General properties of transition metals (A-level only) — AQA A-Level Chemistry
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General properties of transition metals (A-level only) explained
A transition metal is a d-block element that forms at least one stable ion with a partially filled d sub-level, and that single definition does all the work.
Read the full explanation
Scandium is in the block but its only common ion, Sc³⁺, is 3d⁰; zinc's only common ion, Zn²⁺, is 3d¹⁰. One is empty and one is full, so neither element is a transition metal and neither shows the characteristic chemistry. Titanium through to copper all manage an ion with between one and nine d electrons. Two configurations break the filling pattern, chromium as [Ar]3d⁵4s¹ and copper as [Ar]3d¹⁰4s¹, because a half-full or full 3d sub-level is lower in energy. When ions form the 4s electrons leave first, so Fe²⁺ is [Ar]3d⁶ and Fe³⁺ is [Ar]3d⁵.
The characteristic properties include: complex formation formation of coloured ions variable oxidation state catalytic activity.
Four properties define transition metal chemistry, tracing back to a partially filled 3d sub-level. The ions are small and highly charged, accepting lone pairs from ligands to form complexes. The d orbitals are split into two energy levels by ligands; because they are partly filled, an electron can be promoted across the gap by visible light, making the ions coloured. The 4s and 3d sub-levels lie close in energy, allowing different numbers of electrons to be lost for variable oxidation states. This ability, plus adsorption onto surfaces, makes them effective catalysts. While zinc and scandium do form complexes, they lack the partially filled d sub-level required for coloured ions, variable oxidation states, and catalytic activity.
A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.
A ligand is a lone pair donor: it has at least one lone pair and gives that pair to a metal ion to form a co-ordinate, or dative covalent, bond. Both electrons in the bond come from the ligand, which is why the bond is drawn as an arrow pointing from the donor atom to the metal. Ligands are classified by how many donor atoms each one uses. Monodentate ligands such as H₂O, NH₃, Cl⁻ and CN⁻ donate one pair each. Bidentate ligands such as ethanedioate, C₂O₄²⁻, and 1,2-diaminoethane, H₂NCH₂CH₂NH₂, donate two pairs and grip the metal at two points. Multidentate ligands such as EDTA⁴⁻ donate six pairs, from two nitrogen atoms and four oxygen atoms, wrapping right round the ion.
A complex is a central metal atom or ion surrounded by ligands.
A complex is a central metal atom or ion bonded to a set of ligands by co-ordinate bonds. Note atom or ion: Cr(PF₃)₆ is a complex even though the chromium is in oxidation state zero, because PF₃ is a neutral ligand and the whole species is neutral. The overall charge on a complex is the sum of the metal's oxidation state and the charges on the ligands, so [Cu(H₂O)₆]²⁺ carries 2⁺ from copper(II) with six neutral water ligands, while [CuCl₄]²⁻ is 2⁺ from copper(II) plus 4⁻ from four chloride ions. Square brackets enclose everything inside the co-ordination sphere, and any counter ion, such as the third chloride in [Cr(NH₃)₄Cl₂]Cl, sits outside them.
Co-ordination number is number of co-ordinate bonds to the central metal atom or ion.
Co-ordination number counts co-ordinate bonds, not ligands, and the exam tests that distinction directly. [Cu(H₂O)₆]²⁺ has six monodentate water ligands and a co-ordination number of 6. [CuCl₄]²⁻ has four chloride ligands and a co-ordination number of 4. But [Cr(C₂O₄)₃]³⁻ has only three ethanedioate ligands, each bidentate, so its co-ordination number is still 6, and [Cu(EDTA)]²⁻ has a single multidentate ligand donating six pairs, so its co-ordination number is 6 as well. Small ligands such as H₂O, NH₃ and CN⁻ usually allow six around the metal, while bulky ligands such as Cl⁻ usually allow only four.
Your focus
- Define a transition metal in terms of forming at least one stable ion with a partially filled d sub-level.
- Write the electron configuration of any atom or ion from titanium to copper, removing 4s electrons before 3d.
- Justify the exclusion of scandium and zinc by quoting the configurations of Sc³⁺ and Zn²⁺.
Show all 15 objectives
- List the four characteristic properties and trace colour, variable oxidation states, and catalytic activity back to the partially filled d sub-level.
- State the three changes to a complex that alter its colour and give an example of each.
- Contrast Cu²⁺ with Zn²⁺, identifying which characteristic properties Zn²⁺ lacks and explaining why.
- Define a ligand in terms of lone pair donation and name the type of bond it forms.
- Identify the donor atom in H₂O, NH₃, Cl⁻, CN⁻, C₂O₄²⁻ and EDTA⁴⁻ and classify each ligand by denticity.
- Draw a co-ordinate bond with the arrow pointing in the correct direction, from ligand to metal.
- State the meaning of the term complex, covering both a central metal atom and a central metal ion.
- Deduce the oxidation state of the metal in a complex from the overall charge and the ligand charges.
- Write the formula of a complex with square brackets placed correctly around the co-ordination sphere.
- Use a co-ordination number of 6 or 4 to predict the shape and bond angle of a complex: 6 gives octahedral at 90°, 4 usually gives tetrahedral at 109.5°.
- Deduce the co-ordination number of complexes containing monodentate, bidentate and multidentate ligands.
- Predict whether a given ligand produces a co-ordination number of 4 or 6 and justify the prediction by ligand size.
General properties of transition metals (A-level only) exam tips
Marking Points
- Defining a transition metal as one that forms at least one stable ion with a partially filled d sub-level.
