Transition Metals
P7 – Describe Features of Transition Metal Complexes (400 words)
Transition metal complexes are substances made when a transition metal ion joins with other
molecules or ions called ligands. These complexes often have bright colours, can have different
charges (oxidation states), and are useful in many chemical processes, including reactions in
industry. (Solubility of Things, 2025).
Transition Metals
A transition metal is a type of metal found in the middle part of the periodic table (called the d-
block). These metals form ions that usually have incomplete d-orbitals, which gives them their
special properties. They can form coloured compounds and bond with ligands in different ways
(Study Mind, 2025). However, mercury (Hg) is a bit unusual. Even though it’s in the d-block,
mercury in its common +2 charge has a full d-orbital (d10), which means it doesn’t show typical
transition metal behaviour like forming colourful complexes or having multiple charges, so it’s not
always counted as a "true" transition metal (Studocu, 2024).
Ligands
A ligand is a molecule or ion that donates a pair of electrons to the metal ion to form a bond. They
act as Lewis bases (electron pair donors), and the central atom acts as a Lewis acid (electron pair
acceptor). Ligands can be simple, like water (H₂O), ammonia (NH₃), or chloride ions (Cl⁻), or they
can be larger and more complex. Some ligands only attach at one point (called monodentate), while
others can attach at two or more points (bidentate or polydentate) (Libre Texts Chemistry, 2025).
Complexes
When a transition metal bonds with ligands, it forms a complex, through co-ordinate bonding
(Study Mind, 2025). The metal sits in the centre, and the ligands arrange themselves around it. The
number of ligands or attachment points is called the coordination number, and this affects the shape
of the complex (Cognito, 2025). Below is an example of a labelled complex ion.
(Cognito, 2025)
5
2021_22 BTEC Assignment Brief
Issue Date: November 2022
Owner: BTEC Assessment
DCL1 Public (Unclassified)
Version 1.0
, Shapes
One common shape is octahedral, where six ligands surround the metal in a shape like two
pyramids joined at the base. An example is [Fe(H2O)6]3+, where an iron ion is surrounded by six
water molecules. Another shape is tetrahedral, where four ligands form a pyramid-like structure,
like in [CuCl4]2− which has four chloride ions around a copper ion. A third shape is square planar,
where four ligands lie flat around the metal in a square. This is common in metals like platinum, as
seen in [Pt(NH3)2Cl2], a medicine used to treat cancer (Chem Guide, 2014).
(Chem Guide, 2014).
Square Brackets
In chemical formulas, square brackets [ ] are used to show what’s inside the coordination sphere,
which is where the metal and the ligands that are directly attached to it (BYJUS, 2025). Anything
outside the brackets (like a charge) shows the overall charge of the complex (Cognito, 2025). For
example, in [Co(NH3)6]3+ the cobalt ion is surrounded by six ammonia molecules, and the whole
complex has a charge of +3. Below is an example of this.
(Study Mind, 2025).
6
2021_22 BTEC Assignment Brief
Issue Date: November 2022
Owner: BTEC Assessment
DCL1 Public (Unclassified)
Version 1.0
P7 – Describe Features of Transition Metal Complexes (400 words)
Transition metal complexes are substances made when a transition metal ion joins with other
molecules or ions called ligands. These complexes often have bright colours, can have different
charges (oxidation states), and are useful in many chemical processes, including reactions in
industry. (Solubility of Things, 2025).
Transition Metals
A transition metal is a type of metal found in the middle part of the periodic table (called the d-
block). These metals form ions that usually have incomplete d-orbitals, which gives them their
special properties. They can form coloured compounds and bond with ligands in different ways
(Study Mind, 2025). However, mercury (Hg) is a bit unusual. Even though it’s in the d-block,
mercury in its common +2 charge has a full d-orbital (d10), which means it doesn’t show typical
transition metal behaviour like forming colourful complexes or having multiple charges, so it’s not
always counted as a "true" transition metal (Studocu, 2024).
Ligands
A ligand is a molecule or ion that donates a pair of electrons to the metal ion to form a bond. They
act as Lewis bases (electron pair donors), and the central atom acts as a Lewis acid (electron pair
acceptor). Ligands can be simple, like water (H₂O), ammonia (NH₃), or chloride ions (Cl⁻), or they
can be larger and more complex. Some ligands only attach at one point (called monodentate), while
others can attach at two or more points (bidentate or polydentate) (Libre Texts Chemistry, 2025).
Complexes
When a transition metal bonds with ligands, it forms a complex, through co-ordinate bonding
(Study Mind, 2025). The metal sits in the centre, and the ligands arrange themselves around it. The
number of ligands or attachment points is called the coordination number, and this affects the shape
of the complex (Cognito, 2025). Below is an example of a labelled complex ion.
(Cognito, 2025)
5
2021_22 BTEC Assignment Brief
Issue Date: November 2022
Owner: BTEC Assessment
DCL1 Public (Unclassified)
Version 1.0
, Shapes
One common shape is octahedral, where six ligands surround the metal in a shape like two
pyramids joined at the base. An example is [Fe(H2O)6]3+, where an iron ion is surrounded by six
water molecules. Another shape is tetrahedral, where four ligands form a pyramid-like structure,
like in [CuCl4]2− which has four chloride ions around a copper ion. A third shape is square planar,
where four ligands lie flat around the metal in a square. This is common in metals like platinum, as
seen in [Pt(NH3)2Cl2], a medicine used to treat cancer (Chem Guide, 2014).
(Chem Guide, 2014).
Square Brackets
In chemical formulas, square brackets [ ] are used to show what’s inside the coordination sphere,
which is where the metal and the ligands that are directly attached to it (BYJUS, 2025). Anything
outside the brackets (like a charge) shows the overall charge of the complex (Cognito, 2025). For
example, in [Co(NH3)6]3+ the cobalt ion is surrounded by six ammonia molecules, and the whole
complex has a charge of +3. Below is an example of this.
(Study Mind, 2025).
6
2021_22 BTEC Assignment Brief
Issue Date: November 2022
Owner: BTEC Assessment
DCL1 Public (Unclassified)
Version 1.0
