Summary — What This Chapter Covers
- A transition element is a d-block element that forms one or more stable ions with incomplete d orbitals.
- Typical properties: variable oxidation states, catalytic activity, complex ions and coloured compounds.
- Ligands donate a lone pair to the metal ion through dative bonds; complexes are linear, square planar, tetrahedral or octahedral.
- Ligand exchange changes the colour of copper(II) and cobalt(II) complexes.
- E⦵ values predict redox feasibility; MnO4−, Cu2+/I− and other titrations give calculations.
- Colour comes from electrons moving between split, non-degenerate d orbitals; ΔE depends on the ligand and the geometry.
- Complexes show cis/trans and optical isomerism; Kstab measures how stable a complex is.
1. Physical and Chemical Properties of the First-Row Transition Elements
28.1
What counts as a transition element
A transition element is a d-block element which forms one or more stable ions with incomplete d orbitals. Titanium to copper all qualify. Scandium (Sc3+ is 3d0) and zinc (Zn2+ is 3d10) are d-block elements but not transition elements.
| Element | Atom | Common ion |
|---|
| Ti | [Ar] 3d2 4s2 | Ti3+ [Ar] 3d1 |
| V | [Ar] 3d3 4s2 | V3+ [Ar] 3d2 |
| Cr | [Ar] 3d5 4s1 | Cr3+ [Ar] 3d3 |
| Mn | [Ar] 3d5 4s2 | Mn2+ [Ar] 3d5 |
| Fe | [Ar] 3d6 4s2 | Fe2+ 3d6, Fe3+ 3d5 |
| Co | [Ar] 3d7 4s2 | Co2+ [Ar] 3d7 |
| Ni | [Ar] 3d8 4s2 | Ni2+ [Ar] 3d8 |
| Cu | [Ar] 3d10 4s1 | Cu2+ [Ar] 3d9 |
When the atoms form ions, the 4s electrons are lost first, then 3d.
Shapes of two 3d orbitals
3dxy: four lobes in the xy plane, pointing between the x and y axes. 3dz²: two lobes along the z axis with a ring (doughnut) of electron density around the middle in the xy plane.
Four typical properties, and why
- Variable oxidation states: the 3d and 4s sub-shells are similar in energy, so different numbers of electrons can be lost (or shared) for a similar energy cost. For example Fe +2, +3; Mn +2, +4, +6, +7; Cr +3, +6; Cu +1, +2; V +2, +3, +4, +5.
- Catalysts: they have more than one stable oxidation state, so they can accept and donate electrons (being oxidised and reduced back), and they have vacant d orbitals that are energetically accessible, which can form dative bonds with ligands or reactant molecules.
- Complex ions: they have vacant d orbitals that are energetically accessible, which can accept lone pairs from ligands.
- Coloured compounds: explained in section 3 (partially filled, split d orbitals).
Practice Question
Explain why iron can form Fe2+ and Fe3+ ions.
2. Characteristic Chemical Properties of Transition Elements
28.2
Ligands, complexes and coordination number
A ligand is a species that contains a lone pair of electrons that forms a dative covalent bond to a central metal atom or ion.
A complex is a molecule or ion formed by a central metal atom or ion surrounded by one or more ligands.
coordination number
| Type | Dative bonds per ligand | Examples |
|---|
| Monodentate | 1 | H2O, NH3, Cl−, CN− |
| Bidentate |
2 |
2
2
2
2
|