Zinc coordination number

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

Betyllium, because of its small size, almost invariably has a coordination number of 4. This is important in analytical chemistry since it ensures that edta, which coordinates strongly to Mg, Ca (and Al), does not chelate Be appreciably. BeO has the wurtzite (ZnS, p. 1209) structure whilst the other Be chalcogenides adopt the zinc blende modification. BeF2 has the cristobalite (SiOi, p. 342) structure and has only a vety low electrical conductivity when fused. Be2C and Be2B have extended lattices of the antifluorite type with 4-coordinate Be and 8-coordinate C or B. Be2Si04 has the phenacite structure (p. 347) in which both Be and Si... 

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 observation regarding HSAB theory here is that metal ions that are soft in aqueous solution must not only be able to form covalent M-L bonds, but also must have a loose enough coordination sphere to tolerate potentially adverse steric effects such as bulky donor atoms or substituents on a ligand. For a soft Zn(II) ion to be produced in zinc metalloenzymes, its coordination number must drop from six in the... 

For compounds of the composition MX (M = cation, X = anion) the CsCl type has the largest Madelung constant. In this structure type a Cs+ ion is in contact with eight Cl-ions in a cubic arrangement (Fig. 7.1). The Cl- ions have no contact with one another. With cations smaller than Cs+ the Cl- ions come closer together and when the radius ratio has the value of rM/rx = 0.732, the Cl- ions are in contact with each other. When rM/rx < 0.732, the Cl- ions remain in contact, but there is no more contact between anions and cations. Now another structure type is favored its Madelung constant is indeed smaller, but it again allows contact of cations with anions. This is achieved by the smaller coordination number 6 of the ions that is fulfilled in the NaCl type (Fig. 7.1). When the radius ratio becomes even smaller, the zinc blende (sphalerite) or the wurtzite type should occur, in which the ions only have the coordination number 4 (Fig. 7.1 zinc blende and wurtzite are two modifications of ZnS). 

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). 

Further examples where these rules are observed are as follows. Under pressure, some compounds with zinc blende structure, such as AlSb and GaSb, transform to modifications that correspond to the J3-Sn structure. Others, such as InAs, CdS, and CdSe, adopt the NaCl structure when compressed, and their atoms thus also attain coordination number 6. Graphite (c.n. 3, C-C distance 141.5 pm, density 2.26 gem-3) pr Te diamond (c.n. 4, C-C 154 pm, 3.51 gem-3). 

There are over 140 zinc alkyl or aryl structures reported in the CSD in which more than half of the donors are noncarbon. Those selected for discussion show representative structures and coordination numbers, or particularly prevalent ligand systems. 

Zinc is frequently used as a supramolecular building block due to its inert oxidation state and flexibility in coordination number and geometry. 

The limited number of characterized complexes of arsine and arsenide ligands contain some similar structural motifs to those exhibited for equivalent phosphorus ligands. Coordination numbers of two, three, or four are observed with monomeric up to decameric zinc complexes structurally determined by X-ray crystallography. 

EXAFS has been used to determine the second hydration shell of zinc in aqueous solution. Aqueous solutions of zinc nitrate over a range of concentrations were examined and a Zn—O distance of 2.05 A for the first shell of the six-coordinate zinc center found, which is unaffected by concentration. The second hydration shell shows a Zn—O distance which has no systematic trend but an average distance of 4.1 A. The coordination number for the second shell is 11.6 1.6 with unusual behavior for the most concentrated 2.7 M solution, which has a decrease in coordination number to 6.8 1.5 340... 

The crystal structure of zinc phenylphosphonate was solved and demonstrated the anticipated layer structure with the phenyl rings occupying the interlamellar space.409 The structure of zinc ethylphosphonate and (2-aminoethyl)phosphonate both show four-coordinate zinc centers in contrast to the coordination number of six in the phenylphosphonate compound.410... 

A series of zinc diphosphonate complexes were synthesized in the presence of diamines of varying chain length. 1-hydroxyethylidenediphosphonate bridges the metal ions with the proto-nated diamine (ethyl-, butyl-, pentyl-, or hexylenediamine) filling in channels or residing between chains. All four structures are different with one-, two-, and three-dimensional networks formed. The coordination number (4-6) and geometry also varies.418... 

Although numbering even fewer than the structurally characterized selenium zinc complexes there are examples of zinc tellurides and tellurates with typical coordination numbers of three or four and species from monomeric up to polymeric. 

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