Isomerism in coordination complexes

Figure 1 Broad classification of isomerism in coordination complexes.

Figure 16.18 summarizes the types of isomerism found in coordination complexes. The two major classes of isomers are structural isomers, in which the atoms are connected to different partners, and stereoisomers, in which the atoms have the same partners but are arranged differently in space. Structural isomers of coordination compounds are subdivided into ionization, hydrate, linkage, and coordination isomers. 

Na—Np isomerism Perhaps the most immediately obvious source of isomerism in metal complexes of tridentate azo compounds is the involvement of different nitrogen atoms of the azo group in coordination to the metal. This is not relevant in the case of symmetrical azo compounds but two structurally isomeric metal complexes (79) and (80) are conceivable for unsymmetrical azo compounds. These have been designated by Pfitzner78 as Na and Np isomers. 

SCF and CAS SCF calculations on mono and bimetallic transition metal hydride complexes are reported. The importance of including the non dynamical correlation elTects for the study of the cis-trans isomerism in dihydrido complexes and for the study of the CO insertion reaction into the metal hydride bond is stressed. The metal to metal hydrogen transfer in a class of bimetallic d — d hydride complexes is analyzed and the feasibility of the transfer discussed as a function of the coordination pattern around the two metal centers. 

Since the dcm ligand is a C2-symmetric P-diketone, there is no cis/trans (facAner) isomerization in the complex. As a result, the number of outer sphere coordination sites is reduced from 12 to 4 (two for the A and two for the A isomers). Spectral averaging of fewer isomeric complexes may account for the larger differentiation by this ligand. 

The present discussion of isomerism in coordination compounds is not, nor was it intended to be, comprehensive and exhaustive. The examples considered are an eclectic selection, and many important systems may have been neglected through ignorance. An obvious omission is any detailed consideration of polynuclear complexes and it is, of course, a quite arbitrary... 

The previous examples demonstrate optical isomerism in octahedral complexes. Tetrahedral complexes can also exhibit optical isomerism, but only if aU four coordination sites are occupied by different ligands. Square planar complexes do not normally exhibit optical isomerism as they are superimposable on their mirror images. 

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