xSkill: notes, lectures and practice for CBSE and CSE

1.1 The basic vocabulary A coordination compound contains a central metal ion surrounded by ligands. The coordination number is the number of donor atoms attached to the metal, which is not always the same as the number of ligands when a ligand is bidentate. The oxidation state of the metal is found by treating every ligand as the ion or molecule it is outside the complex and making the charge of the whole entity add up. Werner described the primary valence as the ionisable charge and the secondary valence as the coordination number. The effective atomic number adds the metal's own electrons to twice the coordination number, less the electrons lost in forming the ion, and the rare gas value that results was the original justification for coordination.

oxidation state : x = charge of complex - \sum (ligand charges)

Used to find the metal oxidation state

EAN = (Z - x) + 2 (CN)

Effective atomic number for a metal of atomic number Z and oxidation state x

μ = n (n + 2) BM

Spin only magnetic moment from n unpaired electrons

CFSE = (- 0.4 n_{t_{2 g}} + 0.6 n_{e_{g}}) Δ_{o}

Crystal field stabilisation energy for an octahedral complex

Δ_{t} = \frac{4}{9} Δ_{o}

Tetrahedral splitting is smaller than octahedral splitting for the same ligand set

for d^{4} : Δ_{o} > P gives t_{2 g}^{4} (low spin), Δ_{o} < P gives t_{2 g}^{3} e_{g}^{1} (high spin)

High spin and low spin depend on the competition between splitting and pairing energy

1.2 Hybridisation patterns For coordination number four, a tetrahedral complex uses sp3 hybridisation and a square planar complex uses dsp2 with one inner d orbital. For coordination number six, an inner orbital complex uses d2sp3 and an outer orbital complex uses sp3d2. The magnetic moment separates the two cases: an inner orbital complex with a strong field ligand has fewer unpaired electrons and a lower moment. An inner orbital complex is also called a low spin complex, and an outer orbital one a high spin complex.

CN = 4 : tetrahedral s p^{3}, square planar d s p^{2}

Geometry and hybridisation for four coordination

CN = 6 : inner orbital d^{2} s p^{3}, outer orbital s p^{3} d^{2}

Geometry and hybridisation for six coordination

1.3 Naming order and constants In the IUPAC name the cation is named first and the anion second, ligands are listed alphabetically and the metal follows the ligands with its oxidation state in Roman numerals in brackets. Prefixes di, tri and tetra are used for identical ligands, while bis, tris and tetrakis are used when the ligand name already contains a prefix or is complicated. The complex is enclosed in square brackets, and the whole name is written as one word for the coordination entity. The formation constant, or stability constant, is the equilibrium constant for the formation of the complex from the metal and the ligands, and a larger value means a more stable complex. The chelate effect makes a complex with a bidentate ligand more stable than the comparable complex with monodentate ligands.

M + n L ⇌ M L_{n}, K = \frac{[ M L _{n} ]}{[ M ] [ L ] ^{n}}

Formation constant of a complex

Exam

When a question asks for the magnetic nature of a complex, count the unpaired electrons and quote the moment with its unit. When it asks for the geometry and hybridisation, state both and say whether the complex is inner or outer orbital.

Find the oxidation state before counting d electrons.
Check whether the ligand is strong field or weak field before pairing electrons.
Name ligands alphabetically and put the metal oxidation state in Roman numerals.
For CFSE questions, write the t2g and eg occupancy first.

1.4 A check on every answer Before writing a name, count the ligand charges and confirm that the metal oxidation state reproduces the charge of the complex. Before writing a magnetic moment, confirm the number of unpaired electrons from the d configuration and the ligand field. Before writing a CFSE, write the t2g and eg populations on the page. These three habits catch most of the errors that cost marks in this chapter.