Unsaturated ions

A point of interest at this stop in our tour is that fragmentation of organometallic ions in ESI-MS often proceeds via ligand dissociation (e.g., phosphane loss) to generate coordinatively unsaturated organometallic ions [1-9]. One of the strengths of this technique is that such unsaturated ions are typically proposed as reactive intermediates in catalytic reactions carried out in solution (vide infra), allowing ESI-tandem-MS systems to study directly the gas-phase reactivity of such species. 

Electrospray ionization will often produce ions that are fully coordinated, stable, and nonreactive in the gas phase. These ions may be probed by removal of ligands to form coordinatively unsaturated ions that are generally reactive. The chemical activity of metal cluster ions differs markedly and often shows size specific enhanced reactivity or lack of reactivity. Silver cluster ions Ag are fairly inert similar to Ag+. Platinum cluster ions PL are quite reactive similar to Pt+. Often, large cluster ions only appear to react with one donor molecule such as benzene this may be due to low concentrations of reactants or short reaction times. Similar clusters may react with a larger number of smaller molecules, and so until more information is available, rules for the coordination behavior of metal clusters are as yet not available. 

Many varieties of saturated ions are involved in the fruitful reactions of cracking, all participating in cycles of reactions which preclude deactivation. In order to explain catalyst decay, we must envision the formation of some other type of surface species. This must be a species which occasionally arises from the carbenium ions which normally participate in the mainline reactions. Where else could it come from Such a species - the species we believe to be responsible for decay - is unsaturated carbenium ions. Unsaturated ions may be expected to differ in their desorption and reactivity properties from the more common saturated ions perhaps these differences are sufficient to explain the accumulation of deactivating species. 

Surface excitons require less energy in their formahon than bulk excitons, owing to the reduced Madelung constant of the coordinahvely unsaturated ions at the surface [81]. 

The degree of structural defects can be checked by means of IR spectroscopy. Specifically, IR bands at 880 and 1015 cm" are attributed to the Sb—O—Sb and Sb—O—V stretching vibrations involving co-ordinatively unsaturated ions adjacent to cationic vacancies [58], Cationic vacancies play an important role in the catalytic performance of rutile-type mixed oxides [59-61]. For instance, it was found that an increased concentration of cationic vacancies and isolated V species in V/Sb/(Fe)/0 systems, due to the introduction of increasing amounts of Fe in the lattice, led to a proportionally higher activity [60c]. The formation of V in Cr/V/Sb mixed oxides had the similar effect of increasing propane conversion considerably [62]. 

This decrease of the number of acid sites concerns both Bronsted and Lewis acidic sites. The replacement of the acid P-OH groups by basic P-NHx species during nitridation induces an important decrease in the munber of Bronsted acid sites. The decrease in the number of Lewis acid sites induced by nitridation is due to the neutralisation of the coordinatively unsaturated ions by NHx species and / or the substitution of oxygen atoms by nitrogen atoms aroimd the coordinatively unsaturated cations, which reduces the positive charge and hence their tendency to attract electrons. 

The acetate 211a reacts in 5 minutes at —70° to give 3,4,6-tri-O-acetyl-2-deoxy-a-D-arabino-hexopyranosyl fluoride (212a), and after 24 hours at —70°, likewise by way of the unsaturated ion 215a, to the ion 216a. The high stability of the ions of type 216 is also evident in the pentose series. Thus, compounds 220, 221, 223, and... 

When an H-zeolite is heated to high temperature, water is driven off and coordi-natively unsaturated ions are formed. These are Lewis acids (Eq. 7-5). 

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