Co-ordination isomerism

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

The proposed mechanism of the bond shift isomerization of neopentane is shown in Scheme I Cl-3). There are now good models for each step in the proposed sequence, but no simple transition metal complex can accomplish all steps since there cannot be sufficient co-ordination sites. The first steps involve a,y-dinstallation of the alkane, for which there are good precedents in both platinum and iridium chemistry (4, 5, 6). The... 

Ionisation Isomerism.—This type of isomerism is very common in the metal-ammines. If two or more different acidic radicles are present in a molecule of metal-ammine, the acidic radicles may be firmly fixed in the co-ordination complex or may be outside of this. If they are outside the complex they are easily ionised and easily freed by other acids if, on the other hand, they are within the complex they are not ionised and are difficult to free by other acids. This distribution of the acidic radicles in the complex, or outside of it, gives rise to ionisation isomerism. Tor example, the compound Co(NH3)5Br(S04) is known in two forms, one violet and the other red. The violet modification in aqueous solution contains —S04" ions, which may be precipitated by barium chloride. The red variety gives no — S04" ions in aqueous solution, and barium sulphate is not precipitated by barium chloride. These two substances, bromo-pentammino-eobaltic sulphate and sulphato-pentammino-eobaltic bromide, are ionisation isomers, and are represented as ... 

Hydrate Isomerism.—As its name implies, this depends on the position of water in the molecule, just as in the case of the acido compounds. If two or more molecules of water are present in a molecule of ammine, the water may be present within the co-ordination complex or outside of it. For instance, the compound Cr en2.(H20)2.Br3 exists in isomeric forms. It may have all the water within the complex, in which case the formula will be [Cr en2(H20)2]Br3. In solution the whole of the bromine is precipitated by silver nitrate. On the other hand, the compound may have one molecule of water in the complex and the other outside, in which case the formula is [Cr en2(IT20)Br]Br2.H20, and only two-thirds of the bromine are precipitated by silver nitrate. Another example of this kind occurs in the cobalt series chloro-aquo-tetrammino-cobaltic chloride, [Co(NTI3)4Cl.H20]Cl2, is violet in colour, and is isomeric with dichloro-tetrammino-cobaltie chloride monohydrate, [Co(N1I3)4CI2]C1.H20, which is green. 

Co-ordination Position Isomerism.—Here the isomerism occurs through the arrangement of the groups around each cobalt atom with respect to the bridged linkage. For example, there are two dibromo-hexammino-amino-peroxo-dicobaltic bromides which are represented thus ... 

Ammino-derivatives op Cobalt Salts—Cobaltous Salt Ammines—Cobaltic Salt Ammines—Mononuclear Cobalt-ammines containing One Atom of Cobalt in the Molecule—Cobaltic Salts with Trivalent Cation—Cobalt-ammines Containing Divalent Cation—Cobalt-ammines containing Monovalent Cation—Cobalt-ammines consisting of Non-dissociable Complex— Cobalt-ammines containing Monovalent Anion—Cobalt Salts containing Trivalent Anion—Polynuclear Cobalt-ammines containing Two or more Cobalt Atoms in the Molecule—Cobalt-ammines of Unknown Constitution— Ionisation Metamerism—Polymerisation Isomerism—Valency Isomerism —Co-ordination Position Isomerism—Isomerism due to Asymmetric Cobalt Atoms. 

The complex is additionally stabilised by co-ordination of the phenoxide, and possibly the carboxylate, to the metal ion, illustrating the utility of chelating ligands in the study of metal-directed reactivity. We saw in the previous section the ways in which a metal ion may perturb keto-enol equilibria in carbonyl derivatives, and similar effects are observed with imines. The metal ion allows facile interconversion of the isomeric imines. The first step of the reaction is thus the tautomerisation of 5.28 to 5.29 (Fig. 5-56). Finally, the metal ion may direct the hydrolysis of the new imine (5.29) which has been formed, to yield pyridoxamine (5.30) and the a-ketoacid (Fig. 5-57). 

Amino-acid Complexes. X-Ray crystal structures have been reported for many cobalt(m) amino-acid complexes. Potassium dinitrobis(P-alaninato)cobaltate(m) has octahedral co-ordination about the cobalt, trans nitro-groups, and a trans arrangement of amino N- and carboxylato O-donors from the bidentate P-alaninates.371 In Ca[Co(aspar-tate)2] there are two isomeric ions the cis(N)trans Os) (64) and cis(lSl)trans 06) (65),... 

The condensation of o-aminobenzamide and pyridine-2-carbaldehyde produced the terdentate ligand (129) which partially isomerizes in methanol, in the presence of nickel(n) nitrate, to form the cyclic ligand (130). The nickel complex [Ni( 129)(130)-(H20)](N03)2,4H20 has been X-rayed, and the results show that (129) bonds as a terdentate N20 donor, and (130) as a bidentate N2 ligand, the co-ordinated water molecule completing the irregular octahedron about the nickel.575... 

Rate constants and activation energies for liquid- and gas-phase isomerization of a-pinene have been determined.310 The activity of metal sulphate monohydrates in isomerizing a-pinene is correlated with the strength of co-ordination of the water of crystallization to the metal ion.3" Pyrolysis of chrysanthanol acetate (217 R = Ac) gives citronellal and the (E)- and (Z)-3,7-dimethylocta-l, 6-dien-l -ol acetates in 20, 28, and 3% yields respectively formation of the enol acetates is consistent with a biradical or a concerted pathway.312 Further work directed towards C-l—C-7 bond pyrolysis of pinane derivatives shows C-l—C-7 C-l—C-6 bond cleavage ratios of 4 51 for (217 R = Ac), 13 22 for (217 R = H), 6 7 for (218 R = H), and 43 35 for (218 R = Me) the expected acyclic and cyclic alcohol, aldehyde, and ketone pyrolysis products are obtained.313 The ene reaction between /3-pinene and methyl... 

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