Structure and Bonding in Coordination Compounds 447 For an octahedral complex having the general formula ML4X2 (for example, [Co(NH3)4Cl2]+), there are two possible isomers that have cis and trans structures:NH3 H3N Co H3N NH3 cis-(violet colored) Cl H3N Cl trans-(green colored) Cl H3N Co Cl NH3 NH3
If the complex has the formula ML3X3, there are two possible isomers. For [Co(NH3)3Cl3], the two isomers have structures shown as follows.Cl H3N Co H3N NH3 1, 2, 3- or facial (fac) (a) Cl H3N Cl 1, 2, 6- or meridional (mer ) (b) Cl H3N Co NH3 Cl Cl
In (a), the three chloride ions are on one face of the octahedron. In (b), the three chloride ions are occupying positions around an edge (a meridian) of the octahedron. Therefore, the names include fac and mer to indicate the structures. As the number of different groups in the formula increases, the number of possible isomers increases rapidly. For example, if a complex has the general formula MABCDEF (where M represents a metal and A, B, . . . , represent different ligands), a large number of isomers are possible.
19.3.2 Optical IsomerismFor the dichlorobis(ethylenediamine)cobalt(III) ion, [Co(en)2Cl2]+, two geometrical isomers, cis and trans, are possible. As was described in Chapter 2, a plane of symmetry (mirror plane) divides a molecule into equal fragments. For the trans isomer (shown in Figure 19.1), there is a plane of symmetry that bisects the cobalt ion and the ethylenediamine ligands with one chloride ion on either side. As shown in Figure 19.1, the cis isomer has no plane of symmetry. For molecules that do not possess a plane of symmetry, the mirror images are not superimposable. It is a property of such molecules that they rotate a beam of polarized light. If the beam is rotated to the right (when looking along the beam in the direction of propagation) the substance is said to be dextrorotatory (or simply dextro). Those substances www.elsevierdirect.com
448 Chapter 19Cl en Cl en Co en en Cl trans-isomer Nonsuperimposable cis-isomers Co Cl Mirror plane Cl en Cl Co en
Figure 19.1 The three isomers of [Co(en)2Cl2)]+.
that rotate the plane of polarized light to the left are said to be levorotatory (or simply levo). A mixture of equal amounts of the dextro and levo forms is called a racemic mixture, and it gives no net rotation of polarized light. Isomers of complexes that rotate polarized light in opposite directions are said to exhibit optical isomerism.
19.3.3 Linkage IsomerismAlthough the existence of linkage isomers is usually illustrated by recourse to the original example of the cobalt complexes [Co(NH3)5NO2]2+ and [Co(NH3)5ONO]2+ studied by S. M. Jrgensen, a great many other cases are now known. Several monodentate ligands contain two or more atoms that can function as electron pair donors, and three such ligands are CN, SCN, and NO2, which have the following structures:C N 2 S C N 2 O N 2 O
In writing formulas for complexes containing these ions, the atom through which these ambidentate ligands is attached is usually written closest to the metal, as in the formulas [Pt(SCN)4]2 (in which SCN is S-bonded) and [Co(NH3)3NO2]2+ (in which NO2 is N-bonded). It is not surprising that many of the known cases of linkage isomerism involve the ions shown earlier because each of them forms so many complexes. In a general way, one can often predict which of two linkage isomers is more stable on the basis of the hard-soft interaction principle that depends on the electronic character of the metal and the ligand (see Chapter 5). For example, the sulfur atom in SCN is a large and polarizable (soft) electron pair donor, whereas the nitrogen atom is a smaller and harder donor. Thus, in accord with the similar properties of the metal ions and donor atoms, Pt2+ (soft, large, polarizable) forms [Pt(SCN)4]2, but Cr3+ (hard, small, low polarizability) forms [Cr(NCS)6]3. Although there is more than one possible linkage isomer in each of these cases, usually only one isomer is known. However, in numerous cases, a complex containing an ambidentate www.elsevierdirect.com
Structure and Bonding in Coordination Compounds 449 ligand bound in one way can be converted to the other linkage isomer, but such reactions are usually irreversible. A number of linkage isomerization reactions have been studied in fairly great detail. Far fewer cases of linkage isomerism occur in which the ligands are neutral molecules, although such complexes are theoretically possible for ligands such as pyrazine N-oxide. This molecule has the structureO N
and it could coordinate to metal ions through either the oxygen atom or the nitrogen atom that has an unshared pair of electrons. Compounds containing ligands such as these are usually found to contain the ligand bound in only one way, and the other isomer is unknown. Frequently, such ligands (as well as CN, SCN, and NO2) function as bridging ligands bonded to two metal ions simultaneously. In one such case, [(NH3)5CoNCCo(CN)5] and [(NH3)5CoCNCo(CN)5], both isomers have been well characterized.
