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Mystery of Prussian and Turnbull’s Blue Revealed (Part 2)
  • focuschemistry
  • February 01, 2018
  • Transition Elements Coordination Chemistry

Crystal Structure

In Part 1 of the Prussian blue discussion, we noted that

Prussian Blue (PB) : FeIII[FeII(CN)6]  (also known as ferric ferrocynanide)
Turnbull’s Blue (TB) : FeII[FeIII(CN)6] – (also known as ferrous ferricyanide)

The structure of PB is shown below:

  1. Each Fe2+ (yellow) is surrounded octahedrally by 6 CN ligands.  Dative bonds are formed  from the lone pairs of C (blue) to Fe2+.
  2. Each Fe3+ (pink) is surrounded octahedrally by 6 CN ligands.  Dative bonds are formed from the lone pairs of N (black) to Fe3+.

Note:

  1. Fe2+ : [Ar] 3d6 has 2 paired electrons and 4 unpaired electrons (low spin) while Fe3+,with a configuration of [Ar]3d5 has 5 unpaired electrons (high spin)
  2. If one takes Fe+3 (pink) as reference points, the basic crystal structure is cubic with one Fe+3  at the centre of each face of the cube.  Hence this structure is known as face-centre cubic (FCC) structure.

Then how does the structure of Turnbull’s Blue look like?
Exactly the same as Prussian Blue!  Even though TB and PB formulae look different, they are the same in terms of crystal structure. They are just made using different reagents.  This has been proven by numerous researchers dating back to one of the 1st first few research works in 1969.

Colour of PB (and thus TB)
In what we learnt in the crystal field theory, colours come about in transition metal compounds due to the electron transition from the lower energy orbitals (known as t2g) to the higher energy orbitals (known as eg) in a split 3d subshell when it absorbs
energy in the visible light region of the electromagnetic spectrum.  However, the blue colour of Prussian Blue comes about from the movement of electron from the Fe2+ ion to the Fe3+ ion.  This is known as the inter-valence electron transfer.

Why does this happen?
Fe2+ sits in a hole created by 6 C atoms of 6CN.  As C is not very electronegative, the lone pair of electrons of C donates readily to Fe2+. This results in the CN ligands approaching close to the central Fe2+, and thus increasing electron density the Fe2+.

Fe3+, unlike Fe2+, sits in a hole created by 6N atoms which are poorer electron doner than Fe2+ (N has a higher electronegativity).  The transfer of an electron from a higher electron density centre (Fe2+) to Fe3+ requires energy absorption in the visible light region (~ 680 nm red-orange region) resulting in its blue colouration.

This movement of electrons from Fe2+ to Fe3+ results in a constant change of oxidation states of Fe throughout the entire crystal structure.

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