The structural chemistry of zinc

The structures of many of the simple compounds of the IIB elements have been described in earlier chapters. Cadmium, like zinc, has only one valence state in its normal chemistry and its structural chemistry presents no points of special interest. We therefore confine our attention here to zinc and mercury. 

From the geometrical standpoint the structural chemistry of zinc is comparatively simple. There is only one valence state to consider (Zn ) and in most molecules and crystals the metal forms 4 tetrahedral or 6 octahedral bonds two collinear bonds are formed in the gaseous ZnX2 molecules and presumably in Zn(CH3)2, and some examples of 5-coordination are noted later. 

A comparison of the structures of compounds of Be, Mg, Zn, and Cd reveals an interesting point we exclude Hg from this series because there is practically no resemblance between the structural chemistries of Zn and Hg apart from the fact that one form of HgS has the zinc-blende structure. In the following group of compounds italic type indicates tetrahedral coordination of the metal atom in the crystal in other cases the metal has 6 octahedral neighbours. 

It would seem that the tetrahedral Zn-0 bonds have less ionic character than octahedral ones. Evidently the character of a Zn-0 bond will depend on the environment of the oxygen atom, though a satisfactory discussion of this question is not yet possible. In ZnO the 0 atom is forming four equivalent bonds (a), in Zn2Si04 bonds to 2 Znand 1 Si (b), and in ZnC03 bonds to 2 Zn and 1 C of a COs ion, within which the C—0 bonds are covalent in character, (c). 

A similar difference in M-0 bond character presumably exists in oxy-compounds of divalent lead. Many anhydrous oxy-salts of Pb are isostructural with those of Ba (and sometimes Sr and Ca)  

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This review cannot claim to be a comprehensive account of all the coordination chemistry of zinc since the early 1980s. The approach has been to attempt selection of references by key workers or important results in areas where much work has been carried out. It is hoped that it will be possible by following the key references and articles to gain an overview of achievements and advances in the important areas. In many cases, examples where X-ray structural data is included have been selected preferentially for inclusion. 

The reported structural chemistry of zinc aldehyde complexes is dominated by Vahrenkamp and co-workers. A systematic investigation is reported over three papers published in 1999. All X-ray structures reported in these papers are aryl aldehydes and no aliphatic aldehyde structures with zinc were found in the CSD. Zinc is frequently exploited in preparative organic chemistry and enzyme-catalyzed transformations of organic carbonyl compounds. 

The inorganic section is organized by structure, rather than function, making it easier for the reader to both find relevant topics and to assimilate the information. Following a brief review of some fundamental aspects of elemental zinc, its chemistry, and its place in the periodic table, the organometallic chemistry of zinc is covered in a systematic fashion. 

The structural chemistry of some metal dithiocarbamates, i.e. systematics, coordination modes, crystal packing, and supramolecular self-assembly patterns of nickel, zinc, cadmium, mercury,363 organotin,364 and tellurium,365 366 complexes has been thoroughly analyzed and discussed in detail. Supramolecular self-assembly frequently occurs in non-transition heavier soft metal dithiocarbamates. Thus, lead(II),367 bismuth(III)368 zinc,369 cadmium,370 and (organo)mercury371 dithiocarbamates are associated through M- S secondary bonds, to form either dimeric supermolecules or chain-like supramolecular arrays. The arsenic(III)372 and antimony(III)373 dithiocarbamates are... 

Very thorough and comprehensively referenced surveys of the coordination chemistry of zinc are available. In each of these multivolume compendia, there is an article devoted to zinc complexes, but there are also numerous mentions of zinc complexes in the introductory volumes and in the volumes on applications and uses. Thermodynamic data AH, AS, AGf also some AX° values for e.g. ZnX2 + 2L) are available for about 100 zinc complexes. In this present encyclopedia, we are only able to mention a very small selection of the more important and interesting complexes. Preparations, structures, properties, and uses are dealt with in this section, the solution chemistry of zinc complexes in later sections (Sections 9.2 and 9.3). 

Although the structural chemistry of molybdenum and vanadium phosphates is very diverse these compounds seem to be prototypes for open framework transition and main group metal phosphates. Recent results have shown that zeolite-like compounds like zinc/cobalt phosphates [731, indium phosphates [74], nickel/aluminum phosphates [75], cobalt/boron phosphates [76], manganese and cobalt phosphonates with giant 44-membered rings forming ellipsoidal cavities [77] can be synthesized. All these examples show that the diverse chemistry of octahedral-tetrahedral framework structures is a vast field where... 

IR and Raman spectroscopy have been commonly used and, for example, the effects of pressure on the Raman spectrum of a zinc compound with a N2C12 coordination sphere around the metal, have been investigated.28 IR spectroscopy has been utilized in studies of the hydration of zinc in aqueous solution and in the hydrated perchlorate salt.29 Gas phase chemistry of zinc complexes has been studied with some gas phase electron diffraction structures including amide and dithiocarbamate compounds.30-32... 

The chemistry of zinc ammine complexes is well known. There are X-ray structural examples of both the tetrahedral tetra-ammine and octahedral hexa-ammine.90,91 Four- and five-coordinate mixed ligand complexes are common and participation in coordination networks as terminal ligands is observed.92,93... 

Zinc is a constituent of over 300 enzymes with much research into the coordination of zinc to the protein backbone, and how its chemistry is modulated by the donor set and environment.2 As well as the Lewis acid catalysis properties in enzymes, the structural role in zinc finger proteins has been a major area of research since the late 1990s. A number of reviews on zinc physiology, enzymology, and proteins in general have been published.978-981 There is extensive analysis available to classify the mononuclear sites in zinc proteins and identification of structural relationships of the extended environment.982,983... 

The formation of tetrameric aggregates with a hetero-cubane structure is a structural motif that is observed for several alkylzinc alkoxides in the solid state (Figure 80). In these structures the alkoxy group is -bonded with its oxygen atom to three zinc atoms. The structural elucidation of [MeZnOMe]4 (152) represented the first example of such a structural motif in organozinc alkoxide chemistry . Later, similar structures were observed for alkylzinc alkoxides containing other alkoxy groups (153, = f-... 

By contrast to the zinc and cadmium xanthates, for which organoelement xanthate structures are not known, there exists a rich structural chemistry of organomercury xanthates. 

The coordination chemistry of the hexaphyrin system has provided a number of surprises [190]. The addition of nickel chloride to the isomeric mixture of hexaphyrins 250 leads to a single product. The proposed structure of the resulting nickel complex is shown as compound 254 in Scheme 44. Other metals were also found to react with this and other isomeric mixtures of hexaphyrins to give rise to only one isomeric product. For instance, treatment of either 250 or 252 with zinc chloride gives the bimetallic complexes 255 or 256 as one isomer, respectively (Scheme 45). In contrast, the reaction of 253 under identical conditions gives rise to a similar complex 257, wherein, the positions of the ter substituents are different from those reen in the previous examples (Scheme 46). The reactions of hexaphyrins with zinc would appear to indicate that the two isomers normally... 

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