and the factors contributing to the stability of such bonds have been discussed. Schafer and Schnering Sll) and more recently Keppert and Vrieze 229) have reviewed the lower halide, oxide, and oxyhalide clusters of the heavier transition metals. Cotton 102) has considered the transition metal clusters in terms of structural types, and a similar approach has been adopted in a review of molecular polyhedra of high coordination number 309). 

Transition metal complexes usually favor insertion into the carbon-sulfur bond adjacent to C(alkenyl)-S bond (type a), but some complexes insert into the C(aryl)-S bond (type b) [117]. Ab initio calculations suggest that the most important controlling factor for determining selectivity in the C-S bond cleavage of benzothiophene is the thermodynamic stability of the resulting metal-carbon bond [118],... 

Examples of CO insertion into )7 -allyl transition metal complexes are quite limited. Eq. 7.3 shows that the CO insertion into the )7 -allylpalladium proceeds to give isolable acylpalladium complexes depending on conditions [52], The reluctance to the CO insertion may be caused by the stability of t/ -allyl transition metal complexes. Another factor for the lower reactivity of the allylpalladium complex to provide the acylpalladium complex is the facile decarbonylation of the acyl complex produced by the CO insertion into the r/ -allyl bond. In fact removal of the X ligand in the butenoylpalladium complex having the halide ligand X in Eq. 7.3 by addition of a silver salt causes rapid decarbonylation of the butenoyl entity giving the -allylpalladium complex. 

Harvey has examined these issues by density fimctional calculations, and has concluded that (1) "primary alkyl complexes are usually more stable than secondary and tertiary ones," that (2) "this is an electronic effect, due to the partial carbanionic character of the alkyl group," and that (3) "steric effects,... usually invoked in the literature. .. play only a minor role in many cases." The electronic effect is proposed to parallel that in alkyl lithium reagents. Because of the partial negative charge on the a-carbon in alkyl lithium and transition metal complexes, the stability of the alkyl complexes parallels the stability of the carbanions. When the charge on the a-carbon is small, as in neutral, late transition metal complexes, other factors, such as steric effects and agostic interactions, can dominate the stability of the isomeric alkyl complexes. 

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