Transition Metals and Coordination Chemistry
Transition Metals and Coordination Chemistry
The exam tests the characteristic properties of transition metals, complex ion geometry and charge, ligand substitution, the origin of colour, and the redox chemistry of ions such as Fe2+, Fe3+, MnO4- and Cr2O7 2-.
Characteristic properties
A transition metal has an incomplete d subshell in at least one common oxidation state and forms coloured compounds, variable oxidation states, complex ions and catalytic activity. Scandium and zinc are d-block but not typical: Sc3+ is d0 and Zn2+ is d10, so their compounds are colourless and they show one oxidation state. Variable oxidation states allow electron transfer, which is why Fe2+/Fe3+ and MnO4-/Mn2+ systems are used in titrations.
Complex ions and ligands
A ligand donates a lone pair into a vacant orbital on the metal, forming a coordinate bond. Coordination number 6 gives octahedral geometry; 4 gives tetrahedral or square planar. Monodentate ligands include H2O, NH3, Cl- and CN-; bidentate ligands include ethane-1,2-diamine and ethanedioate; EDTA4- is hexadentate. The complex charge is the metal ion charge plus the sum of ligand charges.
Colour and ligand substitution
Ligands split the d orbitals into two levels separated by delta E. An electron absorbs light of energy h x frequency matching delta E and is promoted; the colour seen is the complement of the absorbed colour. Stronger-field ligands give a larger delta E and a different colour. Zn2+ is colourless because d-d transitions are impossible with a full d subshell.
Worked example
Explain the colour change when excess ammonia is added to aqueous copper(II) sulfate. Initially [Cu(H2O)6]2+ is pale blue. Ammonia acts as a ligand and displaces water: [Cu(H2O)6]2+ + 4NH3 <=> [Cu(NH3)4(H2O)2]2+ + 4H2O. Ammonia is the stronger-field ligand, so delta E increases and the absorbed wavelength changes, and the solution becomes deep blue. Note the coordination number stays 6 and the overall charge stays 2+.
Traps
Writing [Cu(NH3)6]2+; forgetting ligand charges when totalling complex charge; saying the metal reflects its colour rather than transmitting the complement; confusing ligand substitution with redox; and claiming all d-block ions are coloured.
Key ideas
- Transition metals: variable oxidation states, coloured complexes, catalysis, ligand bonding.
- Complex charge = metal charge + ligand charges.
- Colour arises from d-d transitions; the observed colour is complementary to that absorbed.
- Stronger-field ligands increase the d-orbital splitting.
- Zn2+ (d10) and Sc3+ (d0) compounds are colourless.