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UNIT 9: CO-ORDINATION COMPOUND
The compounds which contain dative bonds between metal atom and surrounding species is called co-ordination compounds .The branch of inorganic chemistry which deals with hte study of preparation properties of coordination compound is called co-ordination chemistry.
POINTS TO REMEMBER:
Coordination compounds are compounds in which a central metal atom or ion is linked to a
number of ions or neutral molecules by coordinate bonds or which contain complex ions. Examples- K4[Fe(CN)6]; [ Cu(NH3)4]SO4; Ni(CO)4
2.The main postulates of Werner’s theory of coordination compounds
i) In coordination compounds metals show two types of linkages or valencies- Primary and Secondary.
ii) The primary valencies are ionisable and are satisfied by negative ions.
iii) The secondary valencies are non- ionisable and are satisfied by neutral molecules or negative ions. The secondary valence is equal to the C.N and is fixed for a metal.
iv) The ions or groups bound by secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination nos.
3.Difference between a double salt and a complex
Both double salts as well as complexes are formed by the combination of two or more stable compounds in stoichiometric ratio. However, double salts such as carnallite, KCl.MgCl2.6H2O, Mohr‘s salt, FeSO4.(NH4)2SO4.6H2O, potash alum, KAl(SO4)2.12H2O, etc. dissociate into simple ions completely when dissolved in water. However, complex ions such as [Fe(CN)6]4– of K4[Fe(CN)6], do not dissociate into Fe2+ and CN– ions.
(i) Coordination entity: It constitutes the central metal ion or atom bonded to a fixed number of ions or molecules represented within a square bracket.
(ii) Central atom/ ion: In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion.
iii) Ligands: The neutral or negative ions bound to the central metal or
ion in the coordination entity. These donate a pair/s of electrons to the central metal atom /ion. Ligands may be classified as-
a) Monodentate/Unidentate: Ligands bound to the central metal atom/ion through a single donor atom. Ex- Cl- ; H2O ; NH3 ; NO2-.
b) Didentate: Ligates through two donor atoms. Ex- C2O4 2- (ox); H2NCH2CH2NH2(en)
c) Polydentate: which ligates through two or more donor atoms present in a single ligand. Ex- (EDTA)4-
d) Chelating ligands: Di- or polydentate ligands that uses two or more donor atoms to bind to a single metal ion to form ring- like complexes. (Ox); (edta)
e) Ambidentate ligand: A ligand that can ligate through two different atoms, one at a time. Ex-NO2- ; SCN-
v) Coordination number: The no. of ligand donor atoms to which the metal is directly bonded through sigma bonds only. It is commonly 4 or 6.
vi) Counter ions: The ionisable groups written outside the square bracket. Ex- K+ in
K4[Fe(CN)6] OR 3Cl- in [Co(NH3)6]Cl3
vii) Coordination Polyhedron: The spatial arrangement of the ligand atoms which are directly attached to the central metal atom/ion. They are commonly Octahedral, Square-planar or Tetrahedral
Oxidation number: The charge that the central atom would carry if all the ligands are
removed along with their pairs of electrons shared with the central atom. It is represented in parenthesis.
viii) Homoleptic complexes: Complexes in which a metal is bonded to only one kind of donor groups. Ex- [Co(NH3)6] 3+
ix) Heteroleptic complexes: Complexes in which a metal is bonded to more than one kind of donor groups. Ex- [Co(NH3)4 Cl2]+