IB Chemistry UV/visible spectroscopy, transition metal complexes and splitting of 3d orbitals

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Prepared by Lawrence Kok

Video Tutorial on UV /visible spectroscopy, transition metals complexes and splitting of 3d orbitals.

UV and Visible Spectroscopy

• Spectroscopy – interaction of matter with radiation

• Visible Spectroscopy – electronic transition of valence electrons to higher energy level by absorbing radiation in visible range

• UV Spectroscopy – electronic transition of valence electrons to higher energy level by absorbing radiation in UV range

UV and Visible Spectroscopy

Energy difference bet 2p and 3s is HIGHAbsorb UV range - Colourless

Energy difference bet split 3d is SMALLAbsorb Visible range - Coloured

Pic taken from: http://www.extension.org/pages/13116/electromagnetic-spectrum

UV and Visible Spectroscopy

Pic taken from:http://www.hunancreative.com/24.html

Transition metal complexes are coloured – due to splitting of 3d orbitals

* Metal ion surrounded by ligands

1

* 3d orbitals are degenerate without any ligands

2

Transition metal complexes are coloured – due to splitting of 3d orbitals

* Metal ion surrounded by ligands

1

* 3d orbitals are degenerate without any ligands

2

3d orbitals split by ligands

3Splitting

Explanation how splitting occurs due to electron repulsion

Explanation how splitting occurs due to electron repulsion

Splitting of 3d orbitals by ligands – allow electronic transition from lower to higher state

Colour formation due to splitting of 3d orbitals by ligands

Ti 3+ transmit blue/violet region BUT absorb green/orange/red

Ti 3+ violet due to absorption of green/yellow region to excite electrons to higher orbital level

Cu 2+ transmit blue/violet region BUT absorb green/orange/red

Cu 2+ blue due to absorption of green/yellow to excite electrons to higher level

Transition metal have different colours due to• splitting of 3d orbitals• presence of ligands which causes splitting• partially filled 3d orbitals

CuSO4 (anhydrous) without ligands - Colourless

Why some are colourless ?

Cu 2+ anhydrous – colourlessCu 1+ hydrous – colourlessZn 2+ hydrous – colourlessSc 3+ hydrous – colourlessCu 2+ hydrous - colour

Zn 2+ ion with ligand water - Colourless

Sc 3+ ion with ligand water - Colourless

Cu 2+ ion with ligand water – Colour Blue

Cu 1+ ion with ligand water - Colourless

Factors affecting colour of transition metal complexes are• 1. Nature of element• 2. Oxidation state• 3. Type of ligands• 4. Stereochemistry/shape

Elements - Fe2+, Mn2+ , Cu2+ , Co2+

Oxidation state ion - Fe3+ , Fe2+

Factors affecting colour of transition metal complexes are

Different ligands – H20, NH3 , CI-

Stereochemistry – octahedral , tetrahedral

Factor 3. Different ligands – different level of splitting – different colour

Ligands CI- H2O NH3

Ligands F- H2O NH3

Absorption of UV by organic molecules and chromophores

Absorption of UV radiation by C=C, C=O, N=N, N=O gps

UV Electronic transition Chromophores gps

Pic taken from: http://pubs.rsc.org/en/Content/ArticleLanding/2008/NJ/b809055f

Difference bet UV and Visible absorption by

• Coloured - Absorption in visible range• Splitting of 3d is small – falls in visible range

Transition metals Organic molecules/chromophores

• Colourless - Absorption in UV range• Electronic transition from bonding to antibonding orbital (involving pi or lone pair electron)

Pic taken from:http://www.lycocard.com/index.php/lyco_pub/health/

Absorption of UV by organic molecules and chromophores

More conjugation – more delocalization – absorption in visible range - coloured

Absorption of UV by organic molecules and chromophores

More conjugation – splitting energy less – wavelength absorption increase (visible range)

Summary

Absorption of UV by organic molecules and chromophores

• Organic molecules with extensive conjugated double bonds will be coloured• Absorb in visible range

Azo dyesFood dyes

Sun block

ß carotene

Indicators

Absorption of UV by organic molecules and chromophores

How Phenolphthalein indicator changes colour ?

Acidic Colourless

AlkalinePink

Reason for change of colour• change in conjugation• change in delocalization

Acknowledgements

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Prepared by Lawrence Kok

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