What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

State the main postulates of Werner's theory.

Metals show primary (ionisable) and secondary (non-ionisable) valencies; secondary valencies are fixed in number and directed in space, giving definite geometry.

What is the difference between a unidentate and a bidentate ligand?

A unidentate ligand binds through one donor atom; a bidentate ligand binds through two donor atoms (e.g. ethylenediamine).

Define coordination number with an example.

It is the number of donor atoms bonded to the metal; in \([Co(NH_3)_6]^{3+}\) it is six.

What is a chelate and why is it more stable?

A ring complex formed by a polydentate ligand; ring formation gives extra (chelate) stability.

Name the types of structural isomerism in coordination compounds.

Ionisation, linkage, coordination and hydrate (solvate) isomerism.

What is geometrical isomerism?

Cis-trans isomerism arising from different spatial arrangements of ligands around the metal.

How does Valence Bond Theory explain the shape of complexes?

Through hybridisation of metal orbitals (e.g. \(d^2sp^3\) gives octahedral, \(dsp^2\) gives square planar).

What decides whether a complex is high spin or low spin?

The size of the crystal field splitting relative to the pairing energy, set by the strength of the ligand.

What does the spectrochemical series arrange?

Ligands in order of increasing crystal field splitting strength.

Name three biologically important coordination compounds.

Chlorophyll (Mg), haemoglobin (Fe) and vitamin B12 (Co).

Coordination Compounds

Class 12 Chemistry · Chemistry Part I · Chapter 5

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Summary

Coordination compounds are formed when a central metal atom or ion binds a number of anions or neutral molecules, called ligands, by coordinate bonds in which the ligand donates a lone pair of electrons. Werner's theory introduced primary and secondary valencies: the primary valency is satisfied by negative ions and the secondary valency (the coordination number) fixes the geometry. Key terms include the central atom, ligand, coordination number, coordination sphere and coordination polyhedron; ligands may be unidentate, bidentate or polydentate, and chelating ligands form ring structures of extra stability. Compounds are named by systematic IUPAC rules and exhibit isomerism: structural isomerism (ionisation, linkage, coordination and hydrate types) and stereoisomerism (geometrical cis-trans and optical). Bonding is explained by Valence Bond Theory, which uses hybridisation to predict shape and magnetic behaviour, and by Crystal Field Theory, which treats ligands as point charges that split the d orbitals; the magnitude of splitting decides whether a complex is high spin or low spin and accounts for colour. Coordination compounds are vital in biological systems such as chlorophyll, haemoglobin and vitamin B12, and find use in metallurgy, analysis, medicine and electroplating.

Werner's theory and key termsLigands, coordination number and nomenclatureIsomerism in coordination compoundsValence Bond Theory and hybridisationCrystal Field Theory and colourImportance and applications of complexes

Key terms

Ligand

An ion or molecule that donates a lone pair of electrons to the central metal atom or ion.

Coordination number

The number of ligand donor atoms directly bonded to the central metal ion.

Chelate

A ring structure formed when a polydentate ligand binds the central ion at two or more points.

Coordination sphere

The central metal and its attached ligands, written within square brackets.

Crystal field splitting

The energy difference \(\Delta_o\) between sets of d orbitals split by the ligand field.

Homoleptic complex

A complex in which the metal is bound to only one kind of ligand.

Extra questions & answers

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A ligand is best described as a species that:

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