Download CO-ORDINATI Formulas for coordination

CHAPTER -
9
COCO-ORDINATION COMPOUNDS
Formulas for coordination compounds:
Tetraamineaquachloridocobalt (III) chloride ---- [Co(NH3)4 (H2O) Cl]Cl2
Potassium tetrahydroxozincate (II) ------- K2[Zn(OH)4]
Potassium trioxalatoaluminate (III) -------- K3[A](C2O4)3]
Dichloridobis (ethane-1, 2 – diamine) cobalt (III) -------- [cocl2(en)2]+
Tetracarbonylnickel(0) --------- [Ni(CO]4]
Effective atomic number (EAN) = Z (atomic no.) – (O.N.) + 2 (C.N.)
[Co (NH3)5 SO4] Br and [Co (NH3)5 Br]So4 are ionization isomers.
[Co (NH3)6 ] [Cr (CN)6] and [Cr (NH3)6 [Co (CN)6] are co-ordination
ordination isomers.
[Co (NH3)5 ONO]2+ and [Co (NH3)5 NO2]2+ are linkage isomers.
[Cu (NH3)2Cl2] will form geometrical isomers
Trans- [Cu (Gly)2] will form optical isomers.
Werner theory considers two types of valency in complexes
Primary (O.N.) and
Secondary (C.N.) Ligands/ions attached by secondary valency do not ionize.
Complex ion. It is an electrically charged radical which is formed by the union of simple
cat ion with one or more natural molecules or simple ions.
E.g., [Fe(CN)6]4-, is formed b the union of six CN ି ions with one Fe2+ ion. The ions or
molecules which surround the central metal atom or ion are called ligands.
The non bonded pairs of electrons on the ligands form co-ordinate bonds to the central
metal atom or ion by donating these unshared electron pairs into its vacant orbitals. It is
because of this reason that the term co-ordination compounds is coined for such
substances.
Types of complex on the basis of stability. On the basis of stability of complex ion in
solution, complex salts are classified into two types:
Perfect or penetrating complexes. These are compounds in which complex ion is fairly
stable and is either not dissociated or feebly dissociated in solution state. For
example,
[K4[Fe(CN)6]
→
4K+ + [Fe(CN)6]4Fe2 + +6CN- (feebly dissociated)
The ferrocyanide ion [Fe(CN)6]4- is so unsuficiently dissociated that it may be taken as
practically undissociated and does not give the test of Fe2+ or CN- ions. [Co(NH3)6]3+ is
another such complex.
Imperfect or normal complexes. These are compounds in which complex ion is
less stable and is reversibly dissociated to give enough simple ions and thus gives
their tests. For example,
K2 [Cd(CN)4] → 2K+ + [Cd(CN)4]2Cd2+ + 4CN- (appreciably dissociated)
The IUPAC names of the coordination compounds:
[Pt (NH3)2 Cl(NO2)] ------ Diamminechloridonirito -N- platinum(II)
K3[Cr(C2O4)3] -------- Potassium trioxalatochromate (III)
[cocl2(en)2lcl ---------- Dichloridobis (ethane-1, 2- diamine) cobalt (III)
[Co(NH3)5(CO3)]Cl-
-------
Pentaaminecarbonatocobalt (III) chloride
Hg[Co(SCN)4] ------- Mercury tetrathathiocyanatocobaltate (III)
The valence bond theory (VBT) explains with reasonable success, the formation,
magnetic behavior and geometrical shapes of coordination compounds. It, however, fails
to provide a quantitative interpretation of magnetic behavior and has nothing to say about
the optical properties of these compounds.
In nomenclature
O.N. of the metal in a complex is written
If complex is anion, metal ion ends with ate
Ligands (with ite in the end) are written as ending ito, ato (sulphato oxalate etc.)
Ligands (with ide in the end) are written as ending o (chloro, bromo)
K4 [Fe (CN)6] is potassiumhexacyanoferrate (II)
The Crystal Field theory (CFT) to coordination compounds is based on the effect of
different crystal fields (provided by the ligands taken as point charges), on the
degeneracy of d orbital energies of the central metal atom/ion.
