Coordination compounds geometric isomers

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... 

Geometric isomers occur as cis-trans isomers in compounds with double bonds like in N2F2 and especially when coordination polyhedra have different kinds of ligands. The... 

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 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 ... 

Dichloro-bis (2,4-pentanedionato) zirconium(IV) is monomeric and a weak electrolyte in nitrobenzene solution n.m.r. chemical shifts,7 infrared and Raman spectra,8 and dipole-moment studies21 indicate that this compound exists in solution as the octahedral cis geometrical isomer. Chloro- and bromotris(2,4-pentanedionato) zirconium (IV) are seven-coordinate complexes which are monomeric in benzene and only slightly dissociated in nitrobenzene and 1,2-dichloroethane. Iodotris(2,4-pen-tanedionato) zirconium (IV), however, is appreciably dissociated both in nitrobenzene and in 1,2-dichloroethane.7... 

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. 

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

The proposals of Werner (49) about the structure of coordination compounds were amply justified by the chemical reactions exhibited by them, by the identification of geometrical isomers, and by the resolution of certain compounds into their optical antipodes. Modern methods for the determination of structures and for studying the nature of charged or neutral species in solution have demonstrated the fundamental soundness of Werner s views. 

In geometric isomers, or cis-trans isomers, of coordination compounds, the same ligands are arranged in different orders within the coordination sphere. Geometric isomerism occurs when atoms or groups of atoms can be arranged on two sides of a rigid structure. Cis means adjacent to and trans means on the opposite side of. Cis- and trans-diamminedichloroplatinum(II) are shown below. 

Stereoisomers are compounds that are made up of the same types and numbers of atoms bonded together in the same sequence but with different spatial arrangements. There are two types of stereoisomers geometric isomers and optical isomers. Coordination compounds may exhibit one or both types of isomerism. Note, however, that many coordination compounds do not have stereoisomers. 

The introduction of chelate ligands usually acts to limit the number of geometric isomers, or at least to not extend the number. For example, the coordination of three symmetrical didentate chelates can occur in only one way, in the same sense that six identical monoden-tate ligands can coordinate in only one way. Likewise, M(AA)2B2 compounds (where (AA) refers to a symmetric chelate) has precisely the same number and type of diastereomers as... 

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