Constitutional (Structural) Isomerism

Some of the forms of isomerism have little more than historic interest now as their sig nificance has diminished with the rise in physical methods that makes their identification and origins routine, and no longer involves the demanding experiments of an earlier era to probe their form. Nevertheless, some remain important, and others at least give a flavour of the historical development of the field, and this deserves a brief discussion. Constitutional isomers are characterized by species of the same empirical formula (which was able to be determined at an early date in the development of the field) but clearly different physical properties associated with different atom connectivity.

Figure 4.24 Hydrate isomers: the complexes possible for the empirical formula CrCl₃·6H₂O. In addition, the neutral and 1+ compounds on the left may in principle exist as two geometric isomers, of which only one is shown.

4.4.2.1 Hydrate Isomerism Hydrate isomerism (sometimes called solvate isomerism) was named to identify an at first puzzling observation, which was that some hydrated compounds with the same empir ical formula were obviously different, in colour, charge and number of ions. A classic example is the inert compound CrCl₃·6H₂O, for which three forms were detected. We know these now as the compounds [CrCl₂(OH₂)₄]Cl·2H₂O, [CrCl(OH₂)₅]Cl₂·H₂O and [Cr(OH₂)6]Cl₃, being distinguished by the groups bound as ligands to the chromium(III) ion, as shown in the views of the complex cations (Figure 4.24); a fourth option, the neutral complex [CrCl₃(OH₂)₃]·3H₂O shown at left in the figure, is not detected due to its reactiv ity in solution, converting readily to other species. The commercial form is the dark green [CrCl2(OH₂)₄]Cl·2H₂O (sufficiently venerable to have once been called Recoura’s green chloride), formed by crystallization from a concentrated hydrochloric acid solution. Upon redissolution in water, substitution reactions to release additional coordinated chloride ions commence. 4.4.2.2 Ionization Isomerism Ionization isomerism is another case defined by recognizing that an empirical formula allows some options for the coordination sphere of the metal. It is essentially the same situ ation as hydrate isomerism, but involves ligands other than water. For example, consider the inert cobalt(III) compound CoBr(SO₄)·₅(NH₃) which forms two different compounds, one violet, the other red. We know these now as [CoBr(NH₃)₅](SO₄) and [Co(NH₃)₅(SO₄)]Br which differ in the choice of which anion occupies the coordination sphere, the other remaining as the counter-ion (Figure 4.25).

Figure 4.25

Ionization isomers: the complexes exemplified differ in which anion of the two present is coordinated to the metal.

Figure 4.26

Coordination isomers: the complexes exemplified differ in which of the two sets of ligands present is coordinated to which metal.

Early experimentalists identified their character through simple chemical reactions. For example, with silver ion, it is only [Co(NH₃)5(SO₄)]Br that produces an immediate precipitate of AgBr, as the Co Br bond in the other form is too strong to permit reaction readily. Likewise, reaction with barium ion causes an immediate precipitate of BaSO₄ only with the [CoBr(NH₃)5](SO₄) form, again because the Co OSO₃ bond in the other isomer is too strong to allow reaction. These experiments allowed identification of differing ionic character in the two compounds.

4.4.2.3 Coordination Isomerism For complex salts where there are metal ions present in both the cation and the anion, both functioning as a complex ion, there are, with two types of ligands a number of possible coordination forms. The simplest is where all of one type of ligand associate with one or the other metal centre; for example, CoCr(CN)6·6NH3 can be either [Co(NH₃)₆][Cr(CN)₆]or [Cr(NH₃)₆][Co(CN)₆] where effectively, metal ions ‘swap’ ligands, both being capable of binding to either (Figure 4.26). Mixed-ligand assemblies on each metal offer more options.

4.4.2.4 Polymerization Isomerism The empirical formula obtained from elemental analysis identifies the ratio of components, not their actual number. Thus an empirical formula MA₂B₂ could be considered as any of [MA2B2]n (forn=1,2,3,...) a series of compounds with the same empirical formula and with the molecular formula of each some multiple of the simplest formula. In an era where we can deter mine the molecular mass and/or three-dimensional structure fairly readily, this is not much of an issue, but was of concern in earlier times. A classical example is for the empirical formula Pt (NH₃)₂Cl₂, which exists not only as the n = 1 form [PtCl₂(NH₃)₂], but also as [Pt (NH₃)4] [PtCl₄] (n = 2) and [PtCl (NH₃)₃]₂[PtCl₄] (n = 3) (Figure 4.27).

Figure 4.27 Polymerization isomers: the complexes exemplified have identical empirical formulae but differ in the number of replications of the empirical formula {Pt Cl₂(NH₃)₂} n.

A more unusual version of the n = 2 form possible is the dimeric complex [(NH₃)₂Pt(Cl)₂Pt(NH₃)₂]Cl₂ where two chloride ions bridge between and link the two metal centres with coordinate covalent bonds. Since the syntheses of the two n = 2 species differ, this is more an intellectual exercise than a difficult case. However, this example does serve to remind us that oligomers (small polymers) are frequently met in coordination chemistry.

4.4.2.5 Linkage Isomerism

There are a number of molecular ligands that contain two different atoms carrying lone pairs, both capable of coordination to a metal ion. These are called ambidentate or ambivalent ligands, distinguished by their capability for binding a metal ion through either of the different donor groups (Figure 4.28). A simple example is the thiocyanate anion (SCN) which offers either a N atom (NSC-N isomer) or a S atom (NCS-S isomer) to metal ions; which one a metal ion selects depends on metal-ligand preferences discussed earlier. For example, the 'hard' Co (III) ion prefers to form Co-NCS complexes with the 'hard' N-donor whereas the 'soft' Pd (II) ion prefers the 'soft' S-donor and forms Pd-SCN complexes.

Another classical example is nitrite ion, which offers N or O atoms as donors. This example has been deeply studied, and the way it behaves is fairly well understood. The O-bound isomer converts (isomerizes) to the thermodynamically stable N-bound isomer, sometimes even in the solid state, by an intramolecular process (without the ligand departing the coordination sphere) in inert complexes. Another feature of ambidentate ligands is that they can display a tendency to 'bridge' between two metal ions, with each of the two different donor atoms attached to one of two metal ions.

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مواضيع ذات صلة

Isomerism– Real 3D Effects

Chameleon Complexes

Moulding a Relationship - Ligand Influences

Metallic Genetics– Metal Ion Influences

Higher Coordination Numbers (ML7 to ML9)

Six Coordination (ML6)

Five Coordination (ML5)

Four Coordination (ML4)

Three Coordination (ML3)

Two Coordination (ML2)

Complex Lifetimes– Together Forever

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