28.3 A2 Level

Colour of complexes

8 flashcards to master this topic

Definition Flip

Define the term 'degenerate d orbitals'.

Answer Flip

Degenerate d orbitals are d orbitals that possess the same energy level. In an isolated atom or ion, the five d orbitals are degenerate. This degeneracy is removed when ligands are introduced.

Key Concept Flip

Describe how the d orbitals split in an octahedral complex.

Answer Flip

In an octahedral complex, the five degenerate d orbitals split into two sets. Three d orbitals (dxy, dxz, dyz) are lower in energy, and two d orbitals (dz2, dx2-y2) are higher in energy. This energy difference is denoted as ΔE.

Key Concept Flip

Describe how the d orbitals split in a tetrahedral complex.

Answer Flip

In a tetrahedral complex, the five degenerate d orbitals split into two sets. Two d orbitals (dxy, dxz, dyz) are higher in energy, and three d orbitals (dz2, dx2-y2) are lower in energy. The splitting pattern is the inverse of the octahedral complex.

Key Concept Flip

Explain why transition metal compounds are coloured.

Answer Flip

Transition metal compounds are coloured because electrons absorb specific frequencies of visible light to get promoted from a lower energy d orbital to a higher energy d orbital (d-d transition). The observed colour is the complementary colour to the light absorbed.

Key Concept Flip

How does the identity of a ligand affect the magnitude of ΔE?

Answer Flip

Different ligands cause different degrees of d-orbital splitting, and thus different values of ΔE. Strong-field ligands cause a large splitting (large ΔE), and weak-field ligands cause a small splitting (small ΔE).

Key Concept Flip

Explain how ligand exchange can affect the observed colour of a complex.

Answer Flip

Ligand exchange alters the magnitude of ΔE. This, in turn, changes the frequency of light absorbed and the complementary colour observed.

Example: [Cu(H₂O)₆]²⁺ is blue, while [CuCl₄]²⁻ is yellow-green due to the different ligands.
Key Concept Flip

What happens to the frequency of light absorbed as ΔE increases?

Answer Flip

As ΔE increases, the frequency of light absorbed also increases (E = hf). This means the compound will absorb light towards the blue/violet end of the spectrum and appear yellow/orange, as opposed to absorbing red/orange and appearing blue/green.

Definition Flip

Give an example of a ligand exchange reaction using copper(II) ions and ammonia molecules.

Answer Flip

The reaction between [Cu(H₂O)₆]²⁺ (pale blue) and ammonia (NH₃) initially forms a pale blue precipitate of copper(II) hydroxide. With excess ammonia, the precipitate dissolves, and [Cu(NH₃)₄(H₂O)₂]²⁺ (deep blue) is formed.

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26.2 Homogeneous and heterogeneous catalysts 28.4 Stereoisomerism in transition element complexes