UNIT V
COORDINATION COMPOUNDS
Coordination compounds are aspecial cass of compounds in which the central metal stom is
siltrounded bv ions or molecules beyond their normal valeney. These compounds are widelk
Dresent in the minerals. plants and animals and play many important functions. AMany
biologically important compounds are Coordination compounds in which complicated organic
sDecies are bound to metal ions. The conimon examples are: Haemoglobin which is a
Coordination compound of Iron. Chlorophyll which is a Coordination compound of
Magnesium and Vitamin B12which is a Coordination compound of Cobalt ete. The
Coordinationcompounds are also finding extensive applications in metallurgical processes.
analvtical chemistry.industrial cataly sts. textile dyeing. clectroplating and medicinal chemistry
Coordination compounds are molecular compounds which are formed from the combinat ion of
[wo or more simple stable compounds and retain their identitv in the solid as well as in the
dissolved state. For example. when aqueous ammonia is added to green solution of nickel
chloride. NiCk. the colour changes to [Ni (NH:)]Ch purple crystals. Such a compound is called
Coordination compounds.
NiCL (Green) - 6NH... > Ni (NH:)6] Ch (Purple)
When the compound [Ni(NH:)o]Cl; is dissolved in water