Coordination Chemistry: One Mark Questions, Book Back Answers and Notes

About Coordination Chemistry

Medium ~120 min study

A coordination compound contains a central metal atom or ion surrounded by ions or neutral molecules called ligands, each donating a lone pair to the metal. Many familiar substances are complexes: haemoglobin contains iron, chlorophyll contains magnesium, and vitamin B₁₂ contains cobalt.

The unit builds on what you learnt about d-block elements. Valence bond theory explains the geometry of a complex through hybridisation, and crystal field theory explains colour and magnetism through the splitting of the metal d orbitals by the ligands. With these two models you can say why [Fe(CN)₆]^3- and [FeF₆]^3- have different numbers of unpaired electrons even though both contain Fe³⁺.

Exam questions from this unit regularly ask you to write IUPAC names and formulas, identify isomers, work out hybridisation and magnetic moment, and calculate crystal field stabilisation energy. Knowing which ligands are strong field and which are weak field is needed for most of these.

What you'll learn

• Distinguish a double salt from a coordination compound by its behaviour in water.
• Write IUPAC names for mononuclear complexes and formulas from their names.
• Identify and draw the structural and stereoisomers possible for a given complex.
• Predict the geometry and magnetic nature of a complex using valence bond theory.
• Explain the colour of a complex using crystal field theory.
• Calculate the crystal field stabilisation energy for dn configurations in an octahedral field.

Before you start

• Electronic configurations and oxidation states of d-block elements.
• The Lewis acid-base idea, with the metal acting as an electron pair acceptor.
• Hybridisation and basic molecular shapes from VSEPR theory.

Coordination Chemistry explained

Structure, Bonding and Properties of Complexes

Werner's Theory

Alfred Werner was the first to explain the structure of coordination compounds. He proposed that a metal has two kinds of valency. The primary valency is ionisable and corresponds to the oxidation state of the metal. The secondary valency is non-ionisable and corresponds to the coordination number. Secondary valencies are directed towards fixed positions in space, which gives each complex a definite shape. Werner used this to explain the CoCl₃·xNH₃ series, where the number of chloride ions precipitated by AgNO₃ changes with the formula.

Terms and IUPAC Nomenclature

You need to be clear on the terms central atom, ligand, coordination sphere, counter ion, coordination number, and homoleptic and heteroleptic complexes. In naming, ligands are listed in alphabetical order before the metal, and the oxidation state of the metal is given in Roman numerals. If the complex ion is negative, the metal name ends in -ate, so [Fe(CN)₆]^4- is hexacyanidoferrate(II).

Isomerism

Coordination compounds with the same formula can have different structures. Structural isomers differ in which atoms are bonded to the metal: linkage, ionisation, coordination and solvate (hydrate) isomerism. Stereoisomers have the same bonds but a different arrangement in space. Geometrical isomers are cis and trans forms, found in square planar and octahedral complexes. Optical isomers are non-superimposable mirror images, called enantiomers, and are common in octahedral complexes with bidentate ligands such as [Co(en)₃]³⁺.

Valence Bond Theory

In valence bond theory the metal uses vacant s, p and d orbitals to form hybrid orbitals, and each ligand donates a lone pair into one of them. sp³ hybridisation gives a tetrahedral complex, dsp² gives square planar, and d²sp³ or sp³d² gives octahedral. Complexes using inner (n-1)d orbitals are inner orbital complexes; those using outer nd orbitals are outer orbital complexes. Counting the unpaired electrons left tells you whether the complex is paramagnetic or diamagnetic.

Crystal Field Theory and Colour

Crystal field theory treats the ligands as point charges that interact electrostatically with the metal d electrons. In an octahedral field the five d orbitals split into a lower t_2g set and a higher e_g set. The energy gap is the crystal field splitting energy, \(\Delta\). If \(\Delta\) is larger than the pairing energy, electrons pair in t_2g and a low-spin complex forms; if it is smaller, a high-spin complex forms. Absorbing visible light of energy \(\Delta\) promotes an electron from t_2g to e_g, and the complex shows the complementary colour.

Uses of Coordination Compounds

Complexes have many practical uses. EDTA is used to treat lead poisoning and to estimate hardness of water, and cisplatin is used in cancer treatment. In metallurgy, gold and silver are extracted as cyanide complexes, and nickel is purified through Ni(CO)₄ in the Mond process. In biology, haemoglobin carries oxygen and chlorophyll takes part in photosynthesis.

Common mistakes to avoid

• Confusing coordination number with oxidation state. In K4[Fe(CN)6] the coordination number is 6 but the oxidation state of iron is +2.
• Forgetting the -ate ending for the metal in an anionic complex, for example writing iron instead of ferrate in K4[Fe(CN)6], or cobalt instead of cobaltate in an anionic cobalt complex.
• Mixing up types of isomerism. If different atoms are bonded to the metal, it is structural isomerism; if only the arrangement in space differs, it is stereoisomerism.
• Assuming all ligands split the d orbitals equally. Check the spectrochemical series: CN- and CO are strong field ligands, while I- and F- are weak field ligands.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

How is a double salt different from a coordination compound?

A double salt such as Mohr's salt breaks up completely into its simple ions in water, so the solution gives tests for Fe2+, NH4+ and SO42-. A coordination compound keeps its complex ion intact. K4[Fe(CN)6] in water gives K+ and [Fe(CN)6]4-, and no test for free Fe2+.

How do I find the coordination number of a metal?

Count the donor atoms bonded to the metal. With unidentate ligands this equals the number of ligands. With polydentate ligands, multiply the number of each ligand by its denticity: in [Co(en)3]3+, three bidentate en ligands give a coordination number of 6.

Why are some complexes coloured and others colourless?

Most colour in complexes comes from d-d transitions. The complex absorbs visible light and an electron moves from a lower d level to a higher one; the colour seen is the complementary colour. A metal ion with a d0 or d10 configuration cannot undergo a d-d transition, so its complexes, such as those of Zn2+, are usually colourless.

What is a strong field ligand?

A strong field ligand, such as CN- or CO, produces a large crystal field splitting. When \(\Delta\) is greater than the pairing energy, electrons pair up in the lower t2g orbitals instead of entering the eg orbitals, and a low-spin complex forms.

How does valence bond theory predict magnetic behaviour?

Write the configuration of the metal ion, place the ligand electron pairs into the hybrid orbitals, and count the unpaired electrons that remain in the d orbitals. If none remain, the complex is diamagnetic. If one or more remain, it is paramagnetic, and the spin-only magnetic moment can be calculated from that number.

Why is isomerism so common in coordination compounds?

A metal ion bonds to several ligands pointing in fixed directions. The same set of ligands can be arranged in different positions, a ligand may bond through different donor atoms, and ions can sit inside or outside the coordination sphere. Each of these gives a compound with the same formula but different properties.