Ionic Molecules Containing Complex Ions

SCHEME 8.5 Constructing the ionic molecule (Na+)3P04 from neutral Na and the modular neutral fragment PO4 with the click-claek approach. 

SCHEME 8.6 The ionic molecules made from NH4 combined with Cl and with 804 . Note The different stoichiometry is determined by the different charges of the two anions. The formulae below the names of these materials constitute the implicit way chemists write ionic materials by deleting the charges of the constituent ions. 

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The commonest polymorphic changes of this type are those associated with the onset of free rotation in a crystal structure. Many simple molecular compounds (HC1, HBr and CH4 are examples) show a transition with rising temperature from a complex structure to a simple close-packed arrangement in which the molecules effectively acquire spherical symmetry by free rotation. Similar effects are displayed by a number of ionic crystals containing complex ions of un-symmetrical shape. Thus NaCN, KCN and RbCN have complex structures at low temperatures but transform at higher temperatures to the sodium chloride structure in which the CN ions behave as spherical entities. In some cases rotation may take place only about one axis, so that the molecule or group acquires cylindrical rather than spherical symmetry, and in other cases rotations about different axes may be excited successively at different temperatures. An extreme example of this is ammonium nitrate, in which both cation and anion are capable of... 

Evidently many crystals contain bonds of two or more quite distinct types. In molecular crystals consisting of non-polar molecules the bonds within the molecule may be essentially covalent (e.g. 85 or Sg) or of some intermediate ionic-covalent nature (e.g. Sip4), and those between the molecules are van der Waals bonds. In a crystal containing complex ions the bonds within the complex ion may approximate to covalent bonds while those between the complex ion and the cations (or anions) are essentially ionic in character, as in the case of NaNOg already quoted. In other crystals there are additional interactions between certain of the atoms which are not so obviously essential as in these cases to the cohesion of the crystal. An example is the metal-metal bonding in dioxides with the rutile structure, a structure which in many cases is stable in the absence of such bonding. 

The simplest systems containing ionic bonds are the gaseous molecules of alkali halides and oxides, the structures of which are noted in Chapters 9 and 12 we refer later to the halide molecules in connection with polarization. The importance of the ionic bond lies in the fact that it is responsible for the existence at ordinary temperatures, as stable solids, of numerous metallic oxides and halides (both simple and complex), of some sulphides and nitrides, and also of the very numerous crystalline compounds containing complex ions, particularly oxy-ions, which may be finite (CO3 , NO3, SOl", etc.) or infinite in one, two, or three dimensions. 

Many ionic compounds contain what used to be referred to as water of crystallization . For example, magnesium chloride can exist as a fully hydrated salt which was formerly written MgCla.bHjO, but is more appropriately written Mg(OH2)eCl2, since the water molecules occupy coordination sites around the magnesium ions. This is typical. In most compounds that contain water of crystallization, the water molecules are bound to the cation in an aquo complex in the manner originally proposed by Alfred Werner (1866-1919) in 1893 (Kauffman, 1981). Such an arrangement has been confirmed in numerous cases by X-ray diffraction techniques. 

Cations [(RR C=Se)2I]+ In the case of A-methylbenzothiazole-2(3//)-selone (mbts), one equivalent of iodine provides a molecular adduct in dichloromethane solution 50 crystallisation, however, leads to a ionic compound of the type [(RR C Se)2I]+I3, 43 This compound was the first one reported in the literature featuring a two coordinated iodine(I) complex with two donor molecules containing selenocarbonyl groups. In the solid state two molecules of mbts are linearly coordinating a central I+ ion to give a slightly... 

The strong affinity of ionic liquids for aromatics has been attributed to the formation of liquid clathrates (90-92). Liquid clathrates (93) are semi-ordered liquids containing complex salt hosts. They are formed by associative interactions between aromatic molecules and salt ions, which separate cation-anion-packing interactions to a sufficient degree that localized cage structures are formed. Although the aromatic compounds are highly soluble in the ionic liquid phase, the... 

The solubility of an ionic compound increases dramatically if the solution contains a Lewis base that can form a coordinate covalent bond (Section 7.5) to the metal cation. Silver chloride, for example, is insoluble in water and in acid, but it dissolves in an excess of aqueous ammonia, forming the complex ion Ag(NH3)2 + (Figure 16.13). A complex ion is an ion that contains a metal cation bonded to one or more small molecules or ions, such as NH3, CN-, or OH-. In accord with Le Chatelier s principle, ammonia shifts the solubility equilibrium to the right by tying up the Ag+ ion in the form of the complex ion ... 

The important explanation is still that these substances contain the involved complex ions plus water molecules in a 3-dimensional ionic lattice. If one continues to heat the blue crystal the color of the crystals turns white because the water molecules leave the ionic lattices and only the copper ions and sulfate ions are building up the new ionic lattice (see E9.2) ... 

Indeed, when the deuterated phosphine Me2PD is used, the complexity of many of the reactions are revealed. For example for the P transfer reactions shown in equation 26, the ionic product contains no deuterium, instead it is lost in the neutral C2H3D. Further, it has been noted that the ionic products (R3P2 ) of reactions 24 and 26 could in principle have structure 11 or 12 . The latter structure requires a rearrangement reaction to take place. R3P/ ions are also formed in the gas-phase ion chemistry of Me2PPMe2 (equation 28) and its mixture with PH3 (equations 29 and 30) . The ion-molecule reactions shown in equations 31 and 32 are indicative of structure 12 . ... 

Complex ions are any ions that are built up by the union of simpler ions, or of an ion and one or more neutral molecules. They are usually formed around metallic ions. They may be held together entirely by coordinate hnks, as in Cu(NH3)4 and the other metal-ammonia complexes, or they may be bound by a combination of coordinate links and normal covalent links, as in Hgl4 or Fe(CN)6 . They may even be held together by purely ionic or electrostatic forces, as is the case in FeFe this complex is shown by its large magnetic moment to contain a free ferric ion, yet it is extremely stable, since the small size of the fluoride ion allows for close approach and a strong electrostatic force between Fe" and F . 

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