JEE Chemistry Coordination Compounds Notes Set 01

Werner's Theory

Alfred Werner explained how stable compounds such as CoCl₃·6NH₃ exist. A metal shows two kinds of valency. The primary valency is ionisable and equals the oxidation state of the metal. The secondary valency is non-ionisable and equals the coordination number, which is the number of ligand atoms directly bonded to the metal. Secondary valencies are directed in space, which gives each complex its geometry.

Important Terms

1. Central metal atom or ion: the acceptor of electron pairs, usually a d-block ion.
2. Ligand: an ion or molecule that donates an electron pair. Ligands can be unidentate (Cl⁻, NH₃), bidentate (en, ox²⁻) or polydentate (EDTA, hexadentate).
3. Ambidentate ligands such as NO₂⁻ (N or O) and SCN⁻ (S or N) can bond through two different atoms.
4. Coordination sphere: the metal and its ligands written inside square brackets. The ions outside are counter ions.
5. Charge on the complex = oxidation state of metal + sum of charges of ligands.
6. Chelate: a ring formed when a polydentate ligand binds a metal; chelates are especially stable.

Nomenclature (IUPAC)

1. Name the cation first, then the anion.
2. Inside the sphere, ligands come first in alphabetical order, then the metal.
3. Use di, tri, tetra for simple ligands and bis, tris, tetrakis for complex ligands or those already containing a number.
4. Anionic ligands end in -o (chlorido, cyanido), neutral ligands keep their names (aqua, ammine, carbonyl).
5. Give the oxidation state of the metal in Roman numerals in brackets.
6. In an anionic complex the metal name ends in -ate (ferrate, cuprate, argentate, aurate).

Example. K₃[Fe(CN)₆] is potassium hexacyanidoferrate(III), and [Co(NH₃)₅Cl]Cl₂ is pentaamminechloridocobalt(III) chloride.

Effective Atomic Number

EAN = atomic number of the metal − oxidation state + 2 × (number of ligands donating pairs). Many stable complexes reach the EAN of the next noble gas (36 for the 3d series). For Fe(CO)₅: 26 − 0 + 10 = 36.

Isomerism

1. Structural isomers: ionisation (the ion inside and outside the sphere are exchanged), hydrate (different number of water molecules inside the sphere), linkage (ambidentate ligand bonded through a different atom) and coordination (ligands exchanged between cationic and anionic complexes).
2. Geometrical isomers: cis and trans in square planar MA₂B₂ and octahedral MA₄B₂; fac and mer in MA₃B₃. Tetrahedral complexes do not show geometrical isomerism.
3. Optical isomers: need a non-superimposable mirror image, as in [M(AA)₃] and cis-[M(AA)₂B₂]. The trans form has a plane of symmetry and is optically inactive.
4. Square planar MABCD has 3 geometrical isomers; [Ma₂bc] octahedral type counts are found by drawing the arrangements and removing those with a mirror plane.