Download PREPARATION OF COORDINATION COMPOUNDS Coordination

PREPARATION OF COORDINATION COMPOUNDS
Coordination compounds cannot really be purchased from BDH, Fisher scientific company, Alfa
productions etc. they are rather synthesized from starting materials like CrCl3, CoCl2, NiCl2,
K2PtCl4 either through substitution of H2O and or halide ions in the primary coordination sphere
of the starting materials.
Other preparative methods of coordination compounds include:
a.
SIMPLE ADDITION REACTION example:
i.
BF3 + :NH3 → [BF3NH3] →
This is a gas-phase addition and each reactant is carefully controlled into a large evacuated flask
with a resultant deposit of a white powder (product).
For heterogeneous reactions
ii.
BF3 + Et2O →[BF3 . OEt2]
Here, the ether and Boron trifluoride are condensed separately into an evacuated flask and cooled
in liquid nitrogen. When warmed gently, a controlled reaction takes place.
Other coordination compounds prepared by simple addition reaction include.
iii.
SnCl4 + 2NMe3
trans [SnCl4 (NMe3)2]
iv.
8NH4F + 2H3BO3 + 3H2SO4 → 3(NH4) SO4 + 2 N4 [BF4] = 6H2O
v.
2KCL + TiCl4 → K2[TiCl6]
b.
SUBSTITUTION REACTIONS
Labile complexes can easily be substituted e.g.
i.
[Cu(H2O)]2+ 4NH3 (aq) → [Cu (NH)4]2+ + 4H2O.
Though the reaction shows complete substitution, the reactions occurs in steps and the species
[Cu(H2O)4]2+ will be present in the solution even though at low concentration.
ii.
[Pb (H2O)6]2+ 6SC(NH2) →[Pb(Sc(NH2)2)6]2+ + 6H2O (thiourea)
iii.
[Fe(H2O)6]3+ + 3acac- → [Fe(acaca)3] (insoluble in water).
Some examples of preparations involving substitution reactions of inert complexes are
given below:
i.
[Co(NH3)5CO3]+ + 2HF
ii.
[Co(NO2)6]3- + 2en
[Co(NH3)5F]2+ + F- + CO2 + H2O
Cis[Coen2(NO2)2]+ + 4NO2-
(Cis-dinitrobis(ethylenediamine)cobalt III)
Other preparative methods involving substitution reactions in non-aqueous media are:
i.
[Cr(H2O)6]3+ + 6NCS-
[Cr(NCS)6]3- + 6H2O
ii.
2NEt4Cl + NiCl2
c.
OXIDATION – REDUCTION REACTION
(NEt4)2[Net4[NiCl4]
Substitution reactions cannot be totally relied upon as a preparative method for coordination
compounds especially inert complexes. A better method is to take a compound containing the
metal in a different oxidation state and oxidize or reduce it as appropriate in the present of a
coordinating ligand. This method is used extensively in the preparation of complexes of Co 3+
with Co2+ as the precursor and either air or H2O2 being the oxidizing agent.
i.
CoCl2 + 2NH4Cl + 10NH3 + H2O2
2[Co(NH3)6]Cl3 + 22O
ii.
K2Cr2O7 +H2C2O4 + 2K2C2O4 → 2K3[Cr(C2O4)3] + 6CO2 + 7H2O
iii.
8[Mn(H2O)6]2+ + 2[MnO4]- + 25HF2 → [MnF5(H2O)]2- + 9H+ + 46H2O.
d.
THERMAL DISSOCIATION REACTIONS
Some complexes can be degraded to others through controlled heating, expelling the volatile
ones. Examples,
i.
CuSO4 . 5H2O
CuSO4 . 4H2O
CuSO4
CuSO4 . 3H2O
CuSO4 . H2O
ii.
Cis[Cr(en)2Cl2]2+ can be prepared by heating [Cr(en)3]Cl3. i.e.
[Cr(en)3]Cl3
iii.
