Heterolytic bond fission

Jeveral aspects of the photolytic behavior of aqueous complex ions have been studied in this laboratory over the past few years. One continually interesting question has been the extent to which the photochemistry of a complex depends on the absorption band irradiated. In the case of Co(III) acidopentamines, such as Co(NH3)5Br+2, we found that irradiation of Ajg —> g) bands showing appreciable charge transfer led to redox and aquation reactions which were competitive. It was reasonable to suppose that the common precursor was the species formed by a prompt heterolytic bond fission (7). The ( Aig —> Tig) band was far less photoactive, and in model cases, irradiation led only to aquation. Each excited state or excited state manifold thus tended to show a distinct photochemistry, which meant that conversion from one excited state to another was not important. 

An alternative inclination has been to view the initial chemical act following excitation as a prompt heterolytic bond fission of some one chromium-ligand bond (,2), (15). Both pictures adequately account for the so far not very elaborate data, the second perhaps more in keeping with the principle of scientific parsimony. 

The general observation in the published work has been that for species such as hexamminechromium, thiocyanatopentammine, or the hexa-aquo ion where O18 exchange was looked at, irradiation produced a substitution reaction and nothing else. Moreover the reaction mode was independent of wave length, and the quantum yields did not change much. From a morphological point of view, there are essentially three types of explanations. First all excited states independently lead to the same chemical sequence, and we suppose that the primary act is simply a heterolytic bond fission. 

A concerted reaction involving heterolytic bond-fission of the dia-... 

The observed survival probability depends not only on the survival probability of an ion-pair formed with an initial separation, r0, but also on the distribution of initial separations, w(r0). As will be discussed in Sects. 3 and 4, the rate of loss of energy of ions after heterolytic bond fission (or, alternatively, the range of electrons formed by ionisation of a molecule) depends sensitively on the energetics of the solvent molecules. These separation distances can range up to 10 nm or more and are specifically discussed in Sect. 3. The survival probability of a collection of isolated ion-pairs, P(t), formed at a time t0 — 0, and with a distribution of initial distances w(r0) is... 

If the absorbing species is in a condensed phase, intermolecular interactions may arise that control the fate of the excited-state entity, eg when the excited molecule is in a solvent cage and the initial photochemical reaction relies on the homo-lytic or heterolytic bond fission, diffusion of the initial products (radicals) from the same precursor is inhibited. Instead the products remain in the cage for several vibration periods which enables the back reaction to form the substrate or its isomeric form [4-6],... 

Homoatomic bonds tend to prefer homolytic bond fission, which results in radicals being formed while heteroatomic bonds often undergo heterolytic bond fission, which results in ions being formed. 

The neutral and acid-catalysed hydrolysis of mesitoyl chloride in acetonitrile containing 1% 018-enriched water is not accompanied by O18 exchange (Bender and Chen, 1963). It is suggested that in both cases unimolecular heterolytic bond fission occurs with the formation of an acyl cation. In alkaline solution a tetracovalent intermediate is postulated on the basis of a comparison of the effect of substituents on the rate of hydrolysis of the corresponding benzoate esters. The exchange of O18 in alkaline solution was not determined experimentally. 

MO acting as the acceptor orbital. In the extreme, complete transfer of charge could lead to heterolytic bond fission as in the formation of [I(py)2] (Fig- 17.4 and eq. 17.10). 

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