Ferric Chloride aka Iron(III) Chloride Not just for etching copper Simon Cotton University of Birmingham Molecule of the Month October 2016 Also available: JSMol version.

Why not call it by its proper name?

A lot of people still refer to it as ferric chloride, and this links with its origins, with the Latin ferrum or the French fer.

What’s with the –ic anyway?

Transition metals often have two principal oxidation states. Traditionally, -ous is a suffix used to mark the lower oxidation state and –ic for the higher one. Thus, ferrous means iron (II) and ferric means iron (III). Similarly cobaltous means cobalt (II) and cobaltic means cobalt (III); cuprous means copper (I) and cupric means copper(II); and aurous means gold (I) and auric means gold (III).

Anyway, how do you make it?

If you react iron (e.g. steel wool) with gaseous HCl, you get FeCl₂. If, instead, you use chlorine gas, which is a stronger oxidising agent, you get FeCl₃.

2Fe(s) + 3Cl₂(g) 2FeCl₃ (g)

At the temperature of the reaction (> 200ºC), the iron chloride vapourises out of the hot zone and is collected as a brown-black sublimate.

Apparatus used to make anhydrous FeCl₃ and a photo of it as a brown-black powder.

There are other routes; for example it can be made from refluxing Fe₂O₃ with CCl₄ as the chlorinating agent, and by dehydrating FeCl₃.6H₂O with SOCl₂, thionyl chloride. If you try dissolving Fe₂O₃ in hydrochloric acid, you get the yellow-black hydrate, FeCl₃.6H₂O.

And the structure of FeCl₃?

In the solid state, FeCl₃ adopts the same structure as BiI₃; each iron is octahedrally coordinated by six chlorines, as shown on the right.

It is volatile above around 300ºC, and the vapour phase then contains substantial amounts of Fe₂Cl₆ molecules. At higher temperatures, around 600ºC, the monomeric form of FeCl₃ is present.

FeCl3	Fe2Cl6

How about when you dissolve it in water?

That gets complicated. A yellow-brown hydrate FeCl₃.6H₂O can be crystallised from solution, as shown in the photo, right. In the solid state, this is trans-[Fe(H₂O)₄Cl₂]⁺ Cl^-.2H₂O, containing trans-[Fe(H₂O)₄Cl₂]⁺ ions.

Studies on aqueous solutions of FeCl₃ by both Extended X-Ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) identified several species: [Fe(H₂O)₆]³⁺, [FeCl(H₂O)₅]²⁺ and [FeCl₂(H₂O)₄]⁺, at different solution concentrations.

trans-[Fe(H₂O)₄Cl₂]⁺

Why does the colour change when you dissolve it in water?

Some of the chloride ligands surrounding the iron get replaced by water molecules. This affects the splittings between the energy levels, as water produces bigger splittings than chloride, so this affects the energy of the electronic transitions, just as with cobalt chloride (MOTM June 2016). There are other factors too.

Can you isolate other complexes from these solutions?

Under certain conditions, other hydrates can be isolated from solution. Back in 1892 it was reported that compounds with the composition FeCl₃.3½.5H₂O, FeCl₃.2½H₂O and FeCl₃.2H₂O could all be isolated. FeCl₃.3½H₂O is thought to be cis-[Fe(H₂O)₄Cl₂]⁺ [FeCl₄]^-.3H₂O, while FeCl₃.2½H₂O is known to have the structure cis-[Fe(H₂O)₄Cl₂]⁺ [FeCl₄]^-.H₂O.

FeCl₄^-and cis-FeCl₂(H₂O)₄⁺ ions in FeCl₃.2½H₂O.

FeCl₃.2H₂O is believed to be trans-[Fe(H₂O)₄Cl₂]⁺ [FeCl₄]^- although FeCl₃.2H₂O molecules have been encapsulated by a crown ether, 15-crown-5.

FeCl₃(H₂O)₂ molecules trapped by 15-crown-5.

In addition, the two isomers of [FeCl₃(H₂O)₃] have been isolated when co-crystallised with organic cation-chloride salts.

fac-FeCl3(H2O)3	mer-FeCl3(H2O)3

Although anionic species are not usually found in aqueous solutions of FeCl₃, they are formed when hydrochloric acid is added, and a number of these have been characterised.

FeCl52-	FeCl4(H2O)22-	FeCl5(H2O)-

How is it that you sometimes get cis- and sometimes trans- isomers?

There’s actually only a small difference in stability between the different isomers. Small factors – like intermolecular forces, e.g. hydrogen bonds to a water molecule – can tip the balance one way or the other.

