Question

Indicate the coordination number and the oxidation number of the metal for each of the following complexes: (a) ${\mathrm{K}}_2{\mathrm{PtCl}}_4$ (b) $[\mathrm{Ni}(\mathrm{CO}{)}_4]{\mathrm{Br}}_2$ (c) ${\mathrm{OsO}}_4$ (d) $[\mathrm{Mn}(\mathrm{en}{)}_3]{\left({\mathrm{NO}}_3\right)}_2$ (e) $[\mathrm{Cr}(\mathrm{en}){\left({\mathrm{NH}}_3\right)}_4]{\mathrm{Cl}}_3$ (f) $[\mathrm{Zn}(\mathrm{bipy}{)}_2]{\left({\mathrm{ClO}}_4\right)}_2$

Short answer

(a) Coordination number: 4, Oxidation number: +2 (b) Coordination number: 4, Oxidation number: 0 (c) Coordination number: 4, Oxidation number: +8 (d) Coordination number: 6, Oxidation number: +2 (e) Coordination number: 6, Oxidation number: +3 (f) Coordination number: 4, Oxidation number: +2

Step-by-step solution

Coordination Number

We must count the number of ligands surrounding Pt. There are four Cl ligands, therefore the coordination number is 4.

Oxidation Number

In this complex, Pt has an oxidation state of +2 because the overall charge of ${\mathrm{PtCl}}_4$ is -2 and there are two $\mathrm{K}$ ions, each with a charge of +1 to balance it. Therefore, Pt has an oxidation number of +2. (b) $[\mathrm{Ni}(\mathrm{CO}{)}_4]{\mathrm{Br}}_2$

Coordination Number

There are four CO ligands surrounding the Ni atom, so the coordination number for Ni is 4.

Oxidation Number

The overall charge of the Ni(CO)4 complex is 0, since the Br2 has no charge. Considering that CO is a neutral ligand, the Ni atom has an oxidation number of 0. (c) ${\mathrm{OsO}}_4$

Coordination Number

Os has four O atoms bonded to it, making its coordination number 4.

Oxidation Number

Considering that each O atom has an oxidation number of -2, to balance the charge, Os must have an oxidation number of +8. (d) $[\mathrm{Mn}(\mathrm{en}{)}_3]{\left({\mathrm{NO}}_3\right)}_2$

Coordination Number

There are three en (ethylenediamine) ligands surrounding the Mn atom. Each en ligand has two donor atoms. Therefore, the coordination number for Mn is 3 x 2 = 6.

Oxidation Number

The overall charge of the Mn(en)3 complex is +2 due to the presence of two NO3- ions. The en ligands are neutral, so Mn must have an oxidation number of +2 to balance the charge. (e) $[\mathrm{Cr}(\mathrm{en}){\left({\mathrm{NH}}_3\right)}_4]{\mathrm{Cl}}_3$

Coordination Number

There is one en ligand and four NH3 ligands surrounding the Cr atom. Since en has two donor atoms, the coordination number for Cr is 1 x 2 + 4 = 6.

Oxidation Number

Considering the overall charge of the complex is +3 due to the three Cl- ions; the en and NH3 ligands are neutral, so the oxidation number of Cr is +3. (f) $[\mathrm{Zn}(\mathrm{bipy}{)}_2]{\left({\mathrm{ClO}}_4\right)}_2$

Coordination Number

There are two bipy (bipyridine) ligands surrounding the Zn atom. Each bipy ligand has two donor atoms, so the coordination number for Zn is 2 x 2 = 4.

Oxidation Number

Considering that the overall charge of the Zn(bipy)2 complex is +2, due to the presence of two ClO4- ions, since bipy ligands are neutral, the Zn atom must have an oxidation number of +2.

Key concepts

Coordination Number

The coordination number in coordination chemistry refers to the number of ligand atoms that are directly bound to the central metal atom or ion. It essentially tells us how many bonds the metal atom is coordinating with ligands.
For example:

• In the complex $[{\mathrm{PtCl}}_4{]}^{2-}$, the platinum (Pt) is surrounded by four chlorine (Cl) ligands, so its coordination number is 4.
• In $[{\mathrm{Ni}(\mathrm{CO})}_4]$, nickel (Ni) coordinates with four carbon monoxide (CO) ligands, giving it a coordination number of 4.
• Similarly, the complex $[{\mathrm{Mn}(\mathrm{en})}_3{]}^{2+}$ has six coordination sites filled by three ethylenediamine (en) ligands, since each en binds through two donor atoms, making the coordination number 6.

Understanding the coordination number helps to determine the geometry of the complex, such as tetrahedral or octahedral.

Oxidation Number

The oxidation number is the formal charge of the central atom in a coordination complex. It indicates how many electrons are lost or gained by the metal during the formation of the coordination complex.
Let's look at some examples:

• In $[{\mathrm{PtCl}}_4{]}^{2-}$, each chloride ion has a charge of -1, cumulatively giving the $[{\mathrm{PtCl}}_4{]}^{2-}$ an overall -2 charge, thus Pt must have an oxidation number of +2 to balance it.
• For $[{\mathrm{Ni}(\mathrm{CO})}_4]$, carbon monoxide is a neutral ligand, resulting in the nickel having an oxidation number of 0.
• In $[{\mathrm{Mn}(\mathrm{en})}_3{]}^{2+}$, the en ligands are neutral, and the complex has an overall +2 charge, leading to an oxidation number of +2 for Mn.

This information is crucial for understanding the electron configuration of the metal and predicting the reactivity of the complex.

Ligands

Ligands are ions or molecules that donate electron pairs to the central metal atom in coordination complexes. They play a crucial role in determining the properties of coordination compounds.
Here are some examples:

• In $[{\mathrm{PtCl}}_4{]}^{2-}$, chloride ions (Cl) act as monodentate ligands, meaning each chloride is bonded to the metal through one site.
• In $[{\mathrm{Mn}(\mathrm{en})}_3{]}^{2+}$, ethylenediamine (en) is a bidentate ligand, forming two bonds per ligand to the metal center, increasing stability.
• Carbon monoxide (CO) in $[{\mathrm{Ni}(\mathrm{CO})}_4]$ is a neutral ligand that forms a strong bond with the metal, influencing the complex's structure and function.

Different ligands can cause variations in metal ion color, solubility, and magnetic properties, making them vital to the study of coordination chemistry.

Transition Metals

Transition metals are elements found in the d-block of the periodic table. They are characterized by their ability to form variable oxidation states and to create complex ions with varieties of geometric arrangements.
Some key points about transition metals in coordination chemistry include:

• Many transition metals, like nickel (Ni) and platinum (Pt), form stable coordination compounds due to their available d-orbitals that can accept electron pairs from ligands.
• These metals can exhibit different oxidation states, as seen in complexes like $[{\mathrm{PtCl}}_4{]}^{2-}$ where Pt has an oxidation state of +2, and $[{\mathrm{OsO}}_4]$ where Os has an oxidation state of +8.
• They tend to have high coordination numbers and intriguing geometries, such as tetrahedral or octahedral, attributed to d-orbital hybridization.

The ability of transition metals to form diverse compounds makes them essential to many biological processes and industrial applications, from catalysis to material science.