CBSE Class 12 Chemistry Coordination Compounds Notes Set 09

These Class 12 CBSE Chemistry notes for Unit 5 Coordination Compounds (Set 09) cover double salts versus complexes, the basic terms of coordination chemistry, the classification of ligands, IUPAC rules for writing formulae and names, and Werner's theory with the cobalt(III)-ammonia series. Five solved examples show how to find coordination numbers, oxidation states, names and AgNO₃ precipitation counts.

Important Notes and Formulas

Double Salts and Coordination Compounds

Both are addition compounds, formed when two or more stable compounds combine in a fixed ratio. They differ in what happens on dissolving in water.

Basic Terms Used in Coordination Chemistry

1. Coordination entity: a central metal atom or ion bound to a fixed number of ions or molecules (ligands). It may be neutral, positive or negative, e.g. [Co(NH₃)₆]³⁺, [PtCl₄]²⁻, [Ni(CO)₄].
2. Central atom or ion: the atom or ion to which the ligands are attached. It accepts electron pairs, so it behaves as a Lewis acid.
3. Ligand: an ion or molecule that donates a lone pair to the central atom. Ligands are Lewis bases, e.g. Cl⁻, H₂O, NH₃.
4. Coordination number (CN): the number of ligand donor atoms directly bonded to the central atom. Only sigma bonds are counted. CN of Co in [Co(NH₃)₆]³⁺ is 6, and in [Co(en)₃]³⁺ it is also 6 because each en gives two donor atoms.
5. Coordination sphere: the central atom together with its ligands, written inside square brackets. The ions outside the bracket are the counter ions (ionisation sphere).
6. Oxidation number of the central atom: the charge the central atom would carry if all the ligands were removed with their shared electron pairs. It is written as a Roman numeral in brackets, e.g. Fe(III) in [Fe(CN)₆]³⁻.
7. Homoleptic complex: the metal is bound to only one kind of donor group, e.g. [Co(NH₃)₆]³⁺. Heteroleptic complex: more than one kind of donor group, e.g. [Co(NH₃)₄Cl₂]⁺.

Example of the layout of the formula for [Co(NH₃)₆]Cl₃: the central metal ion is Co³⁺, the ligands are six NH₃ molecules, the coordination number is 6, the coordination sphere is [Co(NH₃)₆]³⁺ and the counter ions are three Cl⁻.

Classification of Ligands

1. A ring formed by a chelating ligand and the metal ion is a chelate ring, and the complex is a chelate. Chelates are more stable than similar complexes of unidentate ligands (the chelate effect). The number of rings made by one chelating ligand equals its denticity minus 1: [Co(en)₃]³⁺ has 3 rings, and [Ca(EDTA)]²⁻ has 5.
2. EDTA⁴⁻ binds through two nitrogen atoms and four oxygen atoms, so it is a hexadentate ligand.
3. Ambidentate ligands lead to linkage isomerism, e.g. M-NO₂ (nitrito-N) and M-ONO (nitrito-O); M-SCN (thiocyanato-S) and M-NCS (thiocyanato-N).

Common shapes of coordination entities (M is the metal, L a unidentate ligand)

Werner's Theory of Coordination Compounds

Alfred Werner (1866-1919), a Swiss chemist, explained the behaviour of complexes of cobalt(III) chloride with ammonia. His main postulates are:

1. A metal shows two types of valency, primary and secondary.
2. Primary valency is the oxidation state of the metal. It is ionisable, satisfied only by negative ions, and non-directional.
3. Secondary valency is the coordination number. It is non-ionisable, satisfied by negative ions or neutral molecules, and it is directional, so it fixes the geometry of the complex.
4. Every metal tends to satisfy both valencies. Six secondary valencies give an octahedral shape, four give a tetrahedral or square planar shape.
5. Ligands held by the secondary valency are inside the square brackets and are not precipitated by their usual reagents.

The green and violet forms of CoCl₃.4NH₃ are the trans and cis isomers of [Co(NH₃)₄Cl₂]Cl. In every compound the coordination number of cobalt is 6 and its oxidation state is +3, so the chloride ions that are not in the brackets are the ones that precipitate with Ag⁺.

A useful check: the number of moles of AgCl formed equals the number of counter chloride ions, and the electrical conductance rises with the number of ions, in the order [Co(NH₃)₃Cl₃] < [Co(NH₃)₄Cl₂]Cl < [Co(NH₃)₅Cl]Cl₂ < [Co(NH₃)₆]Cl₃.

Typical coordination numbers: Ag⁺ is 2; Cu²⁺, Ni²⁺, Zn²⁺ and Pt²⁺ are commonly 4; Fe³⁺, Co³⁺, Cr³⁺ and Fe²⁺ are commonly 6.

IUPAC Rules for Writing the Formula of a Coordination Entity

1. The central atom is written first, then the ligands.
2. Ligands are listed in alphabetical order of their names (not their charges). For abbreviated ligands the first letter of the abbreviation decides the place.
3. Polyatomic ligands and abbreviations are placed in parentheses. The whole coordination entity is enclosed in square brackets.
4. No space is left between the central atom and the ligands.
5. For a charged entity the charge is written outside the bracket as a right superscript, number before sign, e.g. [Co(NH₃)₆]³⁺.
6. In the full formula the cation is written before the anion, and the total positive and negative charges balance.

