Question

Sketch the geometry of (a) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\) (tetrahedral) (b) cis-[ \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\) \(\left.\) (c) trans-[Pt(NH \(\left.\left._{3}\right)_{2} \mathrm{Br}_{2}\right]^{2+}\) (d) \(\operatorname{trans}-\left[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\right]^{3-}\) (e) \([\mathrm{Au}(\mathrm{CN}) \mathrm{Br}]^{+}\)

Short answer

Question: Sketch the geometry of the following coordination complexes and identify the metal center and the type of ligands bonded to the metal center: (a) \([\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\] (b) cis-\(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^+\) (c) trans-\(\left[\mathrm{Pt}(\mathrm{NH}_{3})_{2} \mathrm{Br}_{2}\right]^{2+}\) (d) trans-\(\left[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\right]^{3-}\) (e) \([\mathrm{Au}(\mathrm{CN}) \mathrm{Br}]^{+}\)

Step-by-step solution

(a) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\)

The metal center in this complex is Zn, and it is bonded to two NH3 and two Cl ligands. The coordination number is 4. This implies that the geometry of the complex is tetrahedral, with the Zn atom at the center and the NH3 and Cl ligands connected to the corners of the tetrahedron.

(b) cis-\(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^+\)

In this complex, the metal center is Co, and it is bonded to four H2O and two Cl ligands. The coordination number is 6. The geometry of the complex is octahedral, with the Co atom at the center. Since it is a cis-isomer, the two Cl ligands will be adjacent to each other in the octahedral geometry.

(c) trans-\(\left[\mathrm{Pt}(\mathrm{NH}_{3})_{2} \mathrm{Br}_{2}\right]^{2+}\)

The metal center in this complex is Pt, and it is bonded to two NH3 and two Br ligands. The coordination number is 4. This implies that the geometry of the complex is square planar, with the Pt atom at the center. Since it is a trans-isomer, the two NH3 ligands will be opposite each other in the square planar geometry, and the two Br ligands will also be opposite each other.

(d) trans-\(\left[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\right]^{3-}\)

In this complex, the metal center is Ni, and it is bonded to two oxalate (ox) and two hydroxide (OH) ligands. The coordination number is 6. The geometry of the complex is octahedral, with the Ni atom at the center. Since it is a trans-isomer, the two oxalate ligands will be opposite each other in the octahedral geometry, and the two hydroxide ligands will also be opposite each other.

(e) \([\mathrm{Au}(\mathrm{CN}) \mathrm{Br}]^{+}\)

The metal center in this complex is Au, and it is bonded to one CN and one Br ligand. The coordination number is 2. This implies that the geometry of the complex is linear, with the Au atom at the center, the CN ligand on one side, and the Br ligand on the other side.

Key concepts

Complex Ions

Complex ions are composed of a central metal atom or ion that is bonded to one or more ligands. Ligands are ions or molecules that have lone pairs of electrons and can donate them to the metal, forming coordinate covalent bonds.

A perfect example to illustrate this idea is the complex ion \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\). The central metal, zinc (Zn), forms bonds with two ammonia (NH₃) molecules and two chloride (Cl^-) ions. In this tetrahedral structure, it’s essential to point out that even though there are only four ligands, each forms a significant interaction with the zinc ion.

When creating complex ions, the coordination number, which we will discuss in detail in another section, indicates the total number of interactions between the central metal and the ligands. Considering the coordination number allows us to predict the molecular geometry of the complex, leading to a deeper understanding of its chemical properties and reactivity.

Isomers in Coordination Compounds

Isomers in coordination compounds refer to complexes that have the same formula but different arrangements of atoms. These differences can dramatically influence the chemical properties and biological activities of the compounds.

For instance, the cis and trans isomers, such as cis-\(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^+\) (Step 2) and trans-\(\left[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\right]^{3-}\) (Step 4), show different placements of identical ligands around the central metal. In cis isomers, the similar ligands are next to each other, while in trans isomers, they are opposite from each other.

The properties of these isomers can vary largely due to the positions of the ligands. This affects their shape, dipole moment, and even their interaction with other molecules, which is particularly important in biological systems and pharmaceutical applications.

Coordination Number

The coordination number of a complex ion is the number of points where ligands are attached to the central atom or ion. It is one of the most crucial factors in defining the structure and stability of the complex.

For example, in \(\left[\mathrm{Pt}(\mathrm{NH}_{3})_{2} \mathrm{Br}_{2}\right]^{2+}\) (Step 3), the coordination number is 4, which corresponds to a square planar geometry around the platinum (Pt) center. In contrast, \(\left[\mathrm{Ni}(\mathrm{ox})_{2}(\mathrm{OH})_{2}\right]^{3-}\) (Step 4) has a coordination number of 6, which is consistent with an octahedral geometry.

Understanding coordination numbers is vital for grasping the geometry of complex ions and predicting how they will interact with other chemical species. It is a concept that bridges the geometric structure with the chemical behavior of coordination compounds, enriching a student's grasp of inorganic chemistry.