Introduction And Nomenclature

Molecular or addition compounds are formed when stoichiometric amounts of two or more stable compounds join together. For example:

$KCl+MgC{l}_2+6H_{2}O\to KCl.MgCl.6H_{2}O\left(carnallite\right)$

$K_{2}S{O}_4+A{l}_2{\left(S{O}_4\right)}_3+24H_{2}O\to K_{2}S{O}_4.A{l}_2{\left(S{O}_4\right)}_3.24H_{2}O\left(potassiumalum\right)$

$CuS{O}_4+4N{H}_3+H_{2}O\to CuS{O}_4.4N{H}_3.H_{2}O$

(tetrammine copper (II) sulphate monohydrate)

$Fe{\left(CN\right)}_2+4KCN\to Fe{\left(CN\right)}_2.4KCN\left(potassiumferrocyanide\right)$

These fall into two categories:

(1) Those which lose their identity in solution.

(2) Those which retain their identity in solution.

An aqueous solution of carnallite shows the properties of K⁺, Mg²⁺ and Cl^- ions. Potassium alum solutions similarly show the properties of K⁺, Al³⁺ and $S{O}_4^{2-}$ ions. They are called double salts and exist only in the crystalline state.

The other two examples of addition compounds behave in a very different way from the double salts. When dissolved they do not form Cu²⁺, or Fe²⁺ and CN- ions, but instead give more complicated structures-the cuproammonium ion [Cu(H₂O)₂(NH₃)₄]²⁺ and the ferrocyanide ion [Fe(CN)₆]^4-. These are complex ions and exist are a single entity. Complex ions are indicated by square brackets. Molecular compounds of this type are called coordination compounds.

Co-ordination compounds are the compounds in which the central metal atom is linked to ions or neutral molecules by co-ordinate bonds, e.g.${\left[Cr{\left(H_{2}O\right)}_{5}Cl\right]}^{2+}$. If the species thus formed as given above carries positive charge, it is called a complex ion.

Terminology

The important terms used in the study of coordination compounds are mentioned below.

Central Metal Ion

It is an acceptor atom containing vacant orbitals to which a fixed number of ligands are attached via co-ordinate bonds in definite geometrical arrangement.

Ligands

It is an ion or molecule capable of donating a pair of electrons to the central atom via a donor atom.

Types of ligands

Unidentate ligands. Ligands with only one donor atom, e.g. NH₃, Cl^-, F^- etc.

Bidentate ligands. Ligands with two donor atoms, e.g. ethylenediamine, C₂O₄^2- (oxalate ion) etc.

Tridentate ligands. Ligands which have three donor atoms per ligand, e.g. (dien) diethyl triamine.

Hexadentate ligands. Ligands which have six donor atoms per ligand, e.g. EDTA.

Co-ordination Sphere

Complex ion enclosed in square bracket, it behaves as a single unit.

Ionization Sphere

Part of compound present outside co-ordination sphere, e.g

Co-ordination Number

It is the total number of ligands attached to the central metal atom through co-ordinate bonds or the number of atoms of a ligand attached to the same central atom, e.g. hexadentate ligand should be counted as forming six co-ordinate bonds.

Oxidation number

It is the charge which the central atom appears to have if all the ligands are removed along with the electron pairs that are shared with the central atom.

${\left[Cr{\left(H_{2}O\right)}_{4}C{l}_2\right]}^{+}$

x + (4 x 0) + (-1 x 2) = +1 \left[ \begin{align} \because\,\,the\,\,ligand\,\,{{H}_{2}}O\,\,is\,\,neutral\,\,and\,\,2\,\,C{{l}^{-}}\, \\ \,carries\,\,-2\,\,ch\arg e \\ \end{align} \right]

x + 0 - 2 = +1

x = 3

Chelating Ligands

Multidentate ligand simultaneously co-ordinating to a metal ion through more than one site is called chelating ligand. These ligands produce a ring like structure called chelate. Chelation increases the stability of complex. This effect is called chelation effect.

Example:

NAMING OF CO-ORDINATION COMPOUNDS

Rule 1: To name a co-ordination compound, no matter whether the complex ion is the cation 1`or the anion, always name the cation before the anion.

Rule 2: Part (a) Name the ligand first, in alphabetical order, then the metal atom or ion.

Part (b) For anionic ligands end in "---o---". For anions that end in "---ide---" (e.g. chloride), "---ate---" (e.g. sulfate, nitrate) and "---ite---" (e.g. nitrite), change the ending as follows:

-ide $\to$ -o, -ate $\to$ -ato, -ite $\to$ -ito

Part (c) For neutral ligands, the common name of the molecule is used e.g. H₂NCH₂CH₂NH₂ (ethylenediamine). Important exceptions: For H₂O $\to$ aqua, NH₃ $\to$ ammine, CO $\to$carbonyl and N₂ and O₂ are called dinitrogen and dioxygen.

Rule 3: Greek prefixes are used to designate the number of each type of ligand in the complex

ion, di, tri and tetra-. If the ligand already contains a Greek prefix (e.g. ethylendiamine) if it is polydenate ligands the prefixes, bis-, tris-, tetrakis-, pentakis- are used instead.

Rule 4: After naming the ligand, name the central metal. If the complex ion is a cation, metal is named same as the element. But if the complex is an anion, the name of metal ends with the suffix -ate for Latin name.

Rule 5: The oxidation state of the metal in the complex is given as a roman numeral in parentheses.

Example:

Iron – Ferrate Silver – Agrenate

Copper – Cuprate Gold – Aurate

Head – Plambate Tin – Stannate

Illustration . Write down the molecular formulae of the following co-ordination compounds.

(i) Hexaammine iron (III) nitrate

(ii) Ammonium tetrachlorocuprate (II)

(iii) Sodium monochloropentacyanoferrate (III)

(iv) Potassium hexafluorocabaltate (III)

Solution: (i) $\left[Fe{\left(N{H}_3\right)}_6\right]{\left(N{O}_3\right)}_3$

(ii) ${\left(N{H}_4\right)}_2\left[CuC{l}_4\right]$

(iii) $N{a}_3\left[FeCl.{\left(CN\right)}_5\right]$

(iv) $K_3\left[Co{F}_6\right]$