The splitting of the d orbital provides different electronic arrangements in strong and
weak crystal fields.
In optical isomers, the designation (+) and (-) or (d) and (l) are put before the complete
formula of the complex compound and not before the complex ion showing optical
isomerism e.g., (+) – K3 [Cr(C2O4)3] and not K3 (+) – [Cr(C2O4)3]
Ligands are generally capable of donating one or more lone pairs of electrons. Positively
charged ligands also donate lone pair of electrons.
Therefore are certain ligands which do not have lone pair of electrons but still can act as
ligand e.g. ethylene or C2H4 in Zeise’s salt is one such ligand.
K[PtCl3(η2 – C2H4)]
Zeise’s salt
Oxidation state of Fe in ferrocene is Fe2+. The two cyclopentadienly anion (C5Hହି )
carries one unit negative charge each.
Metal carbonyls are not regarded as ogranometallic compounds in the true sence. This is
because CO is not an organic compound. However metal carbonyls are studied along
with other ogranometallic compounds.
KCN contains a metal-carbon bond but it is not considered as an ogranometallic
compound.
[Fe (CN)6]4- is a complex (co-ordination) ion in which Fe2+ is a metal ion and CN – ligand
(electron pair donor); ligand co-ordinated to metal ion does not ionize in aqueous
solution/medium.
Ligand can be unidentate, bidentate, tridentate etc. depending upon the number of electron
pairs it donates.
NH3, H2O, CO, NO are neutral unidentate
CN-, OH-, Cl- are anionic unidentate
(CH2NH2)2, C2O42-, glycine are bidentate
EDTA is hexadentate
CN-, NO2-, etc. can be joined to metal ion of either side (ambidient)
Bidentate etc form a ring with metal ion in a complex – Chelate, for example nickel
dimethyl glyoximate.
The co-ordination number (C.N.) of a metal atom in a complex is the total number of bonds
the metal atom forms with ligands; (C.N.) = 2 x valency.
In complex compounds the central metal atom or ion can act as Lewis acid and ligands as
Lewis bases.
Transition metals can form complexes in the zero, positive and low negative oxidation states
i.e. the centre of co-ordination can be a neutral atom, cation or even an anion in low negative
oxidation state.
Chelating ligand is a neutral molecule, free radical or ion (cation or anion) with two or more
Ione pairs of electrons on different atoms.
Chelates with five membered rings are very stable if these do not contain a double bond e.g.
in ethylenediamine. However, Chelates with six membered rings are very stable if the ligands
contain double bond e.g. acac.
Masking: Masking is the process in which a substance without physical separation of it is so
transformed that it does not enter into a particular reaction e.g., masking of Cu2+ by CN- ion.
Application of Co-Ordination Compounds
The ability of metal ions to form complexes with a variety of molecular species with
different physic-chemical properties is utilized in many ways. For example:
In analytical chemistry:
Multidentate ligand, EDTA (ethylenediamintetraacetic acid forms highly stable
complexes with metal ions like Ca2+ and Mg2+. This fact is used to estimate the hardness
of water by a simple titration method using standard EDTA solution.
A confirmatory test for the detection of copper (II) involves the formation of a deep-blue
coloured complex, [Cu(NH3)4]2+, on addition of ammonia solution to a solution of copper
(II) salt.
Cu2+ (aq) + 4NH3(aq) → [Cu(NH3)4]2+ (aq)
tetraamminecopper (II) ion
(Deep-blue)
The separation of Group I precipitate (of qualitative inorganic analysis) of AgCl, Hg2Cl2
and PbCl2 involves the addition of aqueous ammonia solution to the precipitate, when
only silver chlorides dissolve due to the formation of the complex ion, [Ag(NH3)2]+ .
AgCl(s) + 2NH3(aq) → [Ag(NH3)2]+ (aq) + Cl- (aq)
Hg2Cl2 and PbCl2 do not form complex ions with NH3 and hence, do not dissolve.
