Isomerism complex ions

This type of isomerism occurs when isomers produce different ions in solution, and is possible in compounds which consists of a complex ion with a counter ion which is itself a potential ligand. The pairs [Co(NH3)5(N03)]S04, [Co(NH3)5(S04)]N03 and [PtCl2(NH3)4]Br2, [PtBr2(NH3)4]Cl2, and the series [CoCl(en)2-(N02)]SCN, [CoCl(en)2(SCN)]N02, [Co(en)2-(N02)(SCN)]C1 are examples of ionization isomers. 

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. 

Two or more species with different physical and chemical properties but the same formula are said to be isomers of one another. Complex ions can show many different kinds of isomerism, only one of which we will consider. Geometric isomers are ones that differ only in the spatial orientation of ligands around the central metal atom. Geometric isomerism is found in square planar and octahedral complexes. It cannot occur in tetrahedral complexes where all four positions are equivalent... 

Taking position 1 in Figure 15.4 as a point of reference, you can see that groups at 2,3, 4, and 5 are equidistant from 1 6 is farther away. In other words, positions 1 and 2,1 and 3, 1 and 4,1 and 5 are cis to one another positions 1 and 6 are trans. Hence a complex ion like Co(NH3)4Cl2+ (Figure 15.5) can exist in two different isomeric forms. In the cis isomer, the two Cl- ions are at adjacent comers of the octahedron, as close together as possible. In the trans isomer they are at opposite corners, as far away from one another as possible. 

Linkage isomerism This is a special type of structural isomerism in which the differences arise from a particular ligand which may coordinate to a metal ion in more than one way. In Table 1-3 we indicated that a ligand such as thiocyanate could bond to a metal through either the nitrogen or the sulfur atom, and the complex ions [Co(NH3)5(ACS)]2+ and [Co(NH3)5(5CN)]2+ are related as linkage isomers. 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

The results show that a long wavelength intense band at 5300 A (18,900 cm ) has to be ascribed to the ion A and a band at 4600 A (21,700 cm ) to ion B. This assignment was supported by calculations (Verrijn Stuart and Mackor, 1957). The proton addition complexes of isomeric methylbenz[a]anthracenes may be regjffded as examples of ions A and B (Table 6). 

Chromium in the trivalent state forms a variety of salts, the most important and the simplest being the violet salts, which liberate in aqueous solution chromium cation Cr" A green series of chromic salts, isomeric with the violet salts, liberate in aqueous solution some chromium cation, whilst part of the chromium is present as a complex ion. With weak acids, sulphurous, hydrocyanic, or thiocyanic acids, the chromic ion forms complex ions of great stability. Finally, a very large group of salts exists where chromium associated with ammonia forms the complex ion, the chromi-ammines. 

A three-site system for peptide synthesis around a cobalt(III) complex has been studied. Instead of a quadridentate ligand as used in the above experiments, Wu and Busch chose the tridentate ligand diethylenetriamine. The formation of dipeptide and tetrapeptide complexes is shown in Scheme 92.360 The ester carbonyl group in the 0-bonded amide intermediate (127) cannot be activated by coordination because it cannot reach the metal ion. Isomerization to the jV-bonded amide complex (128) occurs with base and enables coordination and therefore activation of the ester carbonyl group. 

The first type of structural isomerism we will consider is coordination isomerism, in which the composition of the complex ion varies. For example, [C NH-d.sSO Br and [Cr(NH3)5Br]S04 are coordination isomers. In the first case S042- is coordinated to Cr3 +, while Br acts as the counter ion in the second case the roles of these ions are reversed. 

Geometrical isomerism also occurs in octahedral complex ions. For example, the compound [Co(NH3)4Cl2]Cl has cis and trans isomers (Fig. 20.12). 

Geometrical isomers are not necessarily optical isomers. For instance, the trans isomer of [Co(en)2Cl2]+ shown in Fig. 20.17 is identical to its mirror image. Since this isomer is superimposable on its mirror image, it does not exhibit optical isomerism and is therefore not chiral. On the other hand, cis-[Co(en)2Cl2]+ is not superimposable on its mirror image thus a pair of enantiomers exists for the complex ion, making the cis isomer chiral. 

Does the complex ion fCo(NH3)Br(en)2] exhibit geometrical isomerism Does it exhibit optical isomerism ... 

The complex ion exhibits geometrical isomerism, since the ethylenediamine ligands can be across from or next to each other ... 

The cis isomer of the complex ion also exhibits optical isomerism since its mirror images,... 

Linkage isomerism isomerism involving a complex ion where the ligands are all the same but the point of attachment of at least one of the ligands differs. (20.4)... 

The reaction between Pd(acac)2 and lithium / -diketiminate, a nitrogen derivative of acac, gives a mixed-ligand and homoleptic complex. Interestingly, these complexes are stable at ambient temperature, but in acetonitrile solution they decompose to elemental Pd presumably due to reaction of Pd(II) and the acac ligand. Fmthermore, in pentane or ether solution the complex can isomerize into the thermodynamically stable form. Both isomers are similar, but their NMR spectra are inequivalent due to an asymmetric structure with nonrotating substituents. Since one of the imine groups is coordinated to the Pd ion, this isomerization results in the formation of a chiral center at the coordinated C atom. 

NMR was used to assign the geometrical isomerism the fac isomers show only two methyl signals, whereas the mer isomers display a complex spectrum. The crystal structure of the A fac isomer was claimed to confirm the assignment (50) however, the complex ion was disordered and the structure refinement was not completed (51). The structure determination of the mer complex has recently been completed (52) The chelate bite angle is 84.3(9)°. 

An excellent resolving agent for many dissymmetric anionic metal complexes is [Co(C204)(en)2]. This complex ion is prepared easily from inexpensive materials and the optical isomers are relatively stable to isomerization in aqueous solutions. Previously, the optical isomers of [Co(C204)(en)2] were separated by means of diastereoisomer formation with [Co(edta)] . This... 

The steric structures presented explain the early detection of isomery (isomerism) of the complex compounds of Co, Pt, etc., as well as predict the number of geometric(al) isomers of the complex ion. 

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