Coordination compounds, hydrates

The physical and chemical properties are less well known for transition metals than for the alkaU metal fluoroborates (Table 4). Most transition-metal fluoroborates are strongly hydrated coordination compounds and are difficult to dry without decomposition. Decomposition frequently occurs during the concentration of solutions for crysta11i2ation. The stabiUty of the metal fluorides accentuates this problem. Loss of HF because of hydrolysis makes the reaction proceed even more rapidly. Even with low temperature vacuum drying to partially solve the decomposition, the dry salt readily absorbs water. The crystalline soflds are generally soluble in water, alcohols, and ketones but only poorly soluble in hydrocarbons and halocarbons. 

Ammonia forms a great variety of addition or coordination compounds (qv), also called ammoniates, ia analogy with hydrates. Thus CaCl2 bNH and CuSO TNH are comparable to CaCl2 6H20 and CuSO 4H20, respectively, and, when regarded as coordination compounds, are called ammines and written as complexes, eg, [Cu(NH2)4]S04. The solubiHty ia water of such compounds is often quite different from the solubiHty of the parent salts. For example, silver chloride, AgQ., is almost iasoluble ia water, whereas [Ag(NH2)2]Cl is readily soluble. Thus silver chloride dissolves ia aqueous ammonia. Similar reactions take place with other water iasoluble silver and copper salts. Many ammines can be obtained ia a crystalline form, particularly those of cobalt, chromium, and platinum. 

Solvates are perhaps less prevalent in compounds prepared from liquid ammonia solutions than are hydrates precipitated from aqueous systems, but large numbers of ammines are known, and their study formed the basis of Werner s theory of coordination compounds (1891-5). Frequently, however, solvolysis (ammonolysis) occurs (cf. hydrolysis). Examples are ... 

This section is almost entirely concerned with the kinetics of solid phase decompositions of classical coordination compounds, since most of the information available refers to these substances. The hydrates, in which the ligands are water only, are correctly classified under the present heading, but as their dehydrations have been so intensively studied, a separate section (Sect. 1) has been devoted to the removal of water from crystalline hydrates. A separate water elimination step also preceeds many decomposition reactions. 

The hydrated ion [Cu(H20)6]2+ is an example of a complex, a species consisting of a central metal atom or ion to which a number of molecules or ions are attached by coordinate covalent bonds. A coordination compound is an electrically neutral compound in which at least one of the ions present is a complex. However, the terms coordination compound (the overall neutral compound) and complex (one or more of the ions or neutral species present in the compound) are often used interchangeably. Coordination compounds include complexes in which the central metal atom is electrically neutral, such as Ni(CO)4, and ionic compounds, such as K4[Fe(CN)6]. 

Figure 16.18 summarizes the types of isomerism found in coordination complexes. The two major classes of isomers are structural isomers, in which the atoms are connected to different partners, and stereoisomers, in which the atoms have the same partners but are arranged differently in space. Structural isomers of coordination compounds are subdivided into ionization, hydrate, linkage, and coordination isomers. 

Coordination compounds Metal salt hydrates Metal and nonmetal oxides Metals and alloys Clays and minerals Soil... 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

Kekule s instability criterion failed completely in the case of many coordination compounds, which were classified as molecular compounds by sheer dint of necessity although they were extremely resistant to heat and chemical reagents. For example, although hexaamminecobalt(III) chloride contains ammonia, it neither evolves this ammonia on mild heating nor does it react with acids to form ammonium salts. Also, addition of a base to its aqueous solution fails to precipitate hydrated cobalt(III) hydroxide. 

CTTS transitions in coordination compounds result in a radial movement of electron density from the metal to the surrounding solution medium. The energies of these transitions generally are very sensitive to environmental parameters such as solvent polarity, temperature and the presence of salts.104 This sensitivity has been used in a diagnostic sense to identify CTTS bands in the spectra of anionic cyanide complexes105 and 1,2-dithiolene complexes of Ni, Pd and Pt.106 Hydrated cations such as Cr2+(aq) and Fe2+ (aqj exhibit absorption bands that are sometimes referred to as CTTS in character. Since the solvent occupies the first coordination sphere of the metal, however, the distinction between CTTS and CTTL transitions in these systems becomes obscured. 

