Coordination Chemistry & Ligands Cheat Sheet

A printable reference covering coordination complexes, ligands, oxidation states, coordination number, geometry, isomerism, and crystal field splitting for grades 11-12.

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Coordination chemistry studies compounds in which a central metal ion is bonded to surrounding ions or molecules called ligands. This cheat sheet helps students identify complex ions, name ligands, determine oxidation states, and predict common shapes. These skills are important for understanding transition metal reactions, colors, magnetism, and biological metal complexes. The core ideas are coordination number, ligand denticity, complex charge, and metal oxidation state. Students also need to connect geometry with common coordination numbers such as $4$ and $6$ . Crystal field splitting explains why many transition metal complexes are colored and why some are paramagnetic. Formation constants such as $K_f$ describe how strongly ligands bind to metal ions.

Key Facts

A coordination complex has a central metal ion bonded to ligands, often written in brackets such as $[\mathrm{Co(NH_3)_6}]^{3+}$ .
The oxidation state of the metal is found by charge balance: $\text{metal charge} + \sum \text{ligand charges} = \text{complex charge}$ .
The coordination number is the number of donor atoms directly bonded to the metal, so $[\mathrm{Co(NH_3)_6}]^{3+}$ has coordination number $6$ .
Monodentate ligands donate one electron pair, while bidentate ligands donate two electron pairs through two donor atoms.
Common geometries include linear for coordination number $2$ , tetrahedral or square planar for coordination number $4$ , and octahedral for coordination number $6$ .
The formation constant is $K_f = \frac{[\mathrm{ML_n}]}{[\mathrm{M}][\mathrm{L}]^n}$ for the equilibrium $\mathrm{M} + n\mathrm{L} \rightleftharpoons \mathrm{ML_n}$ .
In octahedral crystal field splitting, the $d$ orbitals separate into lower-energy $t_{2g}$ orbitals and higher-energy $e_g$ orbitals with energy gap $\Delta_o$ .
A complex is paramagnetic if it has one or more unpaired electrons, and it is diamagnetic if all electrons are paired.

Vocabulary

Coordination complex: A species made of a central metal ion bonded to surrounding ligands through coordinate covalent bonds.
Ligand: An ion or molecule that donates an electron pair to a metal ion to form a coordinate covalent bond.
Coordination number: The number of ligand donor atoms directly bonded to the central metal ion.
Denticity: The number of donor atoms in one ligand that can bond to the same metal ion.
Chelate: A complex formed when a multidentate ligand bonds to a metal ion at more than one site.
Crystal field splitting: The separation of metal $d$ orbital energies caused by the electric fields of surrounding ligands.

Common Mistakes to Avoid

Ignoring ligand charge when finding oxidation state is wrong because neutral ligands such as $\mathrm{NH_3}$ and charged ligands such as $\mathrm{Cl^-}$ affect charge balance differently.
Confusing coordination number with the number of ligands is wrong because one ligand can attach through more than one donor atom, as in a bidentate ligand.
Assuming all coordination number $4$ complexes are tetrahedral is wrong because some $d^8$ metal ions commonly form square planar complexes.
Forgetting brackets around the complex ion is wrong because brackets show which ligands are directly coordinated to the metal, as in $[\mathrm{Cu(NH_3)_4}]^{2+}$ .
Treating color as random is wrong because the observed color depends on $\Delta_o$ , ligand strength, and electronic transitions between split $d$ orbitals.

Practice Questions

1 Find the oxidation state of cobalt in $[\mathrm{Co(NH_3)_5Cl}]^{2+}$ .
2 What is the coordination number of the metal in $[\mathrm{Pt(en)_2Cl_2}]^{2+}$ if $\mathrm{en}$ is bidentate?
3 Write the expression for $K_f$ for $\mathrm{Ag^+} + 2\mathrm{NH_3} \rightleftharpoons [\mathrm{Ag(NH_3)_2}]^+$ .
4 Explain why a complex with strong-field ligands may have fewer unpaired electrons than a similar complex with weak-field ligands.

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