Colour in transition metal complexesEdexcel International A Level Chemistry: Flashcards

What these 12 flashcards ask

Why are most transition metal ions coloured?
State the relationship between energy gap and frequency.
What do ligands do to the d orbitals of the metal ion?
What decides which colour is seen?
Why is Sc³⁺(aq) colourless?
Why is Zn²⁺(aq) colourless?
Why are copper(I) complexes colourless?
Name three changes that can change the colour of a complex.
What happens to the frequency absorbed if ΔE gets larger?
Colour change when Fe²⁺(aq) is oxidised to Fe³⁺(aq)?
Equation: [Cu(H₂O)₆]²⁺ with concentrated hydrochloric acid.
Which d configurations give coloured ions?

Exam questions on Colour in transition metal complexes

A teacher compares two aqueous solutions of the same concentration. The solution containing Cu²⁺(aq) ions is blue, whereas the solution containing Sc³⁺(aq) ions is colourless.
Explain why the Cu²⁺(aq) solution appears blue.2 marks
Aqueous copper(II) ions form the pale blue complex [Cu(H₂O)₆]²⁺. Adding excess aqueous ammonia gives the deeper blue complex [Cu(NH₃)₄(H₂O)₂]²⁺, and adding concentrated hydrochloric acid gives the yellow-green complex [CuCl₄]²⁻.
Explain why replacing water ligands by ammonia ligands changes the shade of blue.2 marks
Complexes of copper(I), such as [CuCl₂]⁻, are colourless, whereas complexes of copper(II), such as [Cu(H₂O)₆]²⁺, are blue. Cobalt(II) forms the pink complex [Co(H₂O)₆]²⁺ and the blue complex [CoCl₄]²⁻.
Explain, with reference to electronic configurations, why copper(II) complexes are coloured but copper(I) complexes are colourless.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).