As a consequence of the fact thal many valence bonds leave residual electrical fields, many molecules in which the "primary" valences are satisfied can combine further with other molecules or with atoms. These higher combinations enter into many important areas of chemical science. They are the basis or the formation of coordination compounds, discussed under that heading. They cause molecular association. They are responsible for the formation of hydrates. They are in many cases Ihe binding forces in nonionic solids, and are of great importance in explaining the structure or larger material aggregates. 

Included in the coordination compounds are the double salts, the complex salts, the oxysalts, and the hydrates. 

The term "cutionic water" may be used to describe the situation in which water appears in coordination compounds apparently joined to cations by covalent bonds. However, the fact that a number of such compounds exhibit "hydrate isomerism" is evidence for cationic bonding, as well as it is lor the existence of other harms of these compounds in which Ihe presence of Water is due to electrostatic attractions or ctystal stability requirements. 

Example CuS04-5H20. hydrated Having water molecules attached, hydrate isomers Isomers that differ by an exchange of an H20 molecule and a ligand in the coordination sphere, hydration (1) (of ions) The attachment of water molecules to a central ion. (2) (of organic compounds) The addition of water across a multiple bond (H to one carbon atom, OH to the other). Example ... 

A large class of coordination compounds, metal chelates, is represented in relation to microwave treatment by a relatively small number of reported data, mainly p-diketonates. Thus, volatile copper) II) acetylacetonate was used for the preparation of copper thin films in Ar — H2 atmosphere at ambient temperature by microwave plasma-enhanced chemical vapor deposition (CVD) [735a]. The formed pure copper films with a resistance of 2 3 pS2 cm were deposited on Si substrates. It is noted that oxygen atoms were never detected in the deposited material since Cu — O intramolecular bonds are totally broken by microwave plasma-assisted decomposition of the copper complex. Another acetylacetonate, Zr(acac)4, was prepared from its hydrate Zr(acac)4 10H2O by microwave dehydration of the latter [726]. It is shown [704] that microwave treatment is an effective dehydration technique for various compounds and materials. Use of microwave irradiation in the synthesis of some transition metal phthalocyanines is reported in Sec. 5.1.1. Their relatives - porphyrins - were also obtained in this way [735b]. 

The terms hydrate, ammoniate, alcoholate, and etherate have long been used by chemists in referring to addition compounds. It seems preferable, however, when such compounds may be appropriately regarded as coordination compounds to refer to them as such ... 

A vexing problem concerning the study of substitution reactions of coordination compounds is this water, which is by far the best solvent for inorganic complexes, is one of the most nucleophilic of the usual solvents. When an attempt is made to carry out a reaction between a complex and an added nucleophile in water, the water almost always intercedes, forming a hydrate and in many cases reaction ceases with... 

The oxyanions of phosphorus and arsenic take up the major portion of a recent large volume of Gmelin (C9), which includes more than 100 compounds, hydrates and phases containing phosphate groups.263 To pursue the detail of the latter is an exercise in phosphorus chemistry, so we concentrate on a survey of the available data on the Mn" coordination polyhedra. 

A number of thorium nitrate complexes have been synthesized and studied. Hydrated thorium nitrate, Th(N03)4.5H20, contains [Th(N03)4.(H20)3] molecules and was one of the first 11-coordinate compounds to be recognized (Figure 11.6). 

The most numerous cases of homogeneous catalysis are by certain ions or metal coordination compounds in aqueous solution and in biochemistry, where enzymes function catalytically. Many ionic effects are known. The hydronium ion H3O and the hydroxyl ion OH catalyze hydrolyses such as those of esters ferrous ion catalyzes the decomposition of hydrogen peroxide decomposition of nitramide is catalyzed by acetate ion. Other instances are inversion of sucrose by HCl, halogenation of acetone by H and OH , hydration of isobutene by acids, hydrolysis of esters by acids, and others. 

---