Does FeCl₃ form other complexes?

Yes, thousands of them. The diagram below shows a variety of the structural possibilities.

This all looks confusing.

There are some general principles. Most complexes are octahedral. Some result from adding other ligands onto the FeCl₃ unit – the more bulky the ligand is, the fewer can be fitted round the iron. In some cases it is more complicated – quite a few complexes with the simple ‘FeL₂Cl₃’ formula are not what they appear to be – something called ‘self’-ionisation’ occurs with the formation of ions, [FeL₄Cl₂]⁺ and [FeCl₄]^-. You can see this happening with Ph₃PO, DMSO (Me₂SO) and DMF (HCONMe₂), for example. Then there is the possibility of isomerisation. Many FeL₃Cl₃ complexes (L = py, pyridine; pyrazole; 1/3 terpyridyl) are the mer-isomer, as shown.

Many factors have to be taken into account to explain the final result, which is why it seems complicated. But rewarding in its richness.

But what use is ferric chloride?

In the laboratory, anhydrous FeCl₃ can be used as an alternative to anhydrous AlCl₃ in Friedel-Crafts reactions, like the alkylation or acylation of aromatic rings. Being a small, highly charged metal, it polarises reagents like halogenoalkanes (alkyl halides), which generates carbocations (carbenium ions) that then attack benzene rings, as in the synthesis here.

Is that it?

No, in the world out there, it is used in etching copper to make printed circuit boards like the one shown on the right.

Ah, yes, I remember reading that ferric chloride is quite acidic.

The Fe³⁺ ion is small and has a high charge, which makes it very polarising. That draws the electron pairs in the Fe-O bonds towards the iron, in turn polarising and weakening the O-H bonds, favouring their ionisation.

But that is not why it is used in etching.

OK, tell me why.

The use of FeCl₃ solutions in etching copper printed-circuit-boards (as in the photo, right) uses a different piece of chemistry, redox, rather than acid-base, and the presence of chloride is important.

Because Fe³⁺ is quite strongly oxidising, it strips electrons from copper atoms. In an overview, it forms Cu²⁺ ions and itself is reduced to Fe²⁺.

2 Fe³⁺(aq) + Cu (s) ⇌ 2 Fe²⁺(aq) + Cu²⁺(aq)

In more detail, oxidation at the surface forms a Cu(I) species, which diffuses away and is oxidised to copper(II) in the solution.

Cu (s) + Fe³⁺(aq) + 3Cl^- (aq) CuCl₃^2- (aq) + Fe²⁺(aq)

CuCl₃^2- (aq) + Fe²⁺(aq) Cu²⁺(aq) + Fe²⁺(aq) + 3Cl^- (aq)

By forming soluble complexes with copper(II), chloride prevents formation of insoluble copper(I) compounds, which would prevent further attack at the copper surface. Iron chloro complexes are also involved.

One more thing. Do they use ferric chloride in Etch-A-Sketch?

No.

Bibliography

• Chapman and Hall Combined Chemical Dictionary compound code number HPN21-R

Synthesis of FeCl₃

• B. R. Tarr, Inorg. Synth., 1950, 3, 191; A. R. Pray, Inorg. Synth., 1957, 5, 153; E. R. Epperson, S. M. Horner, K. Knox and S. Y. Tyree, Inorg. Synth., 1963, 7, 163.

Structure of FeCl₃

• N. Wooster, Z. Kristallogr., 1932, 83, 35-41; K. Geierberger, Z. Anorg. Allg. Chem. 1949, 258, 361–363; S. Hashimoto, K. Forster and S. C. Moss, J. Appl. Crystallogr. 1989, 22, 173–180 (structure in solid state).
• M. Hargittai, J. Tremmel and I. Hargittai, J. Chem. Soc., Dalton Trans., 1980, 87–89; Z. Varga, M. Kolonits, and M. Hargittai, Inorg. Chem. 2010, 49, 1039–1045 (electron diffraction of vapour)

Hydrated FeCl₃ in solution

• M. Magini and T. Radnai, J. Chem. Phys., 1979, 71, 4255-4262; D. L. Wertz and M. L. Steele, Inorg. Chem., 1980, 19, 1652-1656 (X-ray diffraction of solutions of FeCl₃).
• K. Asakara, M. Nomura and H. Kuroda, Bull. Chem. Soc. Jpn., 1985, 58, 1543-1550 (EXAFS of solutions of FeCl₃).

