P.A. Cox - Inorganic chemistry (793955), страница 45
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As the hydroxide M(OH)2 becomes more soluble in the series Mg<Ca <Sr<Ba precipitation requiresincreasingly high pH (see Topic E4).Complex formation is dominated by class a or ‘hard’ behavior (see Topic E3) and is generally most favorable for thesmaller ions.
Beryllium forms [BeF4]2− and strong complexes with some bidentate ligands such as oxalate. Fromcarboxylic acids unusual complexes such as [Be4O(O2CCH3)6] can be obtained; the structure (1) has a central oxygenatom surrounded by a Be4 tetrahedron with acetate groups bridging the edges (only one shown). The larger ions formcomplexes with chelating ligands such as EDTA.
Complexes with ammonia such as [Mg(NH3)6]2+ can be made innonaqueous conditions but are not stable in water. However, chlorophylls, which are essential for photosynthesis inall green plants, have magnesium coordinated by nitrogen in macrocyclic porphine derivatives: 2 shows the basicframework, which has other organic groups attached; Mg2+ normally has one water molecule also coordinated.Solid compoundsBinary compounds are formed with all nonmetallic elements, many by direct combination.
Beryllium is exceptional asits coordination is almost always tetrahedral, giving structures that may be regarded as polymeric rather than highlyionic. Thus BeO has the wurtzite structure (see Topic D3), BeF2 is similar to SiO2, and BeCl2 (like SiS2) has a chainstructure (3) based on edge-sharing tetrahedra. BeH2 is similar, with bridging hydrogens forming three-center bonds asin B2H6 (see Topic C6).G3—GROUP 2: ALKALINE EARTHS197Compounds of the remaining elements have structures more in line with the expectations of the ionic model (seeTopics D3 and D4). Oxides MO all have the rocksalt structure; as the cation size increases they become increasinglybasic and reactive towards water and CO2, giving M(OH)2 or MCO3, respectively. Peroxides such as BaO2 areformed by the heavier elements in the group (see Topic F7).
Halides show increasing coordination with size, six forMg and seven or eight for the larger ions. MgF2 has the rutile structure and the other MF2 compounds the fluoritestructure. Heavier halides of Mg give layer structures (CdCl2 and CdI2) whereas for the larger cations somewhatdistorted structures are formed (e.g. distorted rutile for CaCl2); these appear to be dictated by the tendency toasymmetrical coordination of the halide ion, with cations too large to form normal layer structures. Fluorides(especially CaF2) have low solubility in water, but other halides are extremely soluble.Binary compounds with less electronegative elements include hydrides, nitrides, sulfides and phosphides.
They aredecomposed by water and can provide convenient routes for the preparation of nonmetal hydrides (see Topics B6, F2).The anions may be polyatomic or polymerized, as with CaC2, which containsand reacts with water to giveacetylene (ethyne) C2H2.The elements form an enormous range of compounds with oxoanions, many of those with calcium (carbonate,silicate, phosphate, sulfate) being common minerals in the Earth’s crust. Hydrated forms are common. Their thermalstability towards decomposition to the oxide is less than that for the alkali metals, and increases with cation size.
ThusBe (like Al) does not form a stable carbonate; the decomposition temperatures for the others range from 400°C forMgCO3 to 1400°C for BaCO3. These trends can be understood using lattice energy arguments, as discussed inTopic D6.Organometallic compoundsBe and Mg form an extensive range of organometallic compounds, those of Ca, Sr and Ba being much more reactive anddifficult to characterize. Beryllium alkyls such as Be(CH3)2 have chain structures (see 3 with X=CH3) with multicenterbonding similar to that in Li4(CH3)4 and Al2(CH3)6 (see Topics G2 and G5).
