Advanced level inorganic chemistry: UV & visible light absorption spectroscopy of copper(II) complex ions

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Doc Brown's Advanced Chemistry: PART 15.5 uv and visible absorption spectra of copper complex ions - transition metal absorption spectroscopy - copper compounds

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses Spectroscopic methods of analysis and molecular structure determination investigating copper complexes

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15.5.1 The origin of colour, the wavelengths of visible light, our perception!

15.5.2 uv-visible spectroscopy theory, spectrometer, examples of absorption & reflectance spectra explained

15.5.3 uv-visible absorption spectra - index of examples: uses, applications, more on the chemistry of colour

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The uv and copper(II) complex ions absorption spectrum of copper complex ions and compounds

(a) Octahedral complexes with water and ammonia ligands

If you add conc. ammonia to pale blue copper(II) sulfate solution, you get a stepwise ligand exchange reaction to give a whole series of copper(II) ion ammine complexes as each water molecule is replaced by an ammonia molecules.

visible light absorption spectra copper(II) complex ions water ammonia ligands hexaaquacopper(II) ion tetraamminecopper(II) ion hexaamminecopper(II) ion teraamminecopper(II) ion diaquateraamminecopper(II) ion

[Cu(H2O)6]2+(aq) structure of hexaaquacopper(II) complex ion pale blue-cyan colour absorption spectraThe original [Cu(H2O)6]2+(aq) hexaaquacopper(II) ion, octahedral, gives a pale blue-cyan aqueous solution, that absorbs strongly in the red with a λmax of 780 nm

 The [Cu(NH3)4(H2O)2]2+(aq) diaquatetraamminecopper(II) ion, octahedral, is dark blue, absorbing in yellow-orange-red region, with a λmax of 650 nm.

Structure shown on the right.

 [Cu(NH3)4(H2O)2]2+(aq) structure of diaquatetraamminecopper(II) complex ion dark blue colour absorption spectraThis ion exhibits E/Z isomerism (cis/trans geometrical isomerism), in this case, the more stable isomer is the E isomer (trans).

It worth noting (for 'concept' reasons), that in E/Z isomers of transition metal complex ions, the central metal ion experiences a slightly different ligand field effect, hence a slightly different colour is observed.

For more on these complexes see The chemistry of copper

[Cu(NH3)6]2+(aq) structure of hexaamminecopper(II) complex ion dark indigo-violet colour absorption spectraThe final substitution product is [Cu(NH3)6]2+(aq), the hexaamminecopper(II) ion, octahedral, an even deeper violet-indigo-blue. Unfortunately I could find a λmax for this ion?

However, seems obvious, from colour observations, the greater the amount of NH3 replacing the H2O ligand, the deeper the blue colour of the resulting copper(II) complex ion, so I would expect the λmax to be less than 650 nm?, with the violet and deep blue light is transmitted and less of the yellow-orange transmitted?.

The ammonia causes a large splitting of the 3d orbitals than the water ligand, increasing the energies of absorptions at shorter wavelengths, giving a more violet-blue colour.

electronic energy diagram shows the field splitting effect of the 6 water/ammonia ligands on the 3d orbitals of the central copper ion (Cu2+) in octahedral or square planar compleex copper)II) ions

The above electronic energy diagram shows the octahedral field splitting effect of the 6 water/ammonia ligands on the 3d orbitals of the central copper ion (Cu2+) which has a 3d9 configuration - meaning there is partially filled 3d orbital in the complex ion (compare this with the 3d10 configuration of a Cu+ ion).

The ∆Eelec excitation energy, promoting an electron from a lower 3d orbital to a higher 3d orbital is within the copper(II) complex ions light photon range, so the ion is coloured.

At this pre-university level, the diagram applies to the square planar complexes of Cu2+ with H2O or NH3 i.e. for a 4 ligand square complexes like [Cu(NH3)4]2+ or [Cu(H2O)4]2+, the electronic diagram is the same for an octahedral complex described above.

 

(b) A tetrahedral/square planar copper(II) ion complex with a chloride ion ligand

In the presence of excess chloride ions, the octahedral hexaaquacopper(II) ion forms the tetrahedral (or square planar?) ion, which theoretically is colourless, but isn't in reality!

[Cu(H2O)6]2+(aq) + 4Cl(aq) rev [CuCl4]2(aq) + 6H2O(l)

electronic energy diagram shows the field splitting effect of the chloride ion ligands on the 3d orbitals of the central copper ion (Cu2+) in the terachlorocuprate(II) complex ion

The above electronic energy diagram shows the tetrahedral field splitting effect of the 4 chloride ion ligands on the 3d orbitals of the central copper ion (Cu2+).

