Question

Indicate what type of isomerism may be found in each of the following cases. If no isomerism is possible, so indicate. (a) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{CuCl}_{4}\right]\) (b) \(\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{SCN}\right]^{4-}\) (c) \(\left[\mathrm{NiCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{+}\) (d) \(\left[\mathrm{PtBrCl}_{2}(\mathrm{py})\right]\) (e) \(\left[\mathrm{Cr}(\mathrm{OH})_{3}\left(\mathrm{NH}_{3}\right)_{3}\right]^{-}\)

Short answer

The solution to the exercise involves the following types of isomerism: (a) Coordination Isomerism, (b) Linkage Isomerism, (c) No Isomerism, (d) Ionisation Isomerism, (e) No Isomerism

Step-by-step solution

Identification

Identify the metal, ligands and their arrangement in the complex. Note down their charge, number and nature (whether they are same or different)

isotomerism of \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{CuCl}_{4}\right]\)

This compound shows coordination isomerism - it's possible to swap the metal ions between the two coordination spheres, forming \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]\left[\mathrm{ZnCl}_{4}\right]\)

isotomerism of \(\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{SCN}\right]^{4-}\)

This compound can exhibit linkage isomerism, since the \(\mathrm{SCN}\) ligand can bind to \(\mathrm{Fe}\) through either the sulfur or nitrogen atom, yielding the isomer \(\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NCS}\right]^{4-}\)

isotomerism of \(\left[\mathrm{NiCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{+}\)

No isomerism is possible in this compound because all ligands are identical.

isotomerism of \(\left[\mathrm{PtBrCl}_{2}(\mathrm{py})\right]\)

This compound would show ionisation isomerism as \(\mathrm{Br}\) can be replaced by \(\mathrm{Cl}\) or \(\mathrm{py}\) to form the isomer \(\left[\mathrm{PtCl}_{2}\left(\mathrm{py}\right)_{2}\right]Br\)

isotomerism of \(\left[\mathrm{Cr}(\mathrm{OH})_{3}\left(\mathrm{NH}_{3}\right)_{3}\right]^{-}\)

No isomerism is possible in this compound because all ligands are identical.

Key concepts

Linkage Isomerism

Linkage isomerism occurs in complex ions where a ligand has the potential to attach to the central metal atom in more than one way. This is typically seen with ambidentate ligands which have two different donor atoms. For example, the thiocyanate ion (SCN-) can attach to the metal through the sulfur (S) or nitrogen (N) atom.

In our exercise, the iron complex text([Fe(CN)_5 SCN]^{4-})
illustrates this concept, as the SCN ligand could bind to Fe either through the S to form text([Fe(CN)_5 SCS]^{4-})
or through the N to form text([Fe(CN)_5 NCS]^{4-})
. These are two distinct linkage isomers. Understanding linkage isomerism is essential for students, as the binding site can significantly influence the chemical properties of the complex.

Ionization Isomerism

Ionization isomerism occurs when ions inside and outside the coordination sphere can be exchanged without changing the overall charge of the complex. This typically results in compounds that have different properties like solubility or reactivity, despite having the same formula. For example, a sulfate ligand inside the coordination sphere could be replaced by a bromide ion, resulting in a compound that precipitates differently.

In an example from the exercise,text([PtBrCl_2(py)])
can exhibit ionization isomerism by having either Br or py move in or out of the coordination sphere, creating different ions while keeping the complex balanced. Students often find this conceptually challenging, yet its comprehension is pivotal for understanding reactions in solution chemistry.

Complex Ions

Complex ions are formed when a central metal atom or ion is surrounded by molecules or ions called ligands. These ligands can provide lone pairs of electrons, which coordinate to the metal center to create a coordination complex. These species are crucial in many aspects of inorganic chemistry, including catalysis, electronic spectra, and biological systems.

For instance, the zinc complex from the exercise,text([Zn(NH_3)_4][CuCl_4])
is made up of two complex ions: text [Zn(NH_3)_4]^2+
and text[CuCl_4]^2-
. Each one consists of a central metal ion and the surrounding ligands. The coordination number, or the number of ligand attachments to the metal, can vary, leading to diverse structural arrangements that are paramount for the stability and function of the complex.

Transition Metal Complexes

Transition metal complexes consist of a transition metal ion surrounded by ligands. The unique electronic structure of transition metals allows them to form these versatile complexes, which play a central role in many chemical processes, including industrial catalysis and the transport of oxygen in blood.

The exercise presents different transition metal complexes, showcasing their diversity. One notable example is the nickel complex,text([NiCl(NH_3)_5]^+)
which features a central nickel ion coordinated by five ammonia ligands and one chloride ligand. These complexes can have a variety of geometries such as octahedral, square planar, or tetrahedral, influencing their electronic properties and reactivity. The understanding of these geometries and the dative covalent bonding in transition metal complexes is fundamental for students studying coordination chemistry.