Transition Metal Complexes – Color, Coordination Number, and Geometry
ChemistryInorganic ChemistryAges 17–18Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Inorganic Chemistry
A beaker holds the aqua complex of a transition metal ion (Cu²⁺, Co²⁺, Ni²⁺, Fe³⁺, Cr³⁺) in its real color. Drag the ligand-concentration slider (NH₃, concentrated Cl⁻, SCN⁻, or OH⁻) so the ligand gradually replaces the water molecules in the inner coordination sphere: the solution's color shifts to that of the new complex, and a rotatable 3D model shows the correct octahedral, tetrahedral, or square-planar shape, together with the complex formula and coordination number. The d-orbital splitting screen: choose an ion (Ti³⁺, Cr²⁺/Cr³⁺, Mn²⁺, Fe²⁺/Fe³⁺, Co³⁺, Cu²⁺) and a ligand along the spectrochemical series I⁻ < Cl⁻ < H₂O < NH₃ < en < CN⁻ < CO to see Δo = hν, the wavelength absorbed and the complementary color, high or low spin from comparing Δo with the pairing energy, the unpaired electrons and the spin-only moment μ = √(n(n+2)) μB measured on a Gouy balance. The Test tubes screen: add NaOH, NH₃ or Na₂CO₃ drop by drop and then in excess to Cu²⁺, Fe²⁺, Fe³⁺, Co²⁺, Cr³⁺, Al³⁺ or an unknown solution, read the pH and record the precipitates, amphoteric behaviour and CO₂.
Lesson: Coordination complexes: central atom, ligands, coordination number, geometry, and color
What it shows
A transition metal complex has a central metal ion bonded to ligands, molecules or ions that donate electron pairs. In water the ion exists as an aqua complex; an added ligand replaces water molecules and can change the coordination number, the geometry and the color. Six ligands usually give an octahedral shape, four a tetrahedral or square-planar one. The color arises because the ligands split the d-orbital energies and the complex absorbs part of visible light, so a new ligand gives a new color, such as the deep blue of Cu²⁺ with excess NH₃. The d splitting screen shows the t₂g and e_g levels separated by Δo, which grows along the spectrochemical series and with the charge of the ion; light of energy hν ≈ Δo is absorbed, and comparing Δo with the pairing energy decides between high and low spin. On the Test tubes screen, the amounts of reagent decide what forms: hydroxides precipitate, some dissolve again in excess, and 3+ ions, more acidic because of their higher charge density, release CO₂ from carbonate.
How to use
Choose a Central metal ion and a Ligand added, then drag the Ligand concentration added slider up from 0 M. Watch the solution color change as ligands replace water, and read the formula, coordination number and geometry in the equation line. With Show 3D complex model on, drag to rotate the ball-and-stick model. Some combinations give a precipitate or no clear change, which is also worth discussing. On the d splitting tab, pick a Metal ion and a Ligand (or tap the series strip), compare Δo with P, read the color seen, and turn on the Gouy balance magnet to measure the moment. On Test tubes, choose a Solution and a Reagent, add drops, then excess, and press Record; try Unknown to identify an ion.
Parameters you can change
- Central metal ion Cu²⁺, Co²⁺, Ni²⁺, Fe³⁺, Cr³⁺
- Ligand added Excess NH₃, Concentrated Cl⁻ (HCl), SCN⁻, OH⁻
- Ligand concentration added 0–6 M
- Show 3D complex model
- Screen Colour and geometry, d-orbital splitting (Δo), Test tubes
- Metal ion (splitting screen) Ti³⁺, Cr²⁺, Cr³⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co³⁺, Cu²⁺
- Ligand in the spectrochemical series I⁻, Cl⁻, H₂O, NH₃, en (ethylenediamine), CN⁻, CO
- Gouy balance magnet on
- Solution (test-tube screen) Cu²⁺, Fe²⁺, Fe³⁺, Co²⁺, Cr³⁺, Al³⁺, Unknown
- Reagent (test-tube screen) NaOH, NH₃, Na₂CO₃
Questions to explore
- In the complex formed by Cu²⁺ with excess NH₃, which species is the central ion and which are the ligands?
- Why does the solution change color when a new ligand replaces the water molecules?
- Which geometries are possible for a complex with coordination number 4?
- Why do Fe³⁺, Cr³⁺ and Al³⁺ solutions release CO₂ with Na₂CO₃ while Cu²⁺, Fe²⁺ and Co²⁺ form carbonates?
