The Zintl-Klemm Concept

The Zintl-Klemm concept evolved from the seminal ideas of E. ZintI that explained the structural behavior of main-group (s-p) binary intermetaUics in terms of the presence of both ionic and covalent parts in their bonding description [31, 37]. Instead of using Hume-Rother/s idea of a valence electron concentration, ZintI proposed an electron transfer from the electropositive to the electronegative partner (ionic part) and related the anionic substructure to known isoelectronic elemental structures (covalent part), e.g., TK in NaTl is isoelectro-nic with C, Si and Ge, and consequenUy a diamond substructure is formed. ZintI hypothesized that the structures of this class of intermetallics would be salt-like [16b, 31 f, 37e]. 

The successful use of the Zintl-Klemm concept in the synthesis of complex intermetallics is represented by the work of Schaefer and co-workers in Darmstadt [41]. The extensive investigations on Zind phases mainly by von Schnering 

In the spectrum from classical intermetaUics to valence compounds to insulators, a smooth transition in their chemical bonding (metallic to ionic) is observed. At the border between Zind phases and metaUic phases, the typical properties of Zind phases diminish and metallic conductivity appears. However, it is inaccurate to impose and define a sharp boundary between classical Zind phases and the metallic phases (e.g.. Laves and Hume-Rothery phases), and it is in the overlapping regimes where much chemistry stiU remains to be discovered and understood. 

The validity (or lack thereof) of the classical Zind formahsm as applied to less polar intermetallics, involving metals along the Zind border, is nicely probed by electron-poof trelides. Seminal work by Corbett [44] and Belin [48] recognized the proclivity of trelides (Ga, In, H) to form cluster-based anion structures. The apparent electron deficiency in the chemical bonding of these cluster com- 

As in molecular chemistry, an alternative path to compensate for electron deficiency is the formation of multiple bonds, through 7r-interactions, as in unsaturated and aromatic molecular systems. Our work in Houston focuses on probing the efficacy of the ZintI concept in rationaUzing stoichiometries, crystal structures and chemical bonding of complex electron-poof ZintI phases that exhibit novel i-systems. Their chemical bonding is reflected by their unusual crystal structures related to unsaturated hydrocarbons [53]. 

---

The compound LisZn2Ge3 can be expressed as SLi , 2Zn, SGe in terms of oxidation numbers, or as 8 Li , 2 Zn, 3 Ge with regard to the Zintl-Klemm concept and valence considerations. It closely relates to Li5AlSi2 and the corresponding [Li3Al3Si 5] hexagonal layer in which Zn and Ge would play the role of A1 and Si. With the Zn vacancy compensated by a Hthium dumbbell, the [Li4ZrL4Gei5] anionic network is isoelectronic. 

Santamaria-Perez D, Vegas A, Liebau F (2005) The Zintl-Klemm Concept Applied to Cations in Oxides II. The Structures of Silicates 118 79-135... 

G. A. Papoian, R. Hoffmann, Hypervalent bonding in one, two and three dimensions extending the Zintl-Klemm concept to nonclassical electron-rich networks. Angew. Chem. Int. Ed. 39 (2000) 2408. 

A review on mercury and cadmium pentelide halides has appeared,254 with emphasis on discussion of the structures, based on the Zintl Klemm concept, and their relationship to the electronic properties. 

As mentioned in the Introduction, no structural information on these species was available for more than 40 years after the discovery of the first Zintl metal cluster anions, since no pure crystalline phases could be isolated and characterized structurally. Nevertheless, early efforts to rationalize the observed formulas and chemical bonding of these intermetallics and related molecules utilized the Zintl-Klemm concept [75, 76] and the Mooser-Pearson [77] extended (8 — N) rule. In this rule N refers to the number of valence electrons of the more electronegative metal (and thus anionic metal) in the intermetallic phases. 

King, R.B. (2004a) The metallurgist s Periodic Table and the Zintl-Klemm concept. In The Periodic Table into the 21st Century, eds. Rouvray, D.H. and King, R.B (Research Studies Press Ltd, Baldock, Hertfordshire), p. 189. 

Santamarla-Perez D, Vegas A, Liebau F (2005) The Zintl-Klemm Concept Applied to Cations in Oxides II. The Structmes of Silicates. 118 79-135 Schaffer CE (2003) Axel Christian Klixbull Jorgensen (1931-2001) 106 1-5 Schaffer CE, see Anthon C (2004) 107 207-301 Schenker S, see Hauser A (2003) 106 81-96... 

The Zintl-Klemm Concept Applied to Cations in Oxides. 

Early Efforts to Rationalize the Structures of the Zintl Ions The Zintl-Klemm Concept... 

Early efforts to rationalize the observed formulas and chemical bonding of Zintl ions and related species used initially the Zintl-Klemm concept [10, 11] and subsequently the Mooser-Pearson [12] extended (8 — AO rule. In this rule, refers... 

