Chemistry Labs

Inorganic chemistry

Isomerism and nomenclature of coordination complexes

A coordination formula does not uniquely determine a structure: ligands can exchange positions, connect through different atoms, or form different networks. Systematic naming makes the chosen structure unambiguous.

1 Lower secondary2 Upper secondary3 Undergraduate4 Advanced5 Research

IntuitionIntuition: same ingredients, different arrangements

Cisplatin and transplatin both have formula [PtCl₂(NH₃)₂], yet the two chlorides are adjacent in cisplatin and opposite in transplatin. This small geometrical change produces distinct chemical and biological behaviour.

Rotate the pair of square-planar Pt complexes. Cl and N atoms are shown; compare adjacent and opposite chloride ligands.

SchoolSchool level: classify the differences

Definition: Geometrical isomerism

Geometrical isomers have the same bonding connectivity but different spatial arrangement. Cis/trans labels apply in suitable square-planar or octahedral geometries; for octahedral complexes, fac/mer distinguishes arrangements of three identical ligands.

Definition: Linkage and ionisation isomers

A linkage isomer uses an ambidentate ligand through a different donor atom, as in nitro-κN versus nitrito-κO. Ionisation isomers exchange a coordinated anion with a counterion, changing which ions appear in solution.

Common coordination isomerism
TypeWhat changesExample
GeometricalLigand positions[PtCl₂(NH₃)₂] cis/trans
OpticalEnantiomeric handedness[Co(en)₃]³⁺, Δ/Λ
LinkageDonor atom[Co(NH₃)₅NO₂]²⁺

Example: Name an octahedral complex

Give the IUPAC-style name for K₃[Fe(CN)₆].

Solution

Name the cation first: potassium. In the complex anion, six cyanido ligands give hexacyanido; Fe has oxidation state +3, and an anionic iron complex uses ferrate: potassium hexacyanidoferrate(III).

UndergraduateUndergraduate: coordination formula and systematic naming

Determine the coordination entity, ligand identities and denticities before naming. List ligand names alphabetically (ignoring multiplicative prefixes), then the metal with oxidation state in Roman numerals. For an anionic complex the metal name takes the conventional -ate form (ferrate, cuprate).

charge du complexe=eˊtat d’oxydation du meˊtal+∑icharges des ligandsi\text{charge du complexe} = \text{état d’oxydation du métal} + \sum_i \text{charges des ligands}_i

Use κ notation to state the donor atom when ambiguity is possible (e.g. thiocyanato-κN versus thiocyanato-κS). Use μ for bridging ligands and η for hapticity in organometallic coordination; these descriptors refer to different structural information.

Optical isomers are non-superposable mirror images. In octahedral [M(AA)₃] complexes with three bidentate ligands, the right- and left-handed helices are conventionally labelled Δ and Λ. A chiral complex can rotate plane-polarised light, but the sign of rotation is not implied by Δ/Λ.

Example: Oxidation state from charge balance

Find the oxidation state of Co in [Co(NH₃)₅Cl]Cl₂.

Solution

Two outer chloride ions imply a +2 complex cation. NH₃ is neutral and coordinated chloride is −1, so x − 1 = +2 and Co is +3. The salt is pentaamminechloridocobalt(III) chloride.

For square-planar d⁸ Pt(II), cis-[PtCl₂(NH₃)₂] is polar and transplatin has a symmetry plane that cancels bond dipoles. Geometry can therefore affect solubility, substitution pathways, target binding and pharmacology without changing formula.

AdvancedStereochemical descriptors and fluxionality

Cis/trans is not a universal descriptor for every coordination polyhedron. Use ligand-specific locants or established descriptors for lower-symmetry structures; in complex cases, stereochemical configuration can be specified with a complete coordination geometry and donor-atom mapping.

Coordination compounds can be fluxional: Berry pseudorotation interconverts trigonal-bipyramidal sites, while octahedral ligand rearrangements may proceed by dissociative, associative or interchange pathways. A drawing is a static snapshot, not necessarily a single rigid solution structure.

References