Group 15 elements Zintl anions

The main group duster chemistry discussed in this book can be considered to originate from two important, but apparently unrelated developments in inorganic chemistry in the 1930s. The first was the identification of the neutral boron hydrides by Stock [1]. The second was the observation by Zintl and co-workers [2-5] of anionic clusters formed from potentiometric titrations of post-transition metals (i.e., heavy main group elements) with sodium in liquid ammonia. 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

Zintl phases remarks on their definition. We have seen that the Zintl phases may be considered as a group of compounds formed by an electropositive (cationic) component (alkali, alkaline earth metal, lanthanide) and an anionic component (for instance a main group element of moderate electronegativity). The anionic part of the structure may be described in terms of normal valence combination. 

Structures of Soluble Homoatomic Zintl Anions of Group 14 and 15 Elements.95... 

The best known examples of polyanions of group 14 elements are the so-called Zintl anions, e.g. (tetrahedral), (trigonal bipyramidal) and... 

Some representative compounds will now be presented. Some contain discrete Zintl anions while others have extended one-, two-, and three-dimensional structures. It is also possible, as just mentioned, to have Zintl anions consisting of more than one main group element. 

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. 

The anion connectivity of many Zintl phases can be rationalized in terms of Hume-Rothery s (8 V) mle. For example, in BaSi2 (with Si clusters), the Si anion is isoelectronic with the nitrogen group elements, that is, it has five valence electrons. The (8 N) rule correctly predicts that each silicon atom will be bonded to three other sUicon atoms. Similarly, in Ca2Si, Si is isoelectronic with the noble gas elements. Again, the 8 A mle correctly predicts that silicon will occur as an isolated ion. Indeed, this compound has the anti-PbCl2 stmcffire, in which the sUicon is surrounded by nine calcium ions at the comers of a tricapped trigonal prism. 

Reaction of group 15 elements like arsenic with alkali or alkaline earth metals forms Zintl anions of the general type Asn". Characterization of these species has been accomplished dissolving the solids in liquid NH3 or ethylenediamine (H2NCH2CH2NH2) and adding the macrocycle, 2,2,2-crypt (5) to complex the cations and stabilize the polyatomic anions. The fully characterized Zintl arsenic anions As4, As and Asu are shown inFigme 1. ... 

Under the appropriate conditions, the heavier members of the groups IIIB-VIB elements will form a wide variety of interesting cluster compounds called Zintl anions. Some of these contain a group lA element as the cation The main group anion clusters in this class of compounds generally follow normal rules of valence, and most are semiconducting. All require the presence of an easily oxidizable element, such as those of the groups lA and IIA, in their formation. 

Reaction of group 15 elements like arsenic with alkali or alkaline earth metals forms Zintl anions of the general type Characterization of these species... 

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