A confirmatory test for nickel involves the addition of a solution of dimethylglyoxime,
when a scarlet-red coloured precipitate is formed, due to the formation of a chelate
complex. Thus:
OH
CH3 – C = N
+ Ni2+
2
CH3 – C = N
OH
Dimethylglyoxim
e
O – H….…. O
CH3 – C = N
N = C – CH3
Ni
CH3 – C = N
N = C – CH3
O…..….H – O
Chelate complex
(Scarlet-red)
+ 2H+
Macrocyclic effect: This term refers to the greater thermodynamic stability of a complex with
a cyclic polydentate ligands when compared to the complex with a non-cyclic ligands. e.g.,
Zn(II) complex with ligand;
NH
NH
NH
HN
is more stable than with
NH
HN
NH2
H2N
π- acid ligand : Ligands which are capable of accepting an appreciable amount of π-electron
density from the metal atom into empty π or π* orbital of their own are called π-acceptor or
π- acid ligands e.g., CO.
ISOMERISM
Two different types of isomerism i.e., structural and stereoisomerism are possible for
coordination compounds.
Structural isomerism. Various types of structural isomerism are:
Ionisation isomerism arises when the co=ordination compounds with same molecular
formula give different ions in solution e.g., violet [Co(NH3)5Br]SO4 and red
[Co(NH3)5SO4]Br.
Linkage isomerism arises in complexes containing monodentate ligands with more
than one donor atoms e.g., NOି
ଶ may be bonded to metal through nitrogen (nitro) or
through oxygen (nitrito). Such ligands are also known as ambidentate ligands
Co-ordination isomerism involves exchange of ligands between complex cation and
complex anion i.e. between co-ordination spheres in a compound.
Hydrate isomerism arises in the complexes due to different number of water molecules
present in the co-ordination spheres. For example:
[Co(H2O)6]Cl3 ; [Co(H2O)5Cl] Cl2. H2O ; [Co(H2O)4C:2] Cl. (H2O)2 ;
[Co(H2O)3Cl3] (H2O)3 are hydrate isomers.
Co-ordination position isomerism. This type or isomerism is shown by bridged
complexes and involve different placement of ligands. it is a special type of coordination isomerism.
e.g., (NH3)4Co
OH
OH
Co(NH3)2Cl2 SO4 and Cl(NH3)3 Co
OH
OH
Co(NH3)3 Cl SO4
Ligand-isomerism: Consider the following two ligands, which are
H2C – CH – CH3
and
H2C – CH2 – CH2
H2N NH2
NH2
1, 2 – diaminopropane (pn)
NH2
1, 3 – diaminopropane (tn)
When these ligand form compounds, they will exhibit ligand isomerism. Example.
[Co (pn)2Cl2]+ and [Co (tn)2Cl2]+
In extraction of metals:
Extraction of metals like silver and gold is carried out by forming their soluble
cyanide complexes. For example,
4 Au + 16 CN- + 6H2O + O2
4 [Au(CN)2] - + 12 OHdicyanogold (I) ion
(Soluble)
The solution containing cyanide complex is then treated with zinc when gold is
precipited.
2 [Au(CN)2]- + Zn
(Zn(CN)4]2- + 2Au
Co-ordination compounds of silver and gold are used as the constituents of
electroplating baths for the controlled delivery of Ag+ and Au+ ions, during
electroplating or electro-refining of these metals.
The name of a complex compound should not start with a capital letter e.g.,
[Cu(NH3)4]SO4
Tetraamminecopper (II) sulphate
And not Tetraammine copper (II) sulphate
The full name of the complex should be written as one word without any gap.
Formulation of Cobalt(III) chloride-Ammonia Complexes
Color
Formula
Solution conductivity
Corresponds to
Yellow
[Co(NH3)6]3+3Cl-
Purple
[cocl(NH3)5]2+2Cl- 1 :2 electrolyte
Green
[cocl2(NH3)4]+ Cl-
1 :1 electrolyte
Vilote
[cocl2(NH3)4]+ Cl-
1 :1 electrolyte
1 :3 electrolyte
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