Electronic spectroscopy - Homoleptic chromium(III) complexes and the spectrochemical series.

1 November 2003

Experiment 1: Electronic spectroscopy – Homoleptic chromium(III) complexes and the spectrochemical series

The metal d orbitals in an octahedral transition complex are split into two energy sets, t2g (lower) and eg (higher), as a result of metal-ligand interactions. The size of the energy gap, ΔO, depends on various factors related to the actual nature of the metal and the ligands. For a single metal in a single oxidation state, the magnitude of ΔO depends largely on the ligands present; a range of complexes of such a metal can be investigated by spectroscopy, in which the frequency of the absorption maxima correlates to the magnitude of the crystal field parameter, ΔO.

In this experiment, three homoleptic (single ligand-type) octahedral complexes of chromium(III) will be prepared, and their UV spectra compared in order to construct a spectrochemical series showing the magnitude of ΔO caused by different ligands. An unknown complex containing both water and chloride ligands will also be prepared, and its composition determined by the rule of average environment, a simple equation which assumes that the crystal field effects of different ligands are purely additive.

Procedure

The complex [Cr(en)3]Cl3·3H2O (en = ethylenediamine) was prepared as follows. 0.9g granular zinc and 2.66g CrCl3·6H2O was mixed with 10cm3 ethylenediamine (causing great effervesence) followed by 10cm3 methanol in a 50cm3 conical flask. The mixture was refluxed for 1 hour, with stirring, on a hot plate, using a water condenser with drying tube containing CaCl2. The grey-brown solution was cooled and then filtered to collect the grey-green product. The filter cake was washed with five 5cm3 portions of 10% ethylenediamine in methanol then purified by stirring in about 12cm3 slightly acidified warm water, filtering, and adding 100cm3 ethanol. The yellow precipitate was redissolved by warming the filtrate, then cooled in an ice bath and filtered. The dark yellow product was washed with methylated spirit, air dried and dessicator dried.

The complex [Cr(acac)3] (acac = acetylacetonate) was prepared as follows. 1.30g CrCl3·6H2O was dissolved in 30cm3 distilled water in a 100cm3 conical flask with a watch glass. To the solution was added 6 drops of concentrated ammonia, 10g of urea and 4.0cm3 of acetylacetone. The mixture was heated on a steam bath, with occasional stirring; within an hour, dark purple crystals formed on the surface of the liquid. After two hours the mixture was cooled to room temperature and the purple crystalline product collected by filtration, washed with water and air dried. It was then recrystallised from 35cm3 each of acetone and ...