[Anonymoose]

I'm having a problem with predicting this complexation reaction. It reads as: "A drop of potassium thiocyanate solution is added to a solution of iron (III) chloride"

This is my attempt:    6SCN^- + Fe³⁺  --->  [Fe(SCN₆)]^3-




This is what my teachers answer sheet says:  SCN^- + Fe³⁺ ---> FeSCN²⁺

Huh? I thought the amount of ligands (SCN) is supposed to be double the charge number of the other element (Fe). 3 x 2 = 6.

Where did I go wrong?

[agrobert]

The reaction you wrote is for isothiocyanate (SCN^-)

http://en.wikipedia.org/wiki/Isothiocyanate

and the problem reads thiocyanate (^-SCN)

http://en.wikipedia.org/wiki/Thiocyanate

^-SCN will make the complex charge ⁺²

written as [Fe(SCN)]⁺²

so FeCl₃ + ^-SCN --> [Fe(SCN)]⁺²

[Alpha-Omega]

OH MY....where to start.....OK  start with the statement of the problem:

It reads as: "A drop of potassium thiocyanate solution is added to a solution of iron (III) chloride"


FeCl₃ + KSCN <----->  Fe(SCN)₃ + KCl

So Balancing this yields:



And I have no idea what your teacher meant about # of ligands being 2 x 3 (the charge on thwe metal cation-Fe3+.  She may have been tring to tell you that many transition metals form octahedral comlexes.....OK the cation...will have six ligands-attached....that gives the complex octahedral geometry.

But I do know this:

Depending on the nature of the ligand and metal cation, a variable number of ligands may attach to the metal cation to form what is called a coordination complex. The number of ligands attached to the metal cation is called the coordination number of the metal.

Coordination numbers can range from 1 to over 6. Common coordination numbers for many transition metal complexes are 4 or 6. An unknown number, referred to here as "x", of thiocyanate ions, SCN¯, form a coordination complex with the iron(III) ion, Fe3+, according to the reversible reaction:

[Alpha-Omega]

Hey OOPS MY BAD-my balanced eqn was missing it should be :

FeCl₃ + 3KSCN <----> Fe(SCN)₃ + 3KCL

[Mitch]

I would of thought the product should of been: [Fe(SCN)]Cl₂ , since you're only adding a drop of thiocyanate.

[Anonymoose]

The reaction you wrote is for isothiocyanate (SCN^-)

http://en.wikipedia.org/wiki/Isothiocyanate

and the problem reads thiocyanate (^-SCN)

http://en.wikipedia.org/wiki/Thiocyanate

^-SCN will make the complex charge ⁺²

written as [Fe(SCN)]⁺²

so FeCl₃ + ^-SCN --> [Fe(SCN)]⁺²

Is there a special rule with the Thiocyanate? Also, can you explain why the "-" is in front of the SCN making it ^-SCN, as well as its significance? I've never seen it written like that.

And I just realized I didn't even balance my own attempt, whether its right or not. Fix'd!

EDIT: I just found another complexation example my teacher did which is similar to this problem and similar to my answer.

A concentrated solution of ammonium thiocyanate is added to a solution of iron (III) chloride.
Fe³⁺ + SCN^- --->  Fe(SCN)₆^3-

I am quite confused as of this point.

[Mitch]

The location of the charge in the molecular formula indicates onto which atom bonding to the metal center occurs.

[Alpha-Omega]

The reactants and products are all aqueous and the degree of color formation determines the shift in equilibrium...

Oftentimes, dissolved metal ions will react with certain substances to produce brightly colored species
called complex ions. For example, iron(III) reacts with the thiocyanate ion (SCN ) to produce a bright red
complex ion:

Fe3+ + SCN = [FeNCS+2]

This is an equilibrium process that is easy to study, because one can monitor the bright red color of
[FeNCS+2 ] as an indication of the position of the equilibrium: If the solution is very red, there is a lot of
[FeNCS+2] present; if the solution is not very red, then there must be very little [FeNCS+2] present.

[Alpha-Omega]

Fe3+(aq) (light yellow) + SCN- (colorless) ↔ FeSCN2+ (aq) (red)

[AWK]

Thiocyanate reacts with iron(III) to form different complexes. Even if a very small amount of thiocyanate is added complex ions will contain from one to five thiocyanate groups. Only in concentrated SCN- the complex containing 6 SCN- also will form and should be taken into account in solution

[Anonymoose]

Okay, so depending on whether its concentrated or if its just a drop makes a big difference?

[Borek]

Six equilibria present in the mixture:

Fe³⁺ + SCN^- <-> FeSCN²⁺; log K = 3.02
Fe³⁺ + 2SCN^- <-> Fe(SCN)₂⁺; log K = 4.64
Fe³⁺ + 3SCN^- <-> Fe(SCN)₃; log K = 5.0
Fe³⁺ + 4SCN^- <-> Fe(SCN)₄^-; log K = 6.3
Fe³⁺ + 5SCN^- <-> Fe(SCN)₅^2-; log K = 6.2
Fe³⁺ + 6SCN^- <-> Fe(SCN)₆^3-; log K = 6.1

(These values were taken from the Handbook of Chemical Equilibria in Analytical Chemistry, Kotrly and Sucha, Ellis Horwood Ltd. 1985.)

When the SCN^- concentration goes higher you move further down the list. So yes, whether it is concentrated or just a drop makes a big difference.

[Anonymoose]

I understand now, Scooby snacks awarded to all those helpful 

Thanks!