Coordination compounds geometrical

The physical and chemical properties of complex ions and of the coordination compounds they form depend on the spatial orientation of ligands around the central metal atom. Here we consider the geometries associated with the coordination numbers 2,4, and 6. With that background, we then examine the phenomenon of geometric isomerism, in which two or more complex ions have the same chemical formula but different properties because of their different geometries. 

Alfred Werner s research on geometrically isomeric coordination compounds. G. B. Kaufmann, Coord. Chem. Rev., 1975, 15,1-92 (564). 

Reactions in which isomerization of coordination compounds occur in solutions are common, and some reactions of this type in solid complexes have been studied. Generally, there is a change in color of the complex as the crystal field environment of the metal ion changes. Accordingly, some of the color changes that occur when complexes are heated may indicate isomerization, but very few geometrical isomerization reactions in solid complexes have been studied in detail. One such reaction is... 

It is especially important to investigate the molecular structure of coordination compounds in the vapor phase because the relatively weak coordination interactions may be considerably influenced by intermolecular interactions in solutions and especially in crystals. It has been shown that the geometrical variations can be correlated with other properties of the molecular complexes ). In particular the structural changes in the F3B N(CH3)3 and CI3B N(CH3)3 molecules ) relative to the respective monomeric species unambiguously indicated boron trichloride to be a stronger acceptor than boron trifluoride. Data on the geometry and force field have also been correlated ). 

Effects of Isomerism Geometrical isomers of coordination compounds can exhibit different values of redox potential, as accounted for, in various cases, by simple djr orbital level splitting diagrams or by MO calculations. The dependence of the relative stability and redox behavior of the geometrical isomers on the electronic configuration of the metal is also... 

As noted previously, the classification of stereoisomers preferred by contemporary organic chemists is the enantiomer-diastereomer dichotomy17 and this may be quite conveniently applied to coordination compounds. Thus, complexes (9a) and (9b) are enantiomers, but (9a) and (9c), and (9b) and (9c), are diastereomers. Older terminology might have led to the description of A and B as optical antipodes and to (A+B) and C as geometrical isomers. 

The intensity distributions of well resolved vibronic spectra recorded in absorption and emission at low temperature are used to determine the geometric distortions of the electronically excited states of coordination compounds. In particular for complexes of lower symmetry, band analysis is necessary leading to results with which bond distance changes can be calculated. For spectra exhibiting no vibrational fine structure, a new technique is proposed which uses time resolved methods, considering deviations from the Poisson distribution of photons by recording time intervals between two successively emitted photons. 

The most striking feature in diastereoisomerism of metal complexes is the very rapidly growing number of isomers, by introducing different elements of chirality in the basic framework of a coordination compound. The complex formation with l,8-diamino-4-methyl-3,7-diazaoctane (5-Metrien) for example — a ligand with a single asymmetric center — leads not only to four geometric isomers. Three of these show a structure containing all four types of chiral elements mentioned, so that the system offers 28 possibilities of isomers. 

The planar square molecular structure turns out to be common among coordination compounds. The other common structure is octahedral, where the metal is at the center of an eight-sided geometric solid with six vertices, formed by joining two square pyramids at their base. For example, a platinum atom can be linked to four chlorine atoms and two molecules of ammonia in this fashion. If the four chlorine atoms are at the corners of the square, and the ammonia molecules at the apexes, then they form one optical isomer. If one of the ammonia molecules and three of the chlorine atoms form the square, and one chlorine atom and one ammonia molecule occupy the apexes, then we have another optical isomer. The two forms are enantiomers of one another ... 

In the following year, 1905, Werner published his great book on coordination chemistry, and this had a powerful influence on Lewis. Werner proposed that in coordination compounds, atoms or groups of atoms surrounded a central atom to form an electrically charged ion or a neutral compound, and the geometrical or structural theory seemed to fit very nicely with Lewis s ideas. All that was needed (and it was a big all ) was a clearer picture of the electrical nature of atoms. 

Exercise 1.3. Consider a coordination compound formed from BH3 and C2H4. From the HOMO-LUMO properties of each species predict the geometric structure of the Lewis acid-base adduct. Now predict the structure of a compound formed by replacing one CO ligand of Fe(CO)5 with C2H4. Note the parallelism between the main group and transition metal examples. The second compound is a stable and isolatable compound, whereas the first is a transient intermediate in the hydro-boration of ethylene to ethyl borane and has only been characterized as a transient intermediate in a fast-flow system by modulated mass spectrometry. 

Ligand substitution is one of the most characteristic reactions of coordination compounds. If in the early stages of the development of coordination chemistry a ligand substitution reaction served as a synthetic method, then later on, especially after Werner, such reactions were widely employed both to solve structural problems (viz., geometric isomerism), and to elucidate the nature of a trans effect. 

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. 

An example of a biologically active geometric isomer is cisplatin, a coordination compound used in medicine to suppress tumors. The molecule consists of a platinum atom surrounded by two ammonia molecules and two chlorine atoms. The four ligands lie at the comers of a square, with ligands of the same kind as neighbors. 

The tetraamminedinitrocobalt(lll) salts represent the second longest known case of geometric isomerism among coordination compounds. The reddish-yellow trans compounds were first prepared in 1875 by air oxidation of a solution of cobalt(II) chloride containing ammonia, sodium nitrite, and ammonium chloride by Wolcott Gibbs, who called them croceo salts. The brownish-yellow cis compounds were first prepared in 1894 by reaction of tetraamminecarbonato-cobalt(III) salts with sodium nitrite by Jorgensen, who called them flavo salts. The two series correspond completely to the bis(ethylenediamine)dinitrocobalt-(111) salts. 

How many geometric isomers does the octahedral coordination compound [Co(NH3)3Cl3] have. ... 

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