Question

(a) Identify the Lewis acid and the Lewis base in the reaction of ethylenediamine \(\left(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\right)\) with \(\mathrm{Ni}^{2+}\) to form \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\) (b) Identify the ligands and donor atoms. (c) Give the coordination number and geometry of the metal in the complex.

Short answer

(a) Lewis acid: \(\text{Ni}^{2+}\); Lewis base: ethylenediamine. (b) Ligands: ethylenediamine; donor atoms: nitrogen. (c) Coordination number: 6; geometry: octahedral.

Step-by-step solution

Identify the Lewis Acid and Lewis Base

In the given reaction, the ethylenediamine molecule (en) is acting as a Lewis base because it donates electron pairs, while the metal ion \(\text{Ni}^{2+}\) acts as a Lewis acid because it accepts electron pairs from ethylenediamine.

Determine the Ligands and Donor Atoms

Ethylenediamine (\(\text{en}\)) is the ligand that coordinates with \(\text{Ni}^{2+}\). It acts as a bidentate ligand since it has two nitrogen atoms that can each donate a pair of electrons to the metal ion. Thus, nitrogen serves as the donor atom for each ethylenediamine molecule.

Calculate the Coordination Number

In the complex \([\text{Ni}(\text{en})_{3}]^{2+}\), the coordination number of \(\text{Ni}^{2+}\) is 6. This is because there are three ethylenediamine molecules, each providing two nitrogen atoms, leading to a total of six coordinate bonds with the nickel ion.

Decide the Geometry of the Metal Complex

The geometry of a metal complex with a coordination number of 6 is typically octahedral. Thus, \([\text{Ni}(\text{en})_{3}]^{2+}\) has an octahedral geometry.

Key concepts

Coordination Chemistry

Coordination chemistry involves the study of compounds formed between metal ions and ligands. These compounds are known as coordination complexes. Here, the central metal ion, which is typically a transition metal, attracts and binds to molecules or ions called ligands. These ligands donate electrons to the metal, which forms a stable structure together.
In the example of ethylenediamine and nickel, - Ethylenediamine serves as the ligand.- Nickel (\(\text{Ni}^{2+}\)) serves as the central metal ion.The attraction and bonding of these components result in the coordination complex \([\text{Ni(en)}_3]^{2+}\).
Coordination chemistry is essential for understanding many biological and industrial processes, such as catalysis and enzyme activity.

Ligands and Donor Atoms

Ligands are ions or molecules that bind to a central metal ion to form a coordination complex. They donate one or more pairs of electrons to the metal. This electron pair donation is why they act as Lewis bases. In the coordination chemistry of the complex \([\text{Ni(en)}_3]^{2+}\), ethylenediamine acts as a ligand.
Ethylenediamine is specifically called a bidentate ligand because it has two atoms that can donate electrons. These donor atoms are nitrogen atoms in the ethylenediamine molecule.
- Each nitrogen atom donates a pair of electrons. - These donor atoms help form strong bonds with the central nickel ion, stabilizing the complex structure.
This binding is crucial as it determines the structure and properties of the coordination compound.

Coordination Number

The coordination number in coordination chemistry refers to how many ligand atoms are directly bonded to the central metal ion. It's a crucial factor determining the geometry of the complex. In the compound \([\text{Ni(en)}_3]^{2+}\), the coordination number of nickel is 6.
This is because three ethylenediamine ligands bind to the nickel ion, and with each ligand having two nitrogen donor atoms, we get 3 ligands \( \times 2donors = 6donor links.\)
The coordination number helps us understand how ligands are arranged around the metal ion and gives us clues about the resulting 3D structure of the complex.

Octahedral Geometry

Octahedral geometry is a common shape for coordination complexes with a coordination number of 6. This means that the metal ion is bonded to six ligands in a spatial arrangement that looks like an octahedron.
In the \([\text{Ni(en)}_3]^{2+}\) complex, the arrangement of six donor nitrogen atoms from three ethylenediamine ligands results in an octahedral geometry.

• Each pair of nitrogen atoms in ethylenediamine can be thought of as acting along a line that forms part of the octahedron's edges.
• This geometry is highly symmetrical and aids in the complex's stability.

Understanding octahedral geometry is vital in predicting and explaining the behavior and reactivity of coordination complexes.