Coordination compounds shapes

Many complexes and coordination compounds exist as isomers, compounds that contain the same numbers of the same atoms but in different arrangements. For example, the ions shown in (13a) and (13b) differ only in the positions of the Cl ligands, but they are distinct species, because they have different physical and chemical properties. Isomerism is of more than academic interest for example, anticancer drugs based on complexes of platinum are active only if they are the correct isomer. The complex needs to have a particular shape to interact with DNA molecules. 

Bowl-shaped coordination compounds have been assembled spontaneously from 10 small components, such as six (en)Pd2+ units and four 2,4,6-tri(3-pyridinyl)triazene moieties the overall structure approaches 3oA <00JA2665>. A different construct was derived from the 4-pyridinyl isomer and Pt(bpy)2+ and shown to facilitate self-assembly within the coordination... 

During the last ten years, studies of luminescence and photochemistry of polypyridyl Ru(II), Rh(III) and Co(III) complexes, porphyrins and uranyl salts, in the presence of biological macromolecules such as DNA, have been the focus of increasing research work. The interest in such coordination compounds stems from their easily tunable properties. Not only their size and shape but also their... 

The angular functions for the s and p. orbital are illustrated in Fig. 2.5. For an s orbital, cl> is independent of angle and is of constant value. Hence this graph is circular or, more properly, in three dimensions—spherical. For the p. orbital we obtain two tangent spheres. The px and py orbitals are identical in shape but are oriented along the x and y axes, respectively. We shall defer extensive treatment of the d orbitals (Chapter 11) and / orbitals (Chapter 14) until bond formation in coordination compounds is discussed, simply noting here that the basic angular function for tl orbitals is fout-iobed and that for / orbitals is six-lobed (see Fig. 2.91... 

Valence-bond representation Molecular-orbital representation it bonding and multicenter it bonds Shapes of molecules Coordination compounds Isomerism Bonding in metals... 

Fig. 8. tma-molecules bond in a flat adsorption geometry at a copper surface and are resolved as equilateral triangle in STM. The sequence of STM images reveals how the thermal motion of molecules at the surface proceed following tma rotational motions and displacements a Cu atom is captured whereupon a cloverleaf-shaped Cu(tma)4 coordination compound evolves (second image for t = 80s, central Cu atom highlighted in red) [87],... 

Platinum (IV) Structures. The oxidation state of the platinum atom in platinum coordination compounds determines the steric configuration of the molecule platinum(II) structures are planar molecules, while platinum(IV) derivatives assume an octahedral shape. Though it was hoped that these differences could be used to circumvent platinum resistance, the two compounds developed in the clinic, iproplatin and ormaplatin, have not proven useful. In the case of the former, testing in Phase-II trials failed to reveal activity. In the case of ormaplatin, the platinum(IV) configuration is not maintained under biological conditions conversion to a platinum(II) metabolite occurs within minutes [14], A series of novel platinum(IV) and mixed ammine/amine derivatives being developed at the Institute for Cancer Research are described in this volume by Kelland. 

Structural thinking in inorganic chemistry, particularly in the field of coordination compounds, is of more recent development and stems chiefly from the massive body of research by A. Werner in the early twentieth century it was Werner who established the importance of the coordination number, and by chemical means alone gave us our present picture of the shapes of coordination compounds. 

Furthermore, inorganic compounds present coordination geometries different from those found for carbon. For example, although 4-coordinate carbon is nearly always tetrahedral, both tetrahedral and square planar shapes occur for 4-coordinate compounds of both metals and nonmetals. When metals are the central atoms, with anions or neutral molecules bonded to them (frequently through N, O, or S), these are called coordination complexes when carbon is the element directly bonded to metal atoms or ions, they are called organometaUic compounds. 

This chapter describes a sampling of the different shapes of coordination compounds. Because of the complex factors involved in determining shapes of coordination compounds, it is difficult to predict shapes with any confidence except when compounds of similar composition are already known. It is possible, however, to relate some structures to the individual factors that interact to produce them. This chapter also describes some of the isomers possible for coordination compounds and some of the experimental methods used to study them. Structures of some organometallic compounds are even more difficult to predict, as will be seen in Chapters 13 through 15. 

The overall shape of a coordination compound is the product of several interacting factors. One factor may be dominant in one compound, with another factor dominant in another. Some factors involved in determining the structures of coordination complexes include the following ... 

Because of their shapes, there can be different coordination compounds having exactly the same atoms and bonds, but arranged differently. Such molecules are called geometric isomers. 

The five-coordination compounds show some more exotic possibilities. PF5 has a trigonal bipyramidal shape with inequivalent axial and equatorial positions. The lone pair in SF4 chooses an equatorial position since it can do less damage there— it make a 90° angle with two F atoms, whereas an axial position would make 90° angles with three F atoms. This leaves SF4 with a shape resembling a distorted. seesaw. The two lone pairs in CIF3 are most stable when located in two equatorial positions, separated by 120". This leaves Cll 3 in a distorted tee shape. The eomplex that forms between 1 and l> in iK iieous solution is a linear ion. 

Self-assembly methodologies have been used for the designed synthesis of a range of discrete, metallocyclic 2-D and 3-D coordination compounds. The topologies of many such compounds resemble well-known geometric shapes, so that they are widely referred to as molecular polygons (2-D) or molecular polyhedra (3-D). 

The calculated moments of inertia of molecules may be used to define the shapes of coordination compounds and cluster molecules. This shape analysis may be related either to electronic factors responsible for distorting the geometry of the molecule or used to define the location of the molecule on the rearrangement coordinate linking alternative polytopal forms. 

This example illustrates the complexity of the excited states dynamics in this class of molecules. The early stage dynamical behaviour (in the first ps) may be tailored by the metal centre, the a-diimine group or the surrounding ligands. These chemical factors govern the character and electronic localization of the excited states, their relative position, the presence of critical geometries. Moreover the shape and relative positions of the PES may be also modified by the other experimental conditions like solvent effects. Obviously the development of new theoretical tools able to compute accurate multidimensional PES is needed to investigate the dynamics of photochemical reactions in transition metal coordination compounds. 

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