Be and Mg form biscyclopentadienylcompounds M(C5H5)2; the Mg compound has an η5 sandwich structure like that of ferrocene (Topic H10, Structure 3)but is more reactive and at least partially ionic:. The Be compound is less symmetrical with one ring displaced sideways,presumably because of the small size of Be.By far the most commonly encountered organometallic compounds in group 2 are the Grignard reagents RMgX,formed by reaction of Mg metal with an alkyl or aryl halide RX in an ether solvent. Solid compounds with additionalether molecules coordinated to Mg can be obtained, but the reagents are generally used in solution. They are veryuseful for alkylation and arylation reactions, either for forming C-C bonds in organic chemistry, or for preparingorganometallic compounds of other elements (see Topic B6).Section G—Chemistry of non-transition metalsG4GROUP 12: ZINC, CADMIUM, AND MERCURYKey NotesThe elementsMII solution chemistryMII solid compoundsLower oxidation statesOrganometalliccompoundsRelated topicsGroup 12 elements are found in nature as sulfides.
Reactivity andelectropositive character is much less than in group 2, especially formercury.Zn2+ is amphoteric. All elements form strong complexes, Hg havingan exceptional affinity for soft ligands.Most compounds show a marked deviation from ionic character, bothin structures and properties. Typical coordination numbers are fourfor Zn, six for Cd, and two or four for Hg.HgI compounds containwith coordinated ligands. Similarspecies can be formed with Zn and Cd but are much less stable.Compounds R2M and RMX are least reactive with Hg, and are verytoxic.IntroductiontononComplex formation (E3)transition metals (G1)The elementsGroup 12 elements have the electron configurations (n−1)d10ns2 with n=4, 5 and 6 for Zn, Cd and Hg, respectively.They are formally part of the d block but the electrons of the (n−1)d shell are too tightly bound to be involved directlyin chemical bonding, and these elements show typical post-transition metal behavior.
The extra nuclear chargeassociated with filling the d orbitals leads to high ionization energies in comparison with group 2 elements and hencereduced electropositive character (see Topic G1). This is especially pronounced with mercury, which forms fewcompounds that can be regarded as ionic.The elements are found in nature as sulfides, especially ZnS (zinc blende or sphalerite) and HgS (cinnabar). Overallabundances in the crust are low. Zinc is an important element of life; Cd and Hg are not essential and are very toxic (seeTopics J3 and J6).The elements may be obtained by reduction of sulfides or oxides (e.g.
ZnO with C). Zinc and cadmium are used forcorrosion-resistant coatings. The metals have melting and boiling points that are lower than for group 2 elements, especiallywith Hg, which is one of two elements (Br being the other) existing as a liquid at 25°C. Zn and Cd are more reactivethan Hg, dissolving in non-oxidizing acids and forming oxide films in air. Mercury oxidizes at room temperature butHgO decomposes readily on heating, a reaction historically important in the discovery of oxygen. Mercury dissolvesmany metallic elements to form amalgams, which can be useful reagents (e.g.
sodium amalgam as a reducing agent, beingmuch easier to handle than elemental sodium).SECTION G—CHEMISTRY OF NON-TRANSITION METALS199MII solution ChemistryThe aqua ions M2+ are more acidic than those in the same periods in group 2 (see Topic E2). Zinc (like Be) isamphoteric, dissolving at high pH to form [Zn(OH)4]2−.
The other ions are not amphoteric as they have little tendencyto complex with the hard ion OH−, but Hg2+ is very strongly protolyzed and readily precipitates as HgO unlesscomplexing ligands are present.All the ions can form strong complexes, the overall formation constants for tetrahedral [ML4] species (e.g. [HgCl4]2−) with a selection of ligands L being shown in Table 1 (see Topic E3).
There is an increasing tendency to ‘soft’ class bbehavior in the order Zn<Cd<<Hg. Complexes with Hg2+ are among the strongest known with any element. Inaddition to the [ML4] complexes, mercury can form linear [HgL2] and sometimes [HgL3]. As in the solid compounds,these trends indicate a pronounced tendency to covalent bonding; on grounds of size alone, the large Hg2+ ion couldsupport a coordination number of six or even eight.MII solid compoundsOnly the fluorides have structures and properties expected for ionic compounds with cations of the appropriate size(ZnF2 rutile, the others fluorite; see Topics D3 and D4). In other compounds the characteristic coordination numbersare four for Zn, four or six for Cd, and two or four for Hg.Zn and Cd halides (apart from fluorides) are based on close-packed lattices of halide ions, with Zn occupyingtetrahedral holes and Cd octahedral ones.
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