Structural isomerism ionization isomers

Ionization isomers result from the interchange of an anionic ligand within the first coordination sphere with an anion outside the coordination sphere. 

Examples of ionization isomers are violet [Co(NH3)5Br] [SO4] (prepared by reaction scheme 19.6) and red [Co(NH3)5(S04)]Br (prepared by reaction sequence 19.7). These isomers can be readily distinguished by appropriate qualitative tests for ionic sulfate or bromide, respectively. 

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First of all we have determined geometric structures and ionization potentials of the three most stable isomers [15], respectively, which are in reasonable agreement with ab initio structure calculations [31-33] and experimental ionization data [34]. For all isomeric structures the ct cross section on corresponding to Ofinai of channel (14a), i.e. 

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

Figure 16.18 summarizes the types of isomerism found in coordination complexes. The two major classes of isomers are structural isomers, in which the atoms are connected to different partners, and stereoisomers, in which the atoms have the same partners but are arranged differently in space. Structural isomers of coordination compounds are subdivided into ionization, hydrate, linkage, and coordination isomers. 

H- and C-labeling revealed that the decomposing molecular ions of 1-, 2-, 3-, and 4-octene isomerize to a mixture of interconverting structures within 10 s after ionization. The equilibration of double bond isomers is mainly due to radical-... 

The relative stability of the delocalized, non-vertical radical cation relative to a localized, vertical isomer was demonstrated also in gas phase experiments [404]. The molecular ions of m/e 132 obtained by gas phase ionization of the [4 + 2] dimer exhibited a bimodal decay, a result which was interpreted as evidence for the presence of two isomeric ions with different structures. The possibility that the reactive ion is a species with excess internal energy was discounted, when equivalent decay curves were observed in experiments using 10 eV and 70 eV electron impact ionization energy. In dramatic contrast, the molecular ions derived from the [2 + 2] dimer fail to react apparently the ion population resulting in this experiment is homogeneous [404],... 

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