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Chapter 24: Problem 18

Cyano complexes of transition metal ions (such as \(\mathrm{Fe}^{2+}\) and \(\mathrm{Cu}^{2+}\) ) are often yellow, whereas aqua complexes are often green or blue. Explain the basis for this difference in color.

Short Answer

Expert verified
The color difference between cyano and aqua complexes of transition metal ions such as Fe2+ and Cu2+ is due to the different extent of crystal field splitting induced by the ligands. Cyanide, being a strong field ligand, causes greater splitting and absorbs higher energy light (blue), thus appearing yellow. In contrast, water is a weak field ligand, causes smaller splitting, absorbs lower energy light (red), and therefore the complex usually appears green or blue.

Step by step solution

01

Understanding the Structure of Transition Metal Ions

The 3d transition metal ions such as Fe2+ and Cu2+ have partially filled d-orbitals. The electrons in the d-orbitals can transition between the different energy levels upon absorption of light, causing the complexes to display various colors.
02

Effects of Different Ligands

The type of ligand attached to the metal ion can greatly influence the energy gap between the d-orbitals. This is because ligands can cause what is known as crystal field splitting, where the d-orbitals are split into two energy levels: lower energy 't2g' orbitals and higher energy 'eg' orbitals. The extent of splitting depends on the nature of the ligand.
03

Examining Cyanide and Aqua Complexes

Cyanide is a strong field ligand and will cause greater splitting of the d-orbitals compared to water, which is a weak field ligand. Because of this, a larger energy of light (towards the blue end of the spectrum) is needed to promote electrons from the t2g to the eg in cyanide complexes compare to aqua complexes. As a result, cyanide complexes often appear yellow as they absorb light from the blue end of the spectrum and transmit more of the yellow and red light. Conversely, aqua complexes absorb lower energy light (towards the red end of the spectrum) and hence often appear green or blue as they transmit more of the higher energy, green and blue light.

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Most popular questions from this chapter

The oxidation state of \(\mathrm{Ni}\) in the complex ion \(\left[\mathrm{Ni}(\mathrm{CN})_{4} \mathrm{I}\right]^{3-}\) is \((\mathrm{a})-3 ;(\mathrm{b})-2 ;(\mathrm{c}) 0 ;(\mathrm{d})+2 ;(\mathrm{e})+3\).

Draw plausible structures of the following chelate complexes. (a) \(\left[\operatorname{Pt}(\text { ox })_{2}\right]^{2-}\) (b) \(\left[\mathrm{Cr}(\mathrm{ox})_{3}\right]^{3-}\) (c) \([\mathrm{Fe}(\text { EDTA })]^{2-}\)

Of the following, the one that is a Bronsted-Lowry acid is (a) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} ;\) (b) \(\left[\mathrm{FeCl}_{4}\right]^{-} ;\) (c) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) (d) \(\left[\mathrm{Zn}(\mathrm{OH})_{4}\right]^{-}\).

How many different structures are possible for each of the following complex ions? (a) \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)\left(\mathrm{NH}_{3}\right)_{5}\right]^{3+}\) (b) \(\left[\operatorname{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\left(\mathrm{NH}_{3}\right)_{4}\right]^{3+}\) (c) \(\left[\operatorname{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\left(\mathrm{NH}_{3}\right)_{3}\right]^{3+}\) (d) \(\left[\operatorname{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\left(\mathrm{NH}_{3}\right)_{2}\right]^{3+}\)

The coordination number of \(\mathrm{Pt}\) in the complex ion \(\left[\mathrm{PtCl}_{2}(\mathrm{en})_{2}\right]^{2+}\) is \((\mathrm{a}) 2 ;(\mathrm{b}) 3 ;(\mathrm{c}) 4 ;(\mathrm{d}) 5 ;(\mathrm{e}) 6\).
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