19.3.4 Ionization IsomerismWhen solid complexes are prepared, they usually contain a cation and an anion. An exception is a complex such as [Co(NH3)3Cl3] that is a neutral species. Considering two complexes such as [Pt(en)2Cl2]Br2 and [Pt(en)2Br2]Cl2, it can be seen that they have the same empirical formula but they are not the same compound. In fact, when the first of these is dissolved in water, [Pt(en)2Cl2]2+ and Br ions are obtained. In the second case, [Pt(en)2Br2]2+ and Cl ions result. This is a situation in which different ions exist in solution (and of course in the solid also), and such a type of isomerism is called ionization isomerism. In general, the complexes are stable enough so that rapid exchange of the ions in the coordination sphere and those in the solvent does not occur. If exchange did occur, it would result in the same product being formed in solution regardless of which complex was initially dissolved. Other examples of ionization isomers include the following pairs: CoNH3 4 ClBrNO2 CoNH3 4 Br2 Cl CoNH3 5 ClNO2 PtNH3 4 Cl2 Br2 and CoNH3 4 ClNO2 Br and CoNH3 4 ClBrBr and CoNH3 5 NO2 Cl and PtNH3 4 Br2 Cl2 www.elsevierdirect.com
450 Chapter 19
19.3.5 Coordination IsomerismCoordination isomers exist when there are different ways to arrange several coordinated ligands around two metal ions. For example, isomers are possible for a complex having the composition [Co(NH3)6][Co(CN)6] because formulas for other complexes having the same composition are [Co(NH3)5CN][Co(NH3)(CN)5] and [Co(NH3)4(CN)2][Co(NH3)2(CN)4], both of which still provide a coordination number of 6 for each metal. Other examples of this type of isomerism include the pairs shown here: CoNH3 6 CrCN6 Pten2 PtCl6 CuNH3 4 PtCl4 and and and CrNH3 6 CoCN6 Pten2 Cl2 PtCl4 PtNH3 4 CuCl4
19.3.6 Polymerization IsomerismThis type of isomerism is actually named incorrectly because polymers are usually not involved at all. The name stems from the fact that the composition of a polymer (an aggregate of monomer units) is the same as that of a monomeric unit that has a lower formula weight. In complexes, the term polymerization isomerism refers to the fact that a larger formula unit has the same overall composition as a smaller unit. Thus, [Pd(NH3)4][PdCl4] has the same empirical formula as [Pd(NH3)2Cl2]. With regard only to the empirical formula, [Pd(NH3)4][PdCl4] is a polymer of the compound [Pd(NH3)2Cl2] that has one-half the formula weight but the same composition. Other examples of this type of isomerism are illustrated by the following pairs: CoNH3 3 NO2 3 CoNH3 3 NO2 3 CrNH3 3 CN3 and and and CoNH3 6 CoNO2 6 CoNH3 5 NO2 CoNH3 NO2 5 CrNH3 6 CrCN6
The polymer is in reality only a complex having a higher formula weight and the same composition as a simpler one, but it does not involve repeating units as in the case of polymeric materials.
19.3.7 Hydrate IsomerismBecause many complexes are prepared in aqueous solutions, they are frequently obtained as crystalline solids that contain water of hydration. Water is, of course, also a potential ligand. As a result, isomeric compounds can sometimes be obtained in which water is coordinated in one case but is present as water of hydration in another. Compounds that differ in this www.elsevierdirect.com
Structure and Bonding in Coordination Compounds 451 way are called hydrate isomers. Because water is a neutral molecule, there must be one anion that is also held in a different way in the isomers. An example of this type of isomerism is illustrated by the compounds [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2 H2O. Other examples include the following pairs: CoNH3 5 H2 ONO2 3 Crpy2 H2 O2 Cl2 Cl and and CoNH3 5 NO2 NO2 2 H2 O Crpy2 H2 OCl3 H2 O
Although other types of isomerism in coordination compounds exist, the types described in this section represent the most important types.
19.4 Factors Affecting the Stability of Complexes19.4.1 The Nature of the Acid-Base InteractionIt has been recognized for many years that in a general way the basicity of the ligands has a great influence on the stability of complexes. After all, the formation of the coordinate bond is an acid-base reaction in the Lewis sense. However, as usually measured, basicity is toward the proton in aqueous solution. It sometimes provides a measure of the availability of electrons that might be expected when the ligands form coordinate bonds to metal ions. The basicity of a base, B, toward H+ is measured by the equilibrium constant, KHB, for the reaction H :B H :B KHB 19:1
In an analogous way, the coordination tendency of ligands toward silver ions is measured by the equilibrium constants for the reactions Ag :B Ag :B Ag :B :B B:Ag :B K1 K2 19:2 19:3
In general, the relationship between the Kb values fo