Organic Chemistry Part 8. Chemistry of organo-nitrogen compounds

Part 8.6 Formation of transition metal ion (3d block) complexes with primary aliphatic amines

The formation of ammine complexes with transition metal ions

(a) A brief introduction

Amines can act as electron pair donor ligands with transition metal ions of the d-block e.g. the 1st transition metals series ions of chromium, cobalt, nickel and copper.

The lone pair of electrons of the nitrogen of the amine group can form a dative covalent bond with an empty orbital of the central metal ion e.g. Cr³⁺, Co²⁺, Ni²⁺, Cu²⁺ and other transition metal ions.

The reactions are very similar to that between these ions and ammonia i.e. an initial metal hydroxide complex precipitate that dissolves in excess amine to give (usually) a deep blue soluble complex ion.

For more details on complexes see:

Introduction to the chemistry of complexes ligands,  chemistry of chromium,  chemistry of cobalt,   chemistry of nickel  and  chemistry of copper

 

(b) Complexes with primary aliphatic amines - monodentate ligands

A monodentate ligands have one group with a non-bonding lone pair of electrons that can act as a lone pair donor to form a dative covalent bond with the central metal ion.

In this case the lone pair of electrons on the nitrogen atom of an aliphatic amine.

The reaction of copper(II) ions and ammonia

To start with the more familiar inorganic example of complexing copper(II) ions with ammonia. On adding ammonia solution to a copper(II) sulfate solution, after an initial copper(II) hydroxide precipitate, with excess ammonia, a deep blue solution is formed of the ammine complex ion (ligand substitution is incomplete), the overall changes can be expressed as:

(i) [Cu(H₂O)₆]²⁺_(aq) +  2NH_3(aq) [Cu(H₂O)₄(OH)₂]_(s) + 2NH₄⁺_(aq)

Initially a 'turquoise' precipitate of the hydrated copper(II) hydroxide is formed.

(ii) [Cu(H₂O)₄(OH)₂]_(s) + 4NH_3(aq)  [Cu(NH₃)₄(H₂O)₂]²⁺_(aq) +  2OH^–_(aq) +  2H₂O_(l)

Then the hydroxide precipitate dissolves in excess ammonia to give the soluble deep blue complex ion.

The overall change as equation (iii)

(iii) [Cu(H₂O)₆]²⁺_(aq) +  4NH_3(aq) [Cu(NH₃)₄(H₂O)₂]²⁺_(aq) + 4H₂O_(l)

You can then repeat the experiment with solutions of aliphatic amines e.g. ethylamine(aq)

Copper(II) complexes with aliphatic amines as the ligand

If an aqueous solution of an aliphatic amine is added to a copper(II) salt solution containing the hexaaquacopper(II) complex ion, an initial neutral hydrated 'turquoise'/pale blue coloured copper(II) hydroxide complex precipitate is formed, equation (i).

Then, with excess amine, the copper(II) hydroxide precipitate dissolves and a deep blue solution is formed of the ammine complex ion, equation (ii).

The changes can be expressed as:

(i) [Cu(H₂O)₆]²⁺_(aq) +  2RNH_2(aq) [Cu(H₂O)₄(OH)₂]_(s) + 2RH₃⁺_(aq)

(ii) [Cu(H₂O)₄(OH)₂]_(s) + 4RNH_2(aq)  [Cu(RNH₂)₄(H₂O)₂]²⁺_(aq) +  2OH^–_(aq) +  2H₂O_(l)

and the overall reaction is

(iii) [Cu(H₂O)₆]²⁺_(aq) +  4RNH_2(aq) [Cu(RNH₂)₄(H₂O)₂]²⁺_(aq) +  4H₂O_(l)

These are NOT redox reactions, but ligand exchange substitution reactions with no change in oxidation state.

Both copper(II) complexes are octahedral, co-ordination number 6, overall charge remains at 2+ because both ligands are electrically neutral.

The complexes with an amine RNH₂ (R = alkyl), ethylamine (CH₃CH₂NH₂) and ammonia (NH₃) are shown below.

You get similar reactions with other transition metal ions e.g. the hexaaqua cobalt(II) ions, nickel(II) ions and chromium(III) ions. which all give hydroxide precipitates that dissolve in excess aqueous aliphatic amine solution to give soluble blue transition metal complexes.

The formula of the neutral hydroxide precipitates would be:

[Co(H₂O)₄(OH)₂],  [Ni(H₂O)₄(OH)₂]_(s), and [Cr(H₂O)₃(OH)₃]

and the formula of the final soluble complexes formed from aliphatic primary amines would be

[Co(RNH₂)₄(H₂O)₂]²⁺,  [Ni(RNH₂)₄(H₂O)₂]²⁺  and  [Cr(RNH₂)₄(H₂O)₂]³⁺

where R = alkyl i.e. for ethylamine R = CH₃CH₂ (if R = H, the ligand is ammonia)

(c) Complexes with primary aliphatic amines - polydentate ligands

Polydentate ligands have at least two groups with a non-bonding lone pair of electrons that can both act as a lone pair donors to form a dative covalent bond with the central metal ion.

A classic bidentate ligand (two electron pair donor groups in the same molecule) is 1,2-diaminiethane.

Structural formula H₂NCH₂CH₂NH₂

Please note, it is common to signify this molecule as 'en', shorthand for its old name of ethylenediamine.

I have also used the abbreviation L-L to signify a bidentate ligand such as 1,2-diaminoethane.

The diagram on the right shows three bidentate ligands coordinated to a central metal ion. The coordination number is still 6 (NOT 3), and the bonds do form an 'octahedral'  arrangement around the central transition metal ion.

Hexaaqua Cr(III) ions, Ni(II) ions, Co(II) ions and Cu(II) form this type of complex.

Using H₂NCH₂CH₂NH_2, 'en' or 'L-L' to represent 1,2-diaminoethane, you can write the complex formations as an overall ligand exchange reaction equation:

(i) [Cr(H₂O)₆]³⁺_(aq)  +  3H₂NCH₂CH₂NH_2(aq)   ==>  [Cr(H₂NCH₂CH₂NH₂)₃]³⁺_(aq)  +  6H₂O(l)

(ii) [Ni(H₂O)₆]²⁺_(aq)  +  3H₂NCH₂CH₂NH_2(aq)   ===>  [Ni(H₂NCH₂CH₂NH₂)₃]²⁺_(aq)  +  6H₂O(l)

The diagram shows the structure of the nickel complex and note it exhibits R/S isomerism!

This structure is common to many transition metal ion - bidentate ligand complexes.

 

(iii) [Co(H₂O)₆]²⁺_(aq)  +  3H₂NCH₂CH₂NH_2(aq)   ==>  [Co(H₂NCH₂CH₂NH₂)₃]²⁺_(aq)  +  6H₂O(l)

and 

showing mirror image forms of the complex.

 

(iv) [Cu(H₂O)₆]²⁺_(aq)  +  2H₂NCH₂CH₂NH_2(aq)   ==>  [Cu(H₂NCH₂CH₂NH₂)₂]²⁺_(aq)  +  6H₂O(l)

Note the copper(II) complex can involves just two ligands in a square planar configuration.

(Not sure if Cu²⁺ complexes with 3 molecules of 1,2-diaminoethane?)