- Quoting Sc³⁺ as 3d⁰ or Zn²⁺ as 3d¹⁰ when explaining why those two elements are excluded.
- Stating the correct configurations [Ar]3d⁵4s¹ for chromium and [Ar]3d¹⁰4s¹ for copper.
- Removing the 4s electrons before the 3d electrons when writing the configuration of an ion.
- Naming each property: complex formation, coloured ions, variable oxidation state and catalytic activity.
- Linking the properties of coloured ions, variable oxidation states, and catalytic activity back to the partially filled d sub-level.
- Naming the three changes that alter the colour of a complex: oxidation state, ligand and co-ordination number.
- Providing a correct example, such as [Cu(H₂O)₆]²⁺ being blue or iron catalysing the Haber process.
- one mark for stating that a ligand donates a lone pair of electrons to the metal
- one mark for naming the bond formed as a co-ordinate or dative covalent bond
- one mark for identifying the donor atom in a named ligand, such as the nitrogen in NH₃ or an oxygen in H₂O
- one mark for classifying a ligand as monodentate, bidentate or multidentate by counting its donor atoms
- one mark for central metal atom or ion surrounded by ligands as the definition
- one mark for deducing the oxidation state of the metal from the overall charge and the ligand charges
- one mark for a correctly written formula with square brackets and the overall charge outside them
- one mark for explaining a zero oxidation state, as in Cr(PF₃)₆, by both the ligands and the complex being neutral
- one mark for defining co-ordination number as the number of co-ordinate bonds to the central metal atom or ion
- one mark for multiplying the number of ligands by the donor atoms each supplies when the ligand is bidentate or multidentate
- one mark for the correct co-ordination number of a named complex, such as 6 in [Cr(C₂O₄)₃]³⁻
- one mark for linking a co-ordination number of 4 to larger ligands such as Cl⁻ and 6 to small ligands such as H₂O
Examiner Tips
- 💡Use the word ion in the definition, because atom alone is incorrect and will not score.
- 💡Write the full [Ar] configuration rather than a shorthand and check the electrons total the atomic number.
- 💡If asked about scandium or zinc, name the specific ion and its configuration as your evidence.
- 💡If the question asks for three ways to change a complex's colour, answer exactly: change the ligand, the oxidation state or the co-ordination number.
- 💡Attach a named example to each property, because a bare list usually scores less than a list with evidence.
- 💡Use the partially filled d sub-level as your explanation for colour, variable oxidation states, and catalytic activity.
- 💡Say lone pair and co-ordinate bond explicitly; paraphrases such as joins to the metal do not score.
- 💡Count donor atoms rather than the size of the molecule when deciding how many bonds a ligand forms.
- 💡Remember that the charge on every ligand contributes to the overall charge when you write a formula.
- 💡Write atom or ion in the definition, because atom is what earns credit on zero oxidation state examples.
- 💡Set the arithmetic out: metal oxidation state plus total ligand charge equals the charge shown outside the bracket.
- 💡Decide which species are bonded to the metal before writing the brackets, since anything ionically bound sits outside.
- 💡Count donor atoms: the number of ligands multiplied by their denticity gives the co-ordination number.
- 💡Quote the definition using the words co-ordinate bonds, because number of ligands is not accepted.
- 💡Use the size of the ligand to predict 4 or 6 when a question gives you an unfamiliar complex.
Common Mistakes
- Saying a transition metal has a partially filled d sub-level in the atom, which wrongly admits scandium (atom is 3d¹) and wrongly excludes copper (atom is 3d¹⁰); correct by specifying it must form a stable ion with a partially filled d sub-level.
- Writing Fe³⁺ as [Ar]3d³4s² by taking electrons from 3d before 4s; correct by removing 4s electrons first.
- Giving chromium as [Ar]3d⁴4s² and copper as [Ar]3d⁹4s²; correct by using the anomalous half-full and full 3d configurations.
- Claiming scandium is excluded because it has no d electrons, when the neutral atom is [Ar]3d¹4s²; correct by referencing the Sc³⁺ ion.
- Listing high melting point or good electrical conductivity as a characteristic property, which is true of metals generally; correct by focusing on the four specific transition metal properties.
- Naming a change of concentration as a way to change the colour of a complex; correct by specifying a change in ligand, oxidation state or co-ordination number.
- Saying transition metals are coloured; correct by stating it is their ions and complexes that are coloured.
- Claiming zinc or scandium do not form complexes; correct by noting they do form complexes but lack the other three characteristic properties.
- saying a ligand shares a pair of electrons, which describes an ordinary covalent bond rather than a co-ordinate one
- drawing the arrow of the co-ordinate bond from the metal to the ligand
- calling 1,2-diaminoethane monodentate because it is a single molecule, instead of counting its two nitrogen donor atoms
- giving EDTA⁴⁻ four donor atoms because of its four negative charges, rather than six
- defining a complex as a metal ion only, which loses the mark on neutral complexes such as Cr(PF₃)₆
- omitting the overall charge when writing the formula of a complex ion
- including a counter ion inside the square brackets, for example writing [Cr(NH₃)₄Cl₃]
- forgetting that anionic ligands such as Cl⁻ lower the overall charge when deducing an oxidation state
- counting ligands instead of bonds, giving [Cr(C₂O₄)₃]³⁻ a co-ordination number of 3
- giving [Cu(EDTA)]²⁻ a co-ordination number of 1 because it contains one ligand
- confusing co-ordination number with the oxidation state of the metal
- assuming every complex is six co-ordinate, so describing [CuCl₄]²⁻ as octahedral