Coordination position isomerism

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... 

A). The diars complex has been isolated in two crystalline forms— monoclinic and orthorhombic. While the former has IR support for perchlorate coordination (65), the latter has X-ray crystal structure evidence (61) for very weak association of perchlorates with the metal ion in the axial positions. The PhNC complex exists in three isomeric forms, and the X-ray crystal structure of one of the forms (green), determined by Jurnak et al. (64), has indicated a perchlorate oxygen at the sixth coordination position around the metal ion with a Co—O distance of 2.594 A and a Co—O—Cl angle of 135.6°. The coordinated O—Cl bond (1.396 A) is longer than two of the uncoordinated ones (1.319, 1.327 A) but is, however, shorter than the third one (1.413 A). Karayannis and co-workers (33,34,39,40,52,53) have characterized a series of perchlorate coordinated cobalt(II) complexes containing the ligands adHNO, phzNO, quxNO, quxNC>2, and adH (Table IV). 

Octahedral models of the cyclic amine show that four possible strain-free planar conformations are possible, as well as two bent arrangements which lead to cis orientation of the remaining two coordination positions. Because each of these six unhindered configurations is distinct from one another, mirror images of each form are possible—even for the trans structures. The latter phenomenon would be without precedent, and further investigation may open a new chapter in inorganic isomerism. 

After insertion, the growing chain can swing back to the coordination position occupied before insertion. This isomerization mechanism, represented in Scheme 3, is usually referred to as site isomerization or backskip of the... 

Hvdrofonnybtion is a homogeneous catalysis reaction which employs a coordination complex of cobelt with the olefin. The catalyst precursor is a cobalt salt which is converted tn siru, in the presence of CO and H , to cobalt tetracarbonyl hydride in equilibrium with cobalt tricarbonyl hydride. The latter, which displays a vacant coordination position, forms a complex with the olefm. The olefin undergoes chemical conversions leading to the production of two isomeric aldehydes, and the normal aldehyde is favored in relation to the iso-compound in the ratio of 4/1. 

In metal complexes, ligands may occupy different positions aroimd the central atom. Since the ligands in question are usually either next to one another cis) or opposite each other (trans), this type of isomerism is often also referred to as cis-trans isomerism. Such isomerism is not possible for complexes with coordination numbers of 2 or 3 or for tetrahedral complexes. In those systems, all coordination positions are adjacent to one another. However, cis-trans isomerism is very common for square planar and octahedral complexes, the only two types to be discussed here. Methods of preparation and reactions of some of these compounds are described in Chapter 4. 

A special type of coordination isomerism is one that involves the different placement of ligands in a bridged complex. This is sometimes called coordination position isomerism, and an example is provided by the isomers... 

Coordinated metal centers may also provide the added nuance of stereoisomerism the component center(s) may possess chirality (i.e., be asymmetric or dissymmetric so that they are nonsuperimposable on their mirror image), or in cases where the coordination geometry of a center involves more than one relative orientation of Ug-ands—for example, square planar (where two coordination positions can be disposed at 90° or 180°), trigonal bipyra-midal (90°, 120°, or 180°), or octahedral (90° or 180°)— geometric isomerism/diastereoisomerism can give rise to alternate forms. Geometrical isomerism also arises in bis(bidentate) or tris(bidentate) centers where the bidentate ligands are unsymmetrical. ... 

A heterometaUic dinuclear complex [(Ph3P)2CIPd(CS2)Pt(PPh3)2] [BF4] also shows coordination position isomerism [3] as shown in Figure 13. 

Structure of 4 symmetry. The compound (CF2)4Fe(CO)4 was originally said (130) to have C—O bands at 2160, 2120, and 2100 cm but more recently four bands at 2160, 2108,2088, and 2055 cm have been reported (112). Four bands is the number expected for an octahedral structure (C2 ) in which the CF2 groups bridge adjacent octahedral coordination positions of the iron atom. The bis(perfluoroalkyl)irontetracarbonyls, (Ri )2Fe(CO)4, show several major carbonyl stretches. Four infrared active C—O stretches would be expected for a cis isomer and one for the trans isomer. Evidently, the cis isomers are present, and the presence of the trans isomer also cannot be ruled out 155). A similar conclusion can be made about the isomeric character of the perfluoroalkyliron tetracarbonyl iodides 155). 

Figure A3.8.1 A schematic diagram of the PMF along the reaction coordinate for an isomerizing solute in the gas phase (frill curve) and in solution (broken curve). Note the modification of the barrier height, the well positions, and the reaction free energy due to the interaction with the solvent.

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. 

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