Octahedral molecules geometrical isomers

Among the commonest types of diketo complex are those with the stoichiometries M(dike)3 and M(dike)2. The former all have structures based on an octahedral disposition of the six oxygen atoms. The tris(chelate) molecules then actually have >3 symmetry and exist as enantiomers. When there are unsymmetrical diketo ligands (i.e., those with R R"), geometrical isomers also exist, as indicated in (11-XXVI). Such compounds have been of value in investigations of the mechanism of racemiza-tion of tris(chelate) complexes. Chiral diketonate complexes have found many... 

Where a molecule is fairly symmetrical different structures can be distinguished using vibrational spectroscopy. This method has been used, for example, to characterize geometrical isomers of substituted octahedral complexes. The point groups and symmetry classes of the normal v(C0) vibrations for octahedral complexes M(CO) L. are listed in Table 5.6. Sometimes infrared spectroscopy... 

Additional opportunities for optical activity arise in molecules containing chelating ligands (A-A). For example, a compound of general formula M(A-A)2B2 has two geometric isomers, one of which is chiral. The propeller-like tris-chelated M(A-A)3 compounds are always chiral. Werner and his collaborators took advantage of the ready optical activity of such complexes to provide additional proof that the coordination spheres of his many and varied cobalt compounds were octahedral. After 20 years of work, Werner s coordination theory was thoroughly established, and he received the Nobel Prize in chemistry in 1913. In a final positive proof, Werner and co-workers... 

The term cis describes geometric isomers in which two groups are attached on the same side of a double bond in an organic molecule, or along the same edge of a square in a square-planar complex, or at two adjacent vertices of an octahedral complex. (See also geometric isomerism.) cis-trans isomerism is a type of stereoisomerism. 

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 ... 

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