Class notes Chemistry Notes - Titration, Spectroscopy (AC102)

.
ot
sp
TRANSITION METAL CHEMISTRY 293




og
Double Salts and Complex Salts
A double salt is formed generally when molar proportions of a monovalent cation




bl
sulphate solution in water is mixed with trivalent cation sulphate solution in water and
evaporated to remove excess water, and the salt crystallises as X2SO4Y2(SO4)3.24H2O which is




y.
the general formula of a common alum, where ‘X’ is monovalent cation, like NH4+, K+, Na+, and
‘Y’ is trivalent cation as Al+3, Cr+3, Fe+3 etc. Another important double salt is Mohr’s salt,




it
(NH4)2 SO4.FeSO4.6H2O. A double salt exists as a distinct entity only in the solid crystalline
state, it loses its entity in solution. The double salt is characterised by complete dissociation in




rs
solution and gives the reactions of all the ions formed by the salts in solutions, viz.
K2SO4.Al2(SO4)3.24H2O in solution gives the reactions of K+, Al+3 and SO4–2 ions.




ve
A complex salt does not give all the ions of its constituents in solution. A complex
salt K4[Fe(CN)6] is prepared by adding KCN solution of FeSO4 solution and boiling.




ni
FeSO4 + 2KCN = Fe(CN)2 + K2SO4
Fe(CN)2 + 4KCN = K4[Fe(CN)6]


lu
The solution of K4[Fe(CN)6] does not respond to the tests for Fe+2 or CN– ions, but responds
to a test for a new ion [Fe(CN)6]–4 (ferrocyanide ion). This ion remains intact even in solution
ca
K4 [Fe(CN)6] 4K+ + [Fe(CN)6]–4
gi

and gives its distinctive reactions. So, it can be said, the complexions, like ferrocyanide ion, are
quite stable in the solid state as well as in solution. A complex salt is different from a double
salt in respect of its distinctive character even in solution.
lo



Examples of complex salts
no




[Ag(NH3)2]Cl [Ag(NH3)2]++ + Cl–
[Cu(NH3)4]SO4 [Cu(NH3)4]++ + SO4
ch




K[Ag(CN)2] K+ + [Ag(CN)2]–
te




Instability Constants of Complex Salts
The dissociation of a complex ion, like ionisation of weak electrolyte, follows the law of
a




mass action. The dissociation constant is called instability constant (Kins) and is a measure of
al




stability of a complex, the lower the value of Kins, the higher is the stability of the complex and
vice versa.
er




[M + ] [L− ]
Kins =
[ML]
/k




Complex ions Instability constant
:/




[Ag(NH3)2]+ Ag+ + 2NH3 6.8 × 10–8
tp




[Ag(CN)2]– Ag+ + 2CN– 1.0 × 10–21
[Cu(CN)4]–3 Cu+ + 4CN– 5.0 × 10–28
ht




[Cd(CN)4]–2 Cd+2 + 4CN– 1.4 × 10–17

Werner’s Theory for Formation of Complex Compounds
Werner put forward a theory for the formation of complex compounds. The theory is known
also as co-ordination theory. The postulates of his theory may be summarised as follows:
• Metals possess two types of valencies: (i) Primary or principal or ionisable valencies
(ii) Secondary or subsidiary or non-ionisable valencies.

http://keralatechnologicaluniversity.blogspot.com

C-8\N-ENGCHE\ECH13-1.PM5 293

, .
ot
sp
294 ENGINEERING CHEMISTRY




og
• Every metal atom has a fixed number of secondary valencies or co-ordination number
(C.N.). The maximum number of ions or molecules thereby the central atoms could




bl
form co-ordinate bonds is known as co-ordination number (C.N.).




y.
Ligands
The ions or molecules thus attached to the central metal atom by co-ordinate




it
bonds are called ligands. The ligands may be unidentate, bidentate, tridentate etc. depending
upon the number of co-ordination centres in the ligand.




rs
Types of Ligands




ve
Unidentate ligands—F–, Cl–, H2O
Bidentate ligands—H2NCH2CH2NH2 (en stands for ethylene diamine)




ni
Cu (II) has C.N. = 4 ; Co (III) and Pt (IV) has C.N. = 6.



lu
• Primary valencies of the metal are satisfied by negative ions, but secondary valencies
may be satisfied by negative groups (CN– , Cl–, etc.), neutral molecules (H2O, NH3 etc.)
ca
or even sometimes some positive groups. In every complex formation the co-
ordination number (C.N.) of the central metal atom must be fulfilled.
gi

• The secondary valencies are directed in space about the central metal ion. For metals
with co-ordination number (C.N. = 6), the six valencies are directed to the apices of a
lo



regular octahedron. Again, for metals, with C.N. = 4, the four such valencies are either
directed to the apices of a regular tetrahedron or a square (planar).
no




This gives rise to various types of isomerism. Werner could isolate even optical isomers.
ch




The main drawback for Werner’s theory is: there is no theoretical reason to
have a central atom of the complex ion to possess two kinds of valencies.
te