IUPAC Rules for Naming Mononuclear Coordination Compounds

1. The cation is named first and the anion second, as in any ionic compound.
2. Inside the coordination entity, the ligands are named first in alphabetical order, and then the central atom, which is followed by its oxidation state in Roman numerals in brackets.
3. The prefixes di, tri, tetra, penta, hexa show the number of simple ligands. Bis, tris, tetrakis are used with complex or already-prefixed ligands, e.g. bis(ethane-1,2-diamine). These prefixes are ignored in alphabetical order.
4. Anionic ligands end in -o (chlorido, cyanido, hydroxido, oxalato, sulphato, nitrito-N). Neutral ligands keep their names, except aqua (H₂O), ammine (NH₃), carbonyl (CO) and nitrosyl (NO). Positive ligands end in -ium.
5. If the complex is an anion, the name of the metal ends in -ate. Latin stems are used for some metals: ferrate (Fe), cuprate (Cu), argentate (Ag), aurate (Au), plumbate (Pb), stannate (Sn). A cationic or neutral complex uses the ordinary metal name.
6. For ambidentate ligands the donor atom is indicated, e.g. nitrito-N (NO₂⁻), nitrito-O (ONO⁻), thiocyanato-S, thiocyanato-N (isothiocyanato).
7. Names of the complexes with ions outside the bracket are written as separate words: the cation first, e.g. hexaamminecobalt(III) chloride.

Solved Examples

Example 1. Explain why Fe(NH₄)₂(SO₄)₂.6H₂O is a double salt but K₃[Fe(CN)₆] is a complex.
Answer:
In water Fe(NH₄)₂(SO₄)₂.6H₂O (Mohr's salt) splits completely into Fe²⁺, NH₄⁺ and SO₄²⁻ ions, and the solution gives the tests of all of them. It has lost its identity, so it is a double salt.
K₃[Fe(CN)₆] gives K⁺ and [Fe(CN)₆]³⁻ ions. The complex ion does not give the tests for Fe³⁺ or CN⁻, so it keeps its identity and is a coordination compound.

Example 2. For the compound [Co(en)₂Cl₂]Cl find the coordination number, oxidation state of cobalt, number of chelate rings and the counter ion.
Answer:
Each en is didentate: 2 × 2 = 4 donor atoms, plus 2 Cl⁻, so the coordination number is 6.
Charge on the complex ion = +1 (one Cl⁻ outside). Let the oxidation state of Co be x: x + 2(0) + 2(−1) = +1, so x = +3.
Chelate rings = 2 en × (2 − 1) = 2. The counter ion is Cl⁻.

Example 3. Write the IUPAC names of (i) [Co(NH₃)₅Cl]Cl₂, (ii) K₄[Fe(CN)₆], (iii) [Cr(en)₃]Cl₃ and (iv) [Pt(NH₃)BrCl(py)].
Answer:
(i) Ligands in alphabetical order: ammine, chlorido. Co: x + 0 − 1 = +2, so x = +3. Name: pentaamminechloridocobalt(III) chloride.
(ii) The complex ion [Fe(CN)₆]⁴⁻ is an anion, so the metal is ferrate. 4(+1) + x + 6(−1) = 0 gives x = +2. Name: potassium hexacyanidoferrate(II).
(iii) en is a named ligand with a prefix inside it, so tris is used. Cr is +3. Name: tris(ethane-1,2-diamine)chromium(III) chloride.
(iv) Alphabetical order: ammine, bromido, chlorido, pyridine. The complex is neutral and Pt is +2. Name: amminebromidochloridopyridineplatinum(II).

Example 4. Write the formulae of (i) tetraamminediaquacobalt(III) chloride, (ii) potassium tetrahydroxidozincate(II) and (iii) pentaamminenitrito-N-cobalt(III) chloride.
Answer:
(i) Co³⁺ with four NH₃ and two H₂O (both neutral) gives a +3 complex ion, so three Cl⁻ balance it: [Co(NH₃)₄(H₂O)₂]Cl₃.
(ii) Zn²⁺ with four OH⁻ gives [Zn(OH)₄]²⁻, so two K⁺ are needed: K₂[Zn(OH)₄].
(iii) Co³⁺ with five NH₃ and one NO₂⁻ gives [Co(NH₃)₅(NO₂)]²⁺, so two Cl⁻ are needed: [Co(NH₃)₅(NO₂)]Cl₂.

Example 5. A complex has the formula CoCl₃.5NH₃. One mole of it gives 2 moles of AgCl with excess AgNO₃ solution. Write its structural formula and name, and state the number of ions it gives.
Answer:
Only two Cl⁻ are free, so one of the three Cl⁻ is inside the coordination sphere. With CN 6, the formula is [Co(NH₃)₅Cl]Cl₂.
Name: pentaamminechloridocobalt(III) chloride.
It gives one complex ion [Co(NH₃)₅Cl]²⁺ and two Cl⁻, so 3 ions in all.