Complexes

MDCAT Chemistry · Chemistry · Transition elements. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A complex ion has a central metal ion joined to a fixed number of ligands. A ligand is an ion or molecule with a lone pair that it donates to the metal ion, forming a coordinate bond. The complex is written in square brackets and stays together as one unit in solution.

The coordination number is the number of coordinate bonds to the metal ion, and it decides the shape. A ligand that bonds through two or more atoms forms a ring with the metal, called a chelate.

The ligands split the d orbitals of the metal into groups of different energy. An electron absorbs visible light to jump from a lower d orbital to a higher one, and we see the colour that is left.

Rules to remember

Monodentate ligands (one donor atom): H₂O, NH₃, Cl⁻, CN⁻, OH⁻. Bidentate (two): ethylenediamine (en), oxalate C₂O₄²⁻.
Coordination number = number of donor atoms bonded to the metal, not the number of ligands: [Co(en)₃]³⁺ has 6.
Oxidation number of the metal = charge on the complex ion − total charge of the ligands.
Naming: cation before anion; ligands in alphabetical order with di, tri, tetra, hexa (aqua, ammine, chloro, cyano, hydroxo); then the metal with its oxidation state in Roman numerals. In a complex anion the metal ends in -ate (ferrate, cuprate).
Shapes: coordination number 2 linear; 4 tetrahedral (sp³) or square planar (dsp²); 6 octahedral (d²sp³ or sp³d²).
In an octahedral complex the five d orbitals split into three of lower energy and two of higher energy. Ions with d⁰ or d¹⁰ are colourless.
Stability is greater for a smaller, more highly charged metal ion (Co³⁺ > Co²⁺) and for chelating ligands; for 3d ions Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺.

Common mistake

Students count the ions outside the square brackets as ligands. Only what is inside the brackets is bonded to the metal; the ions outside are free in solution and only balance the charge.

Worked example

For the compound [Cr(H₂O)₄Cl₂]Cl, find the oxidation number and coordination number of chromium, and name the compound.

One Cl⁻ lies outside the brackets, so the complex ion has a charge of +1.
Oxidation number: x + 4(0) + 2(−1) = +1, so x = +3.
Four H₂O and two Cl⁻ are six monodentate ligands: coordination number 6, octahedral.
Name the ligands in alphabetical order (aqua, then chloro), then the metal, then the anion.

Answer: Chromium is +3 with coordination number 6; the name is tetraaquadichlorochromium(III) chloride.

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