Coordination compounds

    Cards (205)

    • A salt that keeps its identity only in solid state is called a double salt.
    • In solution, double salts dissociate into component ions.
    • Examples of double salts include Mohr’s salt [FeSO 4 .(NH 4 ) 2 SO 4 .6H 2 O], Carnalite [KCl.MgCl 2 .6H 2 O], and Potash alum [K 2 SO 4 .Al 2 (SO 4 ) 3 .24H 2 O].
    • A salt that keeps its identity both in solid state and in solution state is called a complex salt.
    • Examples of complex salts include Potassium ferrocyanide {K 4 [Fe(CN) 6 ]}, Cuprammonium sulphate [Cu(NH 3 ) 4 ]SO 4 , K 2 [PtCl 4 ], [Ni(CO) 4 ].
    • A co-ordination entity is formed by the central metal atom or ion along with ligands.
    • For example, [CoCl 3 (NH 3 ) 3 ] is a co-ordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions.
    • In a co-ordination entity, the atom/ion to which a fixed number of ions/neutral molecules are attached is called the central atom or ion.
    • The central atoms/ions in co-ordination entities are also referred to as Lewis acids, since they accept electron pairs from ligands.
    • The negative ions or neutral molecules which are bonded to the central atom in the coordination entity are called ligands.
    • For a species to act as ligand, it can donate at least one pair of electron to the central atom.
    • Examples of ligands include Cl - , Br - , F - , I - , OH - , CN - , NC - , CNO - , NCO - , SO 4 2 - , NO 3 - , CNS - , H 2 O, NH 3 , CO.
    • The atom of the ligand which is directly bonded to the central atom or ion is called co-ordinating atom or donor atom.
    • The energy of the d x2-y2 and d z2 orbitals (called e g orbitals) will be raised and that of the d xy, d yz and d xz orbitals (called t2g orbitals) will be lowered.
    • The degeneracy of the d orbitals has been removed due to the presence of ligands in a definite geometry, a phenomenon termed crystal field splitting.
    • Since the ligands are approaching through the corners, there is no direct interaction between the ligands and the d-orbitals.
    • The repulsion between the electrons in metal d orbitals and the electrons (or negative charges) of the ligands is greater for the d x2-y2 and d z2 orbitals, which are pointing towards the axes, than the d xy, d yz and d xz orbitals, which are directed between the axes.
    • Strong field ligands produce large splitting whereas weak field ligands produce small splitting of d orbitals.
    • For d1, d2 and d3 coordination entities, the d electrons occupy the t2g orbitals singly in accordance with the Hund’s rule.
    • The ‘t2’ orbitals lie closer to the ligands than the ‘e’ orbitals, resulting in the energy of the ‘t2’ orbitals increasing and that of ‘e’ orbitals decreasing.
    • The splitting in tetrahedral field is less than that in octahedral field, with Δt = 4/9 Δo.
    • For d4 ions, two possible patterns of electron distribution arise: the fourth electron could either enter the t2g level and pair with an existing electron, or it could enter into the e g level.
    • The d x2-y2 and d z2 orbitals (called e orbitals) point towards the centre of each faces of the cube and the d xy, d yz and d xz orbitals (called t2 orbitals) point towards the edge centre of the cube.
    • The d orbitals split into two – triply degenerate ‘t2’ orbitals with higher energy and doubly degenerate ‘e’ orbitals with lower energy.
    • If Δo < P, the fourth electron enters one of the e g orbitals giving the configuration t2g3 eg1, ligands for which Δo < P are known as weak field ligands and form high spin complexes.
    • The energy separation in crystal field splitting is denoted by Δo (the subscript o is for octahedral).
    • If Δo > P, the fourth electron occupies a t2g orbital with configuration t2g4 eg0, ligands for which Δo > P are known as strong field ligands and form low spin complexes.
    • A tetrahedron can be considered as a cube in which only alternate corners are occupied by ligands and the metal ion is at the centre of the cube.
    • The energy of the two e g orbitals will increase by 3/5 Δo and that of the three t2g orbitals will decrease by 2/5 Δo.
    • The oxidation number of the central atom in a complex is defined as the residual charge on it, if all the ligands are removed along with their electron pairs that are shared with the central atom.
    • Complexes containing chelating ligands are more stable than those containing unidentate ligands.
    • Ligands are also classified as Ambidentate ligands which contain more than one donor atoms and can coordinate to the central atom through two different atoms, such as NO 2 – , CN – , SCN – , CNO – etc.
    • In the complex ions, [Fe(C 2 O 4 ) 3 ] 3 – and [Co(en) 3 ] 3+, the co-ordination number of both Fe and Co, is 6 because C 2 O 4 2 – and en (ethane - 1,2 - diamine) are bidentate ligands.
    • Chelating Ligands are di- or polydentate ligands that can bind to the central atom through two or more donor atoms and form ring complexes, such complexes are called chelates and these types of ligands are said to be chelating ligands.
    • It is determined only by the number of sigma bonds formed by the ligand with the central atom/ion.
    • The co-ordination number (C.N) of a metal ion in a complex is the total number of ligand donor atoms to which the metal is directly bonded.
    • Generally, the co-ordination number of most of the complexes is 2, 4 or 6.
    • If the donor atom is N, it is written as NO 2 - and is called nitrito (N) and if it is O, it is written as ONO - and is called nitrito(O).
    • The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively termed as the co-ordination sphere.
    • For example, in the complex ion [PtCl 6 ] 2 – the co-ordination number of Pt is 6 and in [Ni(NH 3 ) 4 ] 2+, the co-ordination number of Ni is 4.