The stability of organometallic compounds

Very reactive species such as the silaethene, Me2Si=CHMe which in most situations would have only a transient existence, can be generated in solid inert gas matrices at very low temperatures (4-20 K). Under these conditions the individual molecules are trapped in isolation from each other, so that they can be studied spectroscopically. Above 45 K, however, diffusion through the matrix is sufficiently rapid for dimerization to occur. Silaethene monomers have also been detected by mass spectrometry in the gas phase at low pressures and high temperatures. The entropy term then favours their formation. The environment is therefore critical in determining whether or not a compound can be isolated or studied. Not only thermodynamic, but also kinetic factors must always be taken into account. 

All such endothermic compounds and many more are thermodynamically unstable to reactions such as 

See also pp. 164,280 for transition metal complexes. (g) denotes gaseous state (c) denotes crystalline state. The enthalpies quoted are generally ca. 5 kj moP L 

Another common pathway for decomposition involves ]8-hydrogen transfer (p.217). 

It is potentially reversible, but if the metal hydride is unstable to further decomposition, the process becomes irreversible. If the metal alkyl does not possess a j -hydrogen atom this pathway is suppressed. Ti(CH2Ph)4, for example, is stable at room temperature, whereas Ti(CH2CH3)4 has never been isolated. Other mechanisms such as a-hydrogen transfer, however, may then take over (p. 220). Both a- and jS-hydrogen transfer are aided by the presence of low-lying empty orbitals on the metal atom. 

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Formation of metal-organic compound co-condensates presents a peculiar problem, which is difficult to study. Most experimental methods are inapplicable to the study of the processes that take place at the instant of cocondensation. This brings to the fore theoretical approaches. The state of the art of computational quantum-chemical methods makes it possible to adequately describe the structure of organometallic compounds and estimate their stability and, sometimes, reactivity. 

Most macrocycles composed of half-sandwich complexes are positively charged and water soluble. The water solubility and stability of organometallic compounds are advantageous, and organometallic chemistry in aqueous-phase is growing rapidly [18]. The overall hydrophilicity of the metalla-cycles combined with potential inner hydrophobic interactions with guest molecules have been exploited previously. 

The stabilization of the Ga-Ga bond is, arguably, the most important development of the past decade in the organometallic chemistry of gallium. Perhaps the most significant factor in the stabilization of these compounds is the utilization of sterically demanding ligands. Such ligands are necessary as they provide kinetic stabilization. It is anticipated... 

The characterization and utilization of photochemical processes are rapidly developing into one of the major areas of activity in modern inorganic and physical chemistry. In the past, the photochemistry of classical metal coordination complexes has received the greatest amount of attention, but recently the photochemistry of organometallic compounds has attracted notice In particular, the photochemistry and photophysics of uranyl compounds have been investigated for more than four decades and a great deal has been learned about the primary photoprocesses and the photo-induced reaction mechanisms displayed by these complexes (.3,4). The popularity of uranyl compounds in photochemical studies is derived from their ready availability and stability, their facile redox chemistry and photosensitivity and their rich excited state chemistry. Since current reviews of uranyl photochemistry are expected to appear in the near future, vide infra, further discussion of this topic here will be limited. 

Electron Rule common.4 Insight into the connection between stability of organometallic compounds... 

The stability of organometallic or inorganic compounds in a petroleum matrix is an important consideration in the sampling process. Stability studies on selected organometallic standards in petroleum... 

There is no direct connexion between the reactivity and thermal stability of organometallic compounds. It would be quite invalid to generalize that very reactive compounds are particularly unstable thermally and less reactive ones thermally more stable. 

No sandwich or 7r-complex seems to be involved. One might observe in general that the isolation of organometallic compounds of low stability may often succeed if complex formation with solvent, metallic salt, or an RM type is provided for (See Section IV.A.3). For example, disubstituted platinum (II) and palladium (II) derivatives have been prepared successfully as the bis(triethylphosphine) complexes in the following reaction (19, 20) ... 

The reaction of organometallic compounds with the surfaces of metals are now widely used to obtain tailor-made bimetallic catalysts. The improvement in catalytic activity, selectivity, and stability can result from the modification... 

A knowledge of such BDE s is important for an understanding of the kinetic and thermodynamic stabilities of organometallic compounds and of the thermodynamics of the many organometallic reactions that involve the formation or dissociation of transition metal-alkyl bonds, for example, insertion or oxidative addition-reductive elimination (Equations 2 and 3, respectively)(1-3). 

Transition-metal NPs are easily prepared by the simple reduction of metal compounds or the decomposition of organometallic compounds in the zero oxidation state dissolved in the imidazolium ILs (Scheme 6.1 and Table 6.1). FunctionaUzed imidazoUum or pyridinium ILs such as those containing thiol, alcohol or cyano groups have also been used for the formation and stabilization of Ni, Ag and Au NPs [61-75], Bimetallic nanorods, hyperbranched nanorods, and NPs with different CoPt compositions have also been easily prepared in BMl.NTf2 [76]. 

Thus kinetic stability of organometallic compounds may be associated with a closed shell of electrons, often of essentially spherical symmetry, around the metal atom. 

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