Cis[Cr(en)2Cl2]Cl + en
Trans[Cr(en)2(SCN)2]SCN is prepared by heating [Cr (en)3](SCN)3 to 1300C
SYNTHESIS OF COORDINATION COMPOUNDS
Transition metal chemistry started from the study of coordination complexes by Werner and his
students. To date, coordination complexes remain the most widely studied compounds because
they were the first to be prepared. A great variety of cobalt compounds have been synthesized,
most of which were made possible due to the lability of Co2+ compounds to substitution and
inertness of Co3+ .Conversion of Co2+ to Co3+ was therefore easy with most starting materials
being converted to the Co2+ intermediate.
Most of the preparative work in Co–chemistry usually begins with the halides (Cl-, Br-, & I-)
because of their easeness to substitution by stronger liquids.
SYNTHESIS OF SOME COORDINATION COMPOUNDS
i.
Synthesis of Pentaamine Dichlorocobalt (ii) Chloride
Co(NO3)2.6H2 → [Co(H2O)6](NO3)2.
The starting materials are first converted to an intermediate Co (ii) carbonato complex which is
then oxidized to the final product.
Co(NO3 ) + N3(aq) + (NH4 )2 Co3 + H2 O2
NH3
O
Nh3
+
No3-
C=0
Co
+ NH4 NO3 + H2 O
O
NH3
Unbalanced
NH3
HCl(aq)
NH3
NH3
H2 O
NH3
H2 O
NH3
2+
Co
Co
NH3
NH3
NH3
Cl
NH3
NH3
NH3
HCl(aq)
NH3
Cl
NH3
Cl2
Co
NH3
Nh3
NH3
ii.
Synthesis of Hexaamine Dichromium III Nitrate
NH3
NH3
CrCl3 + 6NH3
NH3
2
NH | NH3
Co
Cl3
NH3
NH3
NH3
brown
HNO3(aq)
NH3
NH3
NH3
Co
NH3
NH3
NH3
iii.
(NO3 )3
Preparation of Cobalt Compounds
Yellow
2+
When a Co2+ salt is placed in aq NH3, the solution gradually turned brown in the presence of air
and finally wine red on boiling. A purple colour cpd is obtained if excess chloride is used for the
same reaction. Some of the possible compounds include the following:
Co Cl3 . 6NH3
Co Cl3 . 5NH3 . 5N2O
Co Cl3 . 5NH3
NH3
NH3
Cl
NH3
NH3
Co
NH3
Cl3
NH3
NH3
Co
yellow
NH3
Co
Cl3
NH3
red
Cl2
NH3
NH3
Because of the ease with which Co-cplexes are prepared, a great variety of them have been
synthesized and so transition metal chemistry stems from the study of co-cplexes.
Much of the preparative work in Co-chemistry usually begins with the halides (Cl-, Br- & I-).
As expected, the halides are the lowest in the spectrochemcial series and so they are easily
substituted by stronger ligands to form many successive classes of complexes.
iv.
Synthesis of Platimium Compounds
Pt (II) cpds are the most abundant and interesting. The halides are also the route through which
Pt compounds are prepared.
Pt . sponge
1. dissolve in
aqua regia
2. Add. Kcl.
(N2H6)(SO4)2
K2PtCl6
(yellow)
K2PtCl4
(red)
or
Pt. Cl4 (red)
M
M2PtCl4
Pt . + xcl2
PtCl2 (Olive green) Pt. Cl2
Using M2PtCl4 and PtCl2 as starting materials, many Pt (II) cpds can be synthesized e.g.
2[PtCl4]2- + 4L → [PtL4][Pt. Cl4] + 4ClPtL2X2 + 2L + 2AgNO3 → [PtL4](NO3)2 + 2Ag X
[Pt(NH3)4]Cl2 + K2PtCl4 → [Pt (NH3)4][PtCl4].
K2 PtCl4 + NH3
Cl
NH3-
Cl
NH3
Pt
Pt
Cl
Cl
Cl
+ 2Kcl
NH3
K2PtCl4 + 4NH3 → [Pt(NH3)4]Cl2 + 2KCl
When [Pt(NH3)4]Cl2 is heated in the presence of excess, Cl-, a trans isomer is produced.
Pt
Cl
NH3
NH3-
Cl
Pt
Cl2
Cl
NH3
-NH3 Cl
Cl
NH3
-NH3
NH3
Cl
Pt
Cl
- trans
NH3