Polyatomic cationic cluster

Simple cations are unknown within Group 16 (besides Po), but several highly colored polyatomic cations (cationic clusters), like S " ", Sg, Se, SCg, Te, and Teg" ", have been isolated in non-aqueous media [15]. Some mixed chalcogen cationic clusters have also been reported. These are all unstable in water. 

The apparent Instability of homopolyatomic anions of elements to the left of group IV (and of polyatomic cations to the left of group V) Is thought to result from a deficiency of bonding electrons In species where the principal bonding Is considered to originate largely from p-type orbitals. The electron deficient polyboranes, with which these clusters have... 

Salts of homopolyatomic cationic clusters now constitute a well-established class of compound— there are at least twenty-six fairly well-characterized examples. It is probable that many other elements will also be shown to form polyatomic cations. As yet reactions of these species have been little studied, and there is obviously a wide open field here awaiting exploration. Many structures of known cations, as well as those that have not yet been prepared, remain to be investigated, and there is a need for theories that can predict the stability and geometry of these cations and provide a description of the bonding. 

The bonding within these polyatomic cations is weak. One can readily calculate bond orders which are small fiactions. There is no suggestion that any of these cations would be stable outside the zeolite, nor that their geometries are as rigid as those of molecules or molecular ions. All of these polyatomic cations conform to the electrostatic requirements of the zeolite framework, somewhat as liquids adopt the shapes of their containers. Perhaps it is reasonable, because the bonding is so weak, not to refer to these as polyatomic cations at all, but rather as electron traps. This more physical description is consistent with the observation that some of these clusters can be prepared (in low concentration) by y-... 

Other less-symmetrical coordination geometries for Se and Te occur in the /t,-Se2 and /t-Te2 complexes and the polyatomic cluster cations Seio and Tee" " ", as mentioned below. 

The term Zintl phase is applied to solids formed between either an alkali- or alkaline-earth metal and a main group p-block element from group 14, 15, or 16 in the periodic table. These phases are characterized by a network of homonuclear or heteronuclear polyatomic clusters (the Zintl ions), which carry a net negative charge, and that are neutralized by cations. Broader definitions of the Zintl phase are sometimes used. Group 13 elements have been included with the Zintl anions and an electropositive rare-earth element or transition element with a filled d shell (e.g. Cu) or empty d shell (e.g. Ti) has replaced the alkali- or alkaline-earth element in some reports. Although the bonding between the Zintl ions and the cations in the Zintl phases is markedly polar, by our earlier definition those compounds formed between the alkali- or alkaline-earth metals with the heavier anions (i.e. Sn, Pb, Bi) can be considered intermetallic phases. 

Sb-H bonds, 489 Sb-halogen bonds, 489 Se-H bonds, 451 Se-halogen bonds, 451 Si-Br bonds, 465 Si-Cl bonds, 464 Si-F bonds, 464 Si-H bonds, 455 Si-1 bonds, 465 Sn-Br bonds, 475-476 Sn-Cl bonds, 475 Sn-F bonds, 475 Sn-H bonds, 473 Sn-1 bonds, 476 Te-H bonds, 453 Te-halogen bonds, 453 Rare gas complexes anions, 1452 cations, 1446-1452 neutrals, diatomic, 1429-1436 polyatomic, 1436-1446 Rb-contarnmg species neutrals, 557-559 Rb clusters, 559-562 Rb clusters, 562-563 Re-containing species neutrals,796-799 Re clusters, 799-801 Re clusters, 801 Rh-contarnmg species neutrals, 882-892 Rh clusters, 892-894 Ru-containing species neutrals, 840-848 Ru clusters, 849-851... 

It has been shown that the simple approach of PIPICO measurements works satisfactory only for small molecules, where the number of possible iission processes is limited. In the case of polyatomic molecules and clusters, this approach leads to ambiguities. Therefore, the use of multicoincidence experiments, where the individual flight times of the correlated cations, tA and are measured separately, gives more detailed information on the fission mechanisms. One important approach, that is discussed in detail below, is Photoelectron-photoion-photoion-coincidence (PEPIPICO) spectroscopy (cf. Secs. 3.3 and 4.3). The results that are obtained from PEPIPICO spectra are three-dimensional shapes of correlated cation pairs (cf. Sec. 4.3), where the relative intensity of these processes is plotted over the flight times of the correlated cations. Alternatively, the intensity of the coincidence signals can be represented in contour plots, as shown in Fig. 6. Note that only one half of the array is shown because of the mirror symmetry with respect to the main diagonal. 

Naked polyatomic clusters of post-transition metals can also be obtained as cations. They correspond to intermediate species between the metals and the compounds in conventional oxidation states. The well known dimercury cation stable in acidic conditions, may be considered a prototype of this kind of species. The best known examples of this class of compounds are the proper cluster species Bi " and Big" (vide infra). 

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