Spectrophotometry: measure the spectrochemical series
Record the absorption spectra of nine nickel(II) and chromium(III) complexes, find the lowest-energy d–d band, convert its wavelength into the splitting Δo and rank the ligands by it.
Aim Obtain Δo from the lowest-energy band of d³ and d⁸ ions, arrange the ligands in the spectrochemical series and compare the two metals.
Procedure
- Choose 0.10 M . Set the monochromator to 400 nm and press “Read and record”; step the wavelength and read again, or press “Scan every 10 nm” to get the whole spectrum from 350 to 1300 nm.
- Find the band at the longest wavelength (lowest energy) and add readings every 2–5 nm around its top to pin down the wavelength of maximum absorption, λmax.
- Enter λmax: the instrument converts it to ν̃ = 10⁷/λ in cm⁻¹ and to kJ/mol, and compares it with the literature value. Add the result to the table.
- Repeat for and , then for the six chromium(III) samples (F⁻, H₂O, oxalate, NH₃, en, CN⁻).
- Order the ligands by Δo for nickel and for chromium and compare with the spectrochemical series; then compare Δo for Cr(III) and Ni(II) with the same ligand.
- For each chromium(III) complex read the second band as well, use the Tanabe–Sugano relations to get the Racah parameter B from the two bands, divide by the free-ion 917 cm⁻¹ to get β, and rank the nephelauxetic series.
What to notice
- Nickel shows three bands, the lowest in the near infrared: 1176 nm for the aqua ion, moving to 930 nm and 893 nm with NH₃ and en.
- Chromium shows two visible bands; the lower-energy one moves from 671 nm for F⁻ all the way to 375 nm for CN⁻, and the solutions go from green through violet to yellow-orange and pale yellow.
- Both metals give the same ligand order: F⁻ < H₂O ≈ oxalate < NH₃ < en < CN⁻.
- With the same ligand, Δo for Cr³⁺ is about twice that for Ni²⁺: a more highly charged ion draws the ligands closer and splits the d orbitals more.
- β falls from 0.98 for F⁻ through H₂O 0.79, NH₃ 0.72, en 0.68 and oxalate 0.67 to 0.53 for CN⁻: the softer the ligand and the more covalent its bond to the metal, the further the d-electron cloud spreads and the less the electrons repel each other.
- The bands are broad, so the top can be located only to within a few nanometres; the “instrument noise” is deterministic, so repeating a reading gives the same value.
In a real lab
- Zero the instrument on a blank of the solvent (or of the ligand solution alone) before each measurement, and use matched cuvettes; glass and plastic absorb below about 340 nm, so the ultraviolet needs quartz.
- d–d transitions are weak (ε only 1–100 L mol⁻¹ cm⁻¹), so 0.01–0.1 M solutions are needed. Most teaching instruments stop at 1100 nm; the 1176 nm band of needs a near-infrared detector, otherwise Δo has to be found from the second band with a Tanabe–Sugano diagram.
- Real bands often carry shoulders (spin–orbit coupling, Jahn–Teller distortion), for example near 15 400 cm⁻¹ for . Commercial gives green ; for violet , dissolve chrome alum instead.
- Nickel salts are sensitisers and possible carcinogens, and ethylenediamine and concentrated ammonia are corrosive and irritating: wear gloves and work in a fume cupboard. Never acidify cyanido complexes (HCN is released); school practicals usually leave them out.