The intermetallic phase [26] Na2Tl illustrates a simple application of the Zintl-Klemm concept to a group 13 metal cluster. Complete electron transfer from Na to T1 leads to the (Na" )2Tl formulation. The Tl dianion is isoelectronic with group 15 elements and thus should form similar tetrahedral structures with six two-center two-electron bonds along the edges of the tetrahedron. Indeed, the Tl anions in the Na2Tl phase form Tl4 tetrahedra, similar to the isoelectronic P4 and As4 units in white phosphorus and yellow arsenic. 

For a general formulation of the Zintl-Klemm concept, consider an intermetallic AmX phase, where A is the more electropositive element, t3 pically an alkali or an alkaline earth metal. Both A and X, viewed as individual atoms, are assumed to follow the octet rule leading to transfer of electrons from A to X, i.e., A AF, X —> X , so that mp = nq. The anionic unit X arising from this electron transfer is considered to be a pseudoatom, which exhibits a structural chemistry closely related to that of the isoelectronic elements [11]. Since bonding also is possible in the cationic units, the numbers of electrons involved in A-A and X-X bonds of various types (caa and exx> respectively) as well as the number of electrons e not involved in localized bonds can be generated from the numbers of valence electrons on A and X, namely and ex, respectively, by the following equations of balance ... 

Electron count according to the (8-N) rule and the Zintl-Klemm concept... 

C2 units are also found in solid-state compounds with C-C separations that depend on formal electron count. These are viewed as deprotonated ethyne, ethylene or ethane using a popular solid-state idea the Zintl-Klemm concept. This concept is based on the simple idea that the metals transfer their valence electrons to the non-metal atoms thereby generating filled anion-centered bands at low energy, well separated from empty cation-based bands. Of course, this concept fails when the electronegativities of the metal and non-metal are not very different,... 

The first question posed by this structure concerns the formal charge distribution between the two Gd, B6 and B2 units. If we naively apply the Zintl-Klemm concept we arrive at [Gd3+]2[B6, B2]6. The [B6]2 cluster with six external bonds obeys the cluster electron-counting rules. Consequently, the B2 fragment must have a charge of —4. This corresponds to saturated eight-electron B centers, and requires non-planar (tetrahedral) B centers. This does not agree with the observed planar B centers. But we know the metal does not need to be fully oxidized. Consider sp2-hybridized B atoms which satisfy the octet rule. This would lead to [Gd2+]2[B62 ][B22 ] and suggests a B=B double bond. OK, but planar B is... 

In the Zintl-Klemm concept, the description of compounds like Li AF, Li Ga , and Li In is in principle the same. There is also a similar relation between MgGa and LiGe. 

There is again a strong disproportionation of the silicon atoms into isolated, (lb), (2b), and (3b)Si. The overall formula can be rewritten as (Ca7LixMg4 x) ° Sij "(Si, according to the Zintl-Klemm concept. Again, there is no possibility of defining a clear electron distribution in terms of the Lewis notation. 

On the silicon-rich side of the Ca/Mg/Si systems new phases have additionally been foimd. According to the Zintl-Klemm concept they must have a higher mean bond order than those discussed previously. 

The inherent basis of these procedures is the Zintl-Klemm concept and the Mooser-Pearson extended (8 — N) rule. Formerly applied only to classical two-center-two-electron bonds, the extended procedures comprise all varieties of bonding (multiple bonds, partial bonds, multicenter systems, radicals, and free electrons). Generally, for a compound AmB , an electron transfer A- A +, B- mp = nq ) to the more electronegative element B forms pseudoelements k, B that show the structural principles of the corresponding isoelectronic elements with the whole spread of homoatomic bond types. Alternatively, one can derive from the number of valence electrons e and cb according to the equation otca + nee + k = 3n the term k = saa + Y. bb - e, which accounts for the... 

Zintl phases have always been understood within the framework of an elegant and simple scheme, the so-called Zintl-Klemm concept. However, only Zintl compounds with semiconducting properties were assumed to fulfil this concept. In the last decade, important steps have been taken towards an expansion of the Zintl-Klemm concept into the metallic regime while still separating Zintl compounds quite clearly from intermetallic phases. This is due to the fact that in intermetallics the clearcut separation of components at which valence states are centered and those at which they are not, is lost. 

Application of the Zintl-Klemm concept allows complete understanding of the composition and defects in the crystal structure of the binary germanium and tin... 

The application of the electron localizability approach allows for studying details of the atomic interactions in intermetalhc clathrates. This may contribute to the understanding of structural features which cannot be achieved within the Zintl— Klemm concept. Another, from chemical point important, outcome of the electron localizabUity approach is the electron-locahzabihty-based oxidation number (ELIBON [77]) which is the real-space equivalent of the traditional oxidation numbers. Apphcation of oxidation numbers on intermetaUic clathrates allows for new ways of understanding of experimentally observed clathrate compositions and for novel redox routes for their preparation. 

---