Hydrates of FeCl₃

• H. W. B. Roozeboom, Z. Phys. Chem., 1892, 10, 477-503 (phase diagram)
• C. Bordeleau and D. R. Wiles, Inorg. Chim. Acta, 1980, 41, 195-199 (Mössbauer spectra)
• J. T. Szymanski, Acta Cryst., 1979, B35, 1958-1963 (structure of FeCl₃.2½ H₂O)
• U. Russo, B. Zarli, G. Favero, G. Valle and G. J. Long, Inorg. Chim. Acta, 1995, 239, 67-75 (structure of FeCl₃.2H₂O.15-crown-5)
• B. D. James, M. Bakalova, J. Liesegang, W. M. Reiff, B. W. Skelton and A. H. White, Inorg. Chem., 2001, 40, 4617-4622 (structures of [FeCl₅(H₂O)]^2- and trans-[FeCl₄(H₂O)₂]^-)
• T. J. Podesta and A. G. Orpen, Cryst. Growth & Des., 2005, 5, 681-693 (structure of fac-[FeCl₃.(H₂O)₃])
• B. D. James, B. W. Skelton and A. H. White, Z. Anorg. Allg. Chem., 2008, 634, 262-266 (structures of mer-[FeCl₃(H₂O)₃] and trans-[FeCl₂(H₂O)₄]⁺ Cl^-.2H₂O)

Other complexes

• M. J. Bennett, F. A. Cotton and D. L. Weaver, Acta Cryst., 1967, 23, 581-586. (structure of FeCl₃(Me₂SO)₂, trans-[Fe(Me₂SO)₄Cl₂]⁺ FeCl₄^-)
• S. A. Cotton and J. F. Gibson, J. Chem. Soc. (A), 1971, 859-864; E. Durcanska, T. Glowiak, J. Kosizek, I. Ondrejkovicova and G. Ondrejovic, Acta Cryst., 1989, C45, 410-412. (structure of FeCl₃(Ph₃PO)₂, trans-[Fe(Ph₃PO)₄Cl₂]⁺ FeCl₄^-)
• H. J. Goodwin, M. McPartlin and H. A. Goodwin, Inorg. Chim. Acta, 1977, 25, L74. (structure of Fe(phen)Cl₃ , cis-[Fe(phen)₂Cl₂] + [FeCl₄]^-)
• A. Hoser, Z. Kaluski, M. Januszczyk, J. Pietrzak and T. Glowiak, Acta Cryst., 1983, C39, 1039-1041 (FeCl₃(py)₄, mer-[ FeCl₃py₃].py)
• K. R. Millington, S. R. Wade, G. R. Willey and M. G. B. Drew, Inorg. Chim. Acta, 1984, 89, 185–191 (FeCl₃(Me₃N)₂)
• J. D. Walker and R. Poli, Inorg. Chem., 1989, 28, 1793-1801 (structures of FeCl₃(PPh₃)₂ and FeCl₃(PMe₃)₂); J. D. Walker and R. Poli, Inorg. Chem., 1990, 29, 756-761 (structures of FeCl₃(PR₃) (R = Cy, t-Bu))
• S. A. Cotton, J. Fawcett and V. Franckevicius, Polyhedron, 2002, 21, 2055-2061 (structures of mer-[ FeCl₃(pyrazole)₃] and mer-[ FeCl₃(terpy)].
• Although they are now dated, surveys of the chemistry of FeCl₃ (including coordination chemistry) can be found in: - D. Nicholls, The Chemistry of Iron, Cobalt and Nickel, Oxford, Pergamon, 1975, esp. pp 1038-1039, 1044-1046 (part of Comprehensive Inorganic Chemistry, ed. J. C. Bailar et al., Oxford, Pergamon, 1973); R. Colton and J. H. Canterford, Halides of the first row transition metals, London, John Wiley and Sons, 1968, esp. pp 273-279.

Ferric chloride etching

• E. B. Saubestre, Ind. Eng. Chem., 1959, 51, 288-290.
• C. Bryce and D. Berk, Ind. Eng. Chem. Res., 1995, 34, 1412-1418
• http://www.rsc.org/images/TM0413-Printed-circuit-boards_tcm18-232762.pdf
• http://www.artmondo.net/printworks/articles/ferric.htm
• http://copperheartdesigns.blogspot.co.uk/2008/04/tutorial-how-to-etch-copper.html
• https://www.quora.com/Whats-the-reaction-between-Iron-III-chloride-and-copper
• http://www.scienceforums.net/topic/15843-iron-iii-chloride-to-dissolve-copper/

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