Bond energy thermally unstable complexes

The red-shift of the C=0 and Si=0 stretching vibration of 100 and 49 cm" , respectively, the calculated bond energy of complex 10 of more than 20 kcal/mol (RHF/6-31G(d,p)), and the calculated non-bonding Si-O distance of 1.98 A, being only slightly longer than the normal Si-0 bond distance, indicate the unusual electronic properties of 10. Evidently dioxasiletane 9 is formed as a thermally or photochemically unstable intermediate which rapidly decomposes to the silanone-formaldehyde complex. 

There are little quantitative data concerning the thermodynamic stability of metal-organo complexes. The low thermal stability of many complexes shows that they are thermodynamically unstable and it seems probable that the large majority of even thermally stable complexes are also thermodynamically unstable with respect to some decomposition products. This thermodynamic instability of metal-organo complexes probably arises from the relative weakness of the metal-carbon bond compared to the bonds which carbon forms with many other elements, in particular with itself. For example, titanium tetrachloride has a negative free energy of formation from its elements because the Ti-Cl bond is strong whilst the Cl-Cl bond is relatively weak. In contrast, tetramethyltitanium decomposes at —40 C and this is presumably because the Ti-C bond is weak in... 

Examples of silver(l) alkyl and alkenyl (including aryl) complexes have been known from as early as 1941 6-9 however, the number of examples is fairly limited with respect to that of the heavier congeners, copper(l) and gold(l). Such a phenomenon can readily be attributed to the relatively low stability of this class of complexes, both photochemically and thermally. Simple homoleptic alkyl and alkenyl complexes of silver(i) are known to be very unstable under ambient temperature and light, and successful isolation of this class is fairly limited and mainly confined to those involving perfluoroorganics.10 The structures and the metal-carbon bond-dissociation energies for... 

Fig. 51. Schematic representation of the thermal decompoation of a o-organo cmnpkx, M— R, by a unimolecuiar dissociation of the M-R bond, showing, Ea, the ener of activation of preliminary decompoation, which is important in determining the kinetic stability of the complex A(j represoits the free energy of decompoa tion. If the products have less energy than the reactants then the complex is thermodynamicalty unstable...

Periluorocarbon-a-transition metal complexes are likely to be thermodynamically unstable on account of the high lattice energies of metal fluorides. It seems reasonable that the relatively high thermal stability of M—Rf complexes, compared to that of M—Rj, complexes, arises from an increased stability to dissociation of the M—Rf bond. A relative increase in the M—Rf bond strength could be due both to an increase in the or-bonding and, possibly, there could be si-bonding in the M—Rf bond [246, 247, 248]. 

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