In salts, depending on the size of the cation, you observe an orange colour for the distorted tetrahedral [CuCl4]2- ion or pale yellow for the same ion in a square planar configuration.

This shows a change in crystal structure for the same ion can cause a difference in the ∆Eelec excitation energy, hence a change in colour of the copper(II) complex ion.

For a 4 ligand square complexes like [Cu(NH3)4]2+ or [Cu(H2O)4]2+, the electronic diagram is the same for an octahedral complex described above, noting the 'middle' of an octahedral complex is square planar.

For more on these complexes see The chemistry of copper

 

(c) Complexes of copper(I) ion

Not all, but many of copper(I) complex ions are colourless - you get colour with copper(I) charge transfer complexes.

The reason why you can get a colourless copper(I) complex ions/compounds is explained in the diagrams below.

So, I'm presenting the 'electronic' argument as to why you might expect copper(I) compounds to be colourless e.g. CuCl, copper(I) chloride solid is white, but readily oxidises in air or water to give a cyan colour of a copper(II) compound.

diagram explaining electronic energy 3d orbital splitting by 4 ligands in a square planar complex of copper(I) compound or complex Cu+ ion

At pre-university level, it is the same electronic diagram for an octahedral or square planar complex of the copper(I) ion and note that the 'middle' section of an octahedral complex is a square planar arrangement of ligands.

Above and below are the field splitting diagrams for a square planar or tetrahedral complex ion based on the copper(I) ion, Cu+.

The Cu+ ion does not normally form an octahedral complex, usually tetrahedral in shape with a coordination number of 4 ligands.

diagram explaining electronic energy 3d orbital splitting by 4 ligands in a tetrahedral complex of copper(I) compound or complex Cu+ ion

The above electronic energy diagrams show the field splitting effect of the 4 or 6 ligands on the 3d orbitals of the central copper ion (Cu+) which has a 3d10 configuration (not the difference from the 3d9 configuration for a Cu2+ ion).

This means there is NO partially filled 3d orbital in the complex ion.

Therefore, there is no partially filled 3d orbital to which an electron can be excited by a copper(II) complex ions light photon.

For more on copper(I) chemistry see The chemistry of copper


Key words & phrases: interpreting the uv-copper(II) complex ions absorption spectra of copper complexes ions compounds, identifying the maximum absorption peaks in the uv-copper(II) complex ions absorption spectra of copper complexes ions compounds, explaining the uv-copper(II) complex ions absorption spectra of copper complexes ions compounds, how to use the copper(II) complex ions absorption spectra of copper complexes ions compounds to explain the different colours of copper complexes ions compounds, applications of the uv-copper(II) complex ions absorption spectra of copper complexes ions compounds absorption spectrum of hexaaquacopper(II) ion, identifying the maximum absorption peaks in the uv-copper(II) complex ions absorption spectrum of hexaaquacopper(II) ion, explaining the uv-copper(II) complex ions absorption spectrum of hexaaquacopper(II) ion, how to use the copper(II) complex ions absorption spectra of hexaaquacopper(II) ion to explain the colour of hexaaquacopper(II) ion, applications of the uv-copper(II) complex ions absorption spectrum of hexaaquacopper(II) ion absorption spectrum of diaquatetraamminecopper(II) complex ion, identifying the maximum absorption peaks in the uv-copper(II) complex ions absorption spectrum of diaquatetraamminecopper(II) complex ion, explaining the uv-copper(II) complex ions absorption spectrum of diaquatetraamminecopper(II) complex ion, how to use the copper(II) complex ions absorption spectra of diaquatetraamminecopper(II) complex ion to explain the colour of diaquatetraamminecopper(II) complex ion, applications of the uv-copper(II) complex ions absorption spectrum of diaquatetraamminecopper(II) complex ion electronic diagrams to explain the 3d ligand field splitting effects of ligands to explain the colours of xyz complex ions excitation of 3d orbital electrons to produce colour in xyz complex ions explaining why copper(I) compounds and complex ions are colourless and copper(II) compounds and complex ions are coloured


Associated links

UV and visible spectroscopy index

General introduction to electron configuration of transition metal ions and colour theory

The chemistry of copper

Index of Advanced A level Notes on the 3d block and Transition Metals

SPECTROSCOPY INDEXES

All Advanced Inorganic Chemistry Notes

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