Complex ions shapes

Central unit Ligand Co-ordination number Ligand type Complex ion Shape... 

The d orbital splitting depends on the oxidation state of a given ion hence twb complex ions with the same shape, ligands and coordination number can differ in colour, for example... 

Many complex ions, such as NH4+, N(CH3)4+, PtCle", Cr(H20)3+++, etc., are roughly spherical in shape, so that they may be treated as a first approximation as spherical. Crystal radii can then be derived for them from measured inter-atomic distances although, in general, on account of the lack of complete spherical symmetry radii obtained for a given ion from crystals with different structures may show some variation. Moreover, our treatment of the relative stabilities of different structures may also be applied to complex ion crystals thus the compounds K2SnCle, Ni(NH3)3Cl2 and [N(CH3)4]2PtCl3, for example, have the fluorite structure, with the monatomic ions replaced by complex ions and, as shown in Table XVII, their radius ratios fulfil the fluorite requirement. Doubtless in many cases, however, the crystal structure is determined by the shapes of the complex ions. 

The structures of ionic compounds comprising complex ions can in many cases be derived from the structures of simple ionic compounds. A spherical ion is substituted by the complex ion and the crystal lattice is distorted in a manner adequate to account for the shape of this ion. 

Although the ligand field theory can be used to rationalize the geometry of some transition metal molecules and complex ions, the study of the shapes of transition metal molecules in terms of the electron density distribution is still the subject of research and it has not reached a sufficient stage of development to enable us to discuss it in this book. 

There are many ligands in addition to water, for example Cl , NH3, CN , N02, and transition metal ions, in particular, form a large number of complex ions with different ligands. The number of ligands surrounding the central atom, or ion, is called the coordination number. The numerical value of the co-ordination number depends on a number of factors, but one important factor is the sizes of both the ligands and central atom, or ion. A number of complex ions are given below in Table 2 9. The shape of complex... 

Several AFM studies examined the effect of buffer conditions on the formation of motifs with DNA molecules. For instance, it has been foimd that the multivalent cations induce the condensation of DNA molecules into higher ordered structures, including toroids and rods [122], More specifically, Zn and Mg ions induce the formation of DNA kinked and perfect circles, respectively [123] (Fig. 16). Also, higher concentrations of spermidine induce the formation of complex flower-shaped structures with single crossover points [122] and increased concentrations of ethanol lead to complex and looped structures [ 124] (Fig. 17). 

Many mineral species have the same or similar chemical basic units within their atomic structure. All common silicate minerals, for example, are characterized by the association of four large oxygen ions (0 ) bonded to a small silicon ion (Si ). The shape of the complex ion is a tetrahedral unit, with the composition (Si04) . The two- and the three-dimensional expressions of the silicate ion are presented in Fig. 2.1, parts A and B, respectively. The three-dimensional figures emphasize the potential variations in orientation between the ions as they have been observed in minerals. 

Coordina- tion Number Shape Ligand Structure/Formula Name of complex ion/neutral complex... 

Much attention is currently devoted to the synthesis and properties of shape-persistent macrocycles[l]. Such compounds are interesting for a variety of reasons including formation of columnar stacks potentially capable of performing as nanopores for incorporation into membranes or for the generation of nanowires[2]. Furthermore, in shape-persistent macrocycles incorporating coordination units, enc/o-cyclic metal-ion coordination may be exploited to generate nanowires[3], whereas e.ro-cyclic coordination can be used to construct large arrays of polynuclear metal complexes[4]. Shape-persistent macrocycles with reactive substituents may also be linked to other units to yield multicomponent, hierarchical structures. 

Consider the electroplating of copper, an established industrial process. It is well known to the expert in the field that fast plating of thick layers can be achieved in a so-called acid bath, which consists of CuSO in H SO (with some additives, which need not concern us at this point). If, on the other hand, one wishes to obtain a smooth and uniform deposit on an intricately shaped body, an alkaline cyanide bath is better. The alkaline bath consists of copper ions in an excess of KCN (kept at high pH, to prevent the formation of volatile and highly poisonous HCN). In this bath copper exists as the negatively charged complex ion [Cu(CN) ]. Now, we recall that to achieve uniform current... 

Perhaps the most obvious connection of polyhedra with practical chemistry and crystallography is that crystals normally grow as convex polyhedra. The shapes of single crystals are subject to certain restrictions arising from the fact that only a limited number of types of axial symmetry are permissible in crystals, as explained in Chapter 2. We shall not be concerned here with the external shapes of crystals but with polyhedra which are of interest in relation to their internal structures and more generally to the structures of molecules and complex ions. 

The special interest of these compounds lies in the shape of the coordination group around the metal atom in the complex ion. 

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