Ammines and hydrates

A comparison of the empirical formulae of hydrates and ammines leads to the following conclusions  

The nitrates of many divalent metals crystallize with 6 H2O (Fe, Mn, Mg, etc.), but a few nitrates have odd numbers of water molecules, e.g. Fe(N03)3. 9 (and 6) H2O, Cu(N03)2. 3 H2O. Where the corresponding ammine is known it has an even number of NH3, as in Cu(NH3)4(N03)2. There are many other series of hexahydrated salts-perchlorates, sulphites, bromates, etc.-and although in such cases the ammines often contain 6 NH3, the structures of ammine and hydrate are quite different-compare, for example, the structure of [Co(H20)6] (0164)2, described on p. 556, with the simple fluorite structure of [Co(NH3)6] (0164)2. 

The reason for the non-existence of ammines containing large odd numbers of NH3 molecules is presumably that some of these molecules would have to be accommodated between the cation coordination groups and the anions (compare the environment of the fifth H26 in OUS64.5 H26 or the seventh H26 in NiS64.7 H26). The NH3 molecule does not have the same hydrogen-bond-forming capability as H26, particularly to other NH3 molecules. 

M—NH3 bonds are essentially electrostatic, and that even in these cases there are significant differences between the crystal structures of the two compounds MX . n NH3 and MX . n H2O. This latter point has already been dealt with the former is best illustrated by a comparison of the ammines and hydrates of the salts of the metals of Groups I and II of the Periodic Table. 

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In the single crystal absorption spectra of CrF2 and CrCl2 the pattern of spin-allowed (quintet-quintet) bands for the ammines and hydrates is repeated. Gaussian analysis of the higher wavenumber bands has allowed assignments of all three transitions and calculation of... 

In spite of such resemblances, however, it is important to note that there are also significant differences between ammines and hydrates. One of these differences is implicit in what we have already said, for we find that in many cases ammines have simpler structures than the corresponding hydrates, owing to the less polar character of the ammonia in comparison with water. Thus although Mg(NH3)6Cl2 has the fluorite structure, this arrangement is not found in Mg(H20)6Cl2 because it is inconsistent with the tetrahedral form of the water molecule. 

A second difference between ammines and hydrates, again arising from the small dipole moment of the ammonia molecule, is that strong bonds cannot be formed between these molecules they are therefore found in ammines only in a co-ordinating and never in a structural capacity. For this reason the ammine counterparts of hydrates with an odd number of water molecules do not exist, and it is interesting to note, as an example of this point, that cupric sulphate forms only the hydrated ammine Cu(NH3)4S04. H20 and not the compound Cu(NH3)4S04. NH3. 

Patil K C, Secco E A (1972), Metal halide ammines. II. Thermal analyses, calorimetry and infrared spectra of fluoride ammines and hydrates of bivalent metals , Can. J. Chem., 50, 567-573. 

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. 

In 1893 Werner founded his new constitutional formula for inorganic compounds, applied the theory to the systematic classification of the chromi-ammines, and found that all the chromi-ammines which had been investigated could be fitted in to his system of classification. Since then the chemistry of the chromi-ammines has been further developed hv Werner, Pfeiffer, and many others relationships have been traced between chromi-ammines, complex salts, and chromic salt hydrates, and numerous cases of isomerism have been discovered in this series of ammines. 

The chromi-ammines show - very clearly the parallelism between hydrated salts and ammino-salts. It has been proved that water may be gradually substituted for ammonia in the metal-ammines, and in the hexammino-salts of chromium all degrees of substitution, with, the... 

Claus first postulate was vigorously attacked by Karl Weltzien (1813—1870),40 while Hugo Schiff (1834—1915)43 attacked not only Claus first postulate but also his second. All of Claus three postulates reappeared modified almost four decades later in Werner s coordination theory. Claus third postulate closely adumbrates Werner s concepts of the coordination number and of the transition series between metal ammines and metal salt hydrates. 

Osmium Di-ammine Hydroxide, OsO(NH3)2(OH)2, results when the tetroxide is dissolved in concentrated aqueous ammonia and heated in a closed vessel to 50° C. On opening the vessel and evaporating the excess of ammonia, osmosammine hydroxide is obtained as a dark brown powder.2 It unites with acids to form salts, and upon being heated readily decomposes. When boiled with alkalies ammonia is evolved and hydrated osmium dioxide remains. ... 

In summary, it is now agreed that coordination concepts are valuable in explaining a wide variety of inorganic phenomena of theoretical and practical nature, such as the stabilization of unusual oxidation states, analytical implications of metal complexes, and the industrial use of com-plexing agents. Aside from the ammines and the hydrates, discussed mostly by Werner, there are many important types of coordination compounds, such as complex cyanides of heavy metals, metal carbonyls (formed in the catalysis of petroleum products and used to produce metals), and others. 

Vapor pressure eudiometer. This apparatus was originally devised as a tool for analytical checking of the course of decomposition of ammines or hydrates, but it is also useful for preparatory purposes, e.g., when determination of a definite stage of decomposition is desired. Figure 85 shows the construction of this device. The substance is enclosed in the smallest possibleflaska and a definite quantity of the volatile component is removed from it. The right leg of the manometer m is calibrated in milliliters from the zero position (both legs at equilibrium) down. The volumes of a, of the various tube sections between stopcocks and 3,... 

In fact, Werner played such a central and almost monopolistic role in coordination chemistry that his name is virtually synonymous with the field. Even today, almost 75 years after his death in 1919, coordination compounds, particularly metal-ammines, are still colloquially called Werner complexes. The coordination theory not only provided a logical explanation for known "molecular compounds, but also predicted series of unknown compounds, whose eventual discovery lent further weight to Werner s controversial ideas. He showed how ammonia could be replaced by water or other groups, and he demonstrated the existence of transition series between ammines, double salts, and hydrates. Werner recognized and named many types of inorganic isomerism such as coordination isomerism, polymerization isomerism, ionization isomerism, hydrate isomerism, salt isomerism, coordination position isomerism, and valence isomerism. He also postulated explanations for polynuclear complexes, hydrated metal ions, hydrolysis, and acids and bases. His view of the two types of chemical... 

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 ... 

Uptake measurements were made [16] at several oxide/solution ratios, reported as surface loading (SL) or m2 oxide surface/liter of solution, as PdCl, 2 concentration was increased and pH was held constant at the optimal value (Figure 6.10a). Each SL indeed indicated a plateau near the steric value [16], For Pt and Pd ammine cations, the maximum surface density over many oxides appears to be a close-packed layer, which retains two hydration sheaths representative results for PTA uptake over silica from a recent paper [19] are shown in Figure 6.10b. The physical limit of cationic ammine surface density thus appears to be 0.84 pmol/m2, or about 1 cationic complex/2 nm2. Cationic uptake, therefore, is inherently half of anion uptake in many cases. 

The measured uptake of CPA and PTA over the three activated carbons [55] is shown in Figure 6.28, and the trends predicted by the RPA model in Figure 6.27 are at least qualitatively observed. However, at high pH, over the two highest-surface-area carbons (CA and KB), uptake is about half of that predicted by the RPA model. The discrepancy was explained [55] by steric exclusion of the large Pt ammine complexes, believed to retain two hydration sheaths [15,19], from the smallest micropores of the high-surface-area activated carbon. 

Figure 10. Plots of the normalized selectivity coefficient (K ) for Cu in zeolite X (, o), Y (A,A) and mordenite (t,v). Empty symbols hydrated ion filled symbols amminated copper. Reproduced with permission from Ref. 123. Copyright 1980, Heyden Son Ltd.

ZnO films for use as buffer layers in photovoltaic cells (see Chap. 9) have been chemically deposited from aqueous solutions of ZnS04 and ammonia [57]. The solution was heated to 65°C, and adherent, compact Zn(OH)2 + ZnO films were formed after one hour. Low-temperature annealing converted the hydroxide to oxide. The solution composition will be important in this deposition. On one hand, increased ammonia concentration will increase the pH and therefore the homogeneous Zn(OH)2 precipitation in solution. However, further increase in ammonia concentration will redissolve the hydroxide as the ammine complex. There will clearly be an optimum ammonia (and zinc) concentration where Zn(OH)2 does form, but slowly enough to prevent massive homogeneous precipitation. The use of ammonia in (hydr)oxide deposition derives, in part at least, from its gradual loss by evaporation if the system is not closed [58], Any open solution of an ammonia-complexed metal ion (which forms an insoluble hydroxide or hydrated oxide) should eventually precipitate the (hydr)oxide for this reason alone. 

Isomerism in the Metal-ammines.—Werner claimed for the coordination theory that in certain cases isomerism should occur, that isomerism being brought about by different causes. lie divided isomerism in the ammines into five groups, namely, structure isomerism, ionisation isomerism, hydrate isomerism, polymerism, and stereoisomerism. 

Hydrate Isomerism.—As its name implies, this depends on the position of water in the molecule, just as in the case of the acido compounds. If two or more molecules of water are present in a molecule of ammine, the water may be present within the co-ordination complex or outside of it. For instance, the compound Cr en2.(H20)2.Br3 exists in isomeric forms. It may have all the water within the complex, in which case the formula will be [Cr en2(H20)2]Br3. In solution the whole of the bromine is precipitated by silver nitrate. On the other hand, the compound may have one molecule of water in the complex and the other outside, in which case the formula is [Cr en2(IT20)Br]Br2.H20, and only two-thirds of the bromine are precipitated by silver nitrate. Another example of this kind occurs in the cobalt series chloro-aquo-tetrammino-cobaltic chloride, [Co(NTI3)4Cl.H20]Cl2, is violet in colour, and is isomeric with dichloro-tetrammino-cobaltie chloride monohydrate, [Co(N1I3)4CI2]C1.H20, which is green. 

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