Alkali metals coordination numbers

The literature is most prolific on neutral An amides, but neutral U and amides are also well documented. There are only few publications on amides,one being on three alkali metal amidouranate(VI)s. ° There is a paper on a compound (18) which couid be classified as of U° and two on mixed valence U /U amides (see Section 5.4 for these types). There are a number of articles on monocationic amidol/ tetraphenylborates, three on dicationic analogues, three on a cationic U salt and four on alkali metal amidoactinate(IV)s and three on corresponding U salts. For crystalline uranium amides containing only monodentate ct-bonded ligands, metal coordination numbers of 3-7 are... 

In recent years this simple picture has been completely transformed and it is now recognized that the alkali metals have a rich and extremely varied coordination chemistry which frequently transcends even that of the transition metals. The efflorescence is due to several factors such as the emerging molecular chemistry of lithium in particular, the imaginative use of bulky ligands, the burgeoning numbers of metal amides, alkoxides, enolates and organometallic compounds, and the exploitation of multidentate... 

The predominantly ionic alkali metal sulfides M2S (Li, Na, K, Rb, Cs) adopt the antifluorite structure (p. 118) in which each S atom is surrounded by a cube of 8 M and each M by a tetrahedron of S. The alkaline earth sulfides MS (Mg, Ca, Sr, Ba) adopt the NaCl-type 6 6 structure (p. 242) as do many other monosulfides of rather less basic metals (M = Pb, Mn, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Th, U, Pu). However, many metals in the later transition element groups show substantial trends to increasing covalency leading either to lower coordination numbers or to layer-lattice structures. Thus MS (Be, Zn, Cd, Hg) adopt the 4 4 zinc blende structure (p. 1210) and ZnS, CdS and MnS also crystallize in the 4 4 wurtzite modification (p. 1210). In both of these structures both M and S are tetrahedrally coordinated, whereas PtS, which also has 4 4... 

An increase in the Me F ratio leads to an increase in the acidity of the initial solution, whereas the acidity of alkali metals increases according to their molecular weight, from Li to Cs. Therefore the additives of fluorides of alkali metals having higher atomic weight provide formation of complex fluorides with lower coordination number of tantalum or niobium. 

Three conceptual steps can be discerned in the definition of the ionic structure of fluoride melts containing tantalum or niobium. Based on the very first thermodynamic calculations and melting diagram analysis, it was initially believed that the coordination numbers of tantalum and niobium, in a molten system containing alkali metal fluorides, increase up to 8. 

When the positions of cations and anions are interchanged, the same structure types result for the CsCl, NaCl and zinc blende type. In the case of the fluorite type the interchange also involves an interchange of the coordination numbers, i.e. the anions obtain coordination number 8 and the cations 4. This structure type sometimes is called anti-fluorite type it is known for the alkali metal oxides (Li20,..., Rb20). 

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