Chelate Compounds
Chelate compounds are complex compounds containing a ring structure in which a ligand
a




molecule or ion forms more than one dative covalent bonds (co-ordinate bonds) with the central
al




metal ion. Chelates are formed by bidentate and polydentate ligands. Chelation increases the
stability of co-ordination complex i.e., chelate complexes are generally more stable than
er




complexes formed by monodentate ligands. Powerful chelating agents trap metal ions and
/k




effectively trap them in solution, as for example en (ethylene diamine), edta (ethylene diamine
tetra-acetic acid). This type of ligands are known as complexones.
:/
tp




edta or EDTA
ht




The ligand molecules which occupy two or more co-ordination centres are usually organic
molecules.
++




Chelate complex with en


http://keralatechnologicaluniversity.blogspot.com

C-8\N-ENGCHE\ECH13-1.PM5 294

, .
ot
sp
TRANSITION METAL CHEMISTRY 295




og
O
CO




bl
CH2
COCH2




y.
O N
CH2




it
+2
M
CH2
O N




rs
CO CH2
CH2




ve
CO
O




ni
Fig. 13.2 Chelate complex of edta with a bivalent metal.
It is to be noted in chelate complexes that the rings formed are five or six membered.

Inner Metallic Complexes
lu
ca
Many well-known organic ligands, which act as chelating agents, possess a neutral donor
atom as well as an acidic group. They form very stable cyclic complexes, which have very low
gi

solubility in water but high solubility in organic solvents. Generally, the acid group of the
ligand satisfies the primary valency of the central metal ion (which therefore does not appear
lo



as an ion i.e., net charge on the complex molecule is zero). Such non-electrolytic cyclic complexes
are known as inner metallic complexes or simply inner complexes.
no




Nomenclature of Coordination Compounds (IUPAC)
ch




Complex compounds are of the following types:
(i) Cationic, (ii) anionic, (iii) non-ionic and (iv) a combination of cationic and anionic
te




complexes.
(i) In naming cationic complexes like [Co(NH3)5Cl]Cl2, the name of the cation is consid-
a




ered first and then the name of the anion.
al




(ii) In naming the anionic complexes like K2[PtCl6] the K+ is written first then the name
of the complex anion.
er




(iii) If the complex compound is non-ionic (neutral) the name of the complex compound
like [Pt (NH3)4Cl2]0 is written as one word.
/k




(iv) In [PtIV(NH3)4Cl2]2+ [PtII(Cl)4]2–, the name of the complex cation is written first then
:/




the complex anion is named.
Naming of the ligands
tp




Neutral ligands
ht




(C2H5)3N — Triethylamine CO — Carbonyl

NH2-NH2 — Hydrazine CS — Thiocarbonyl

CH3NH2 — Methylamine H2O — Aquo or aqua

NO — Nitrosyl NS — Thionitrosyl
(CH3)2NH — Dimethylamine NH3 — Amine


http://keralatechnologicaluniversity.blogspot.com

C-8\N-ENGCHE\ECH13-1.PM5 295

, .
ot
sp
296 ENGINEERING CHEMISTRY




og
Anion ligands




bl
F– — Fluoro CH3COO– — Acetato

Br– — Bromo NH 2– — Imido




y.
Cl– — Chloro OH– — Hydroxo or hydroxyl




it
I– — Iodo S2– — Thio




rs
NH2– — Amido CN – — Cyano




ve
NC– — Iso-Cyano NO2– — Nitro

ONO– — Nitrito SCN – — Thiocyanato




ni
S2O32– — Thiosulphato NO3– — Nitrato



lu
Ambident ligands
ca
Ambidentate or ambident ligands are those which have two or more different donor
atoms in their structures. These ligands can co-ordinate to the metal atom through any of the
their donor atoms.
gi

Example:
lo



(−)
(i) NO 2− ion → O—N— O named as Nitro-O or Nitrito when donor atom is ‘O’
no




–
O 2N named as Nitro-N or Nitrato when donor atom is ‘N’
(ii) SCN– ion → Thiocyanato-S (SCN–)
ch




Thiocyanato-N (NCS–)
te




Indication of the Number of ligands
a




• If a complex contains two or more simple ligands like Cl–, Br– etc. their number is
al




indicated by di- for two; tri- for three; tetra- for four before the names of the ligands.
• If a complex contains two or more complex ligands or organic molecules and which
er




already have di-, tri- in their names their number is indicated by putting the prefixes
bis- for two; tris- for three; tetrakis- for four. The name of the ligands or organic molecule
/k




is written in the bracket. Examples of complex ligands are—Ethylene diamine (en) etc.

Order of Naming the Ligands
:/




If the co-ordination sphere of a given complex compound contains various types of ligands,
tp




these ligands are named alphabetically.
For example: [Co(NH3)4Cl(NO2)]+ ion is named as tetra-amine chloro nitro cobalt (III) ion.
ht




For anionic complexes the name of central metal atom ends in ‘ate’ and the oxidation
state of the metal is written in Roman numerals (0, I, II).
For example:
Cr — Chromate Co — Cobaltate
Cu — Cupreate Al — Aluminate
Pt — Platinate Ni — Nicklate

http://keralatechnologicaluniversity.blogspot.com

C-8\N-ENGCHE\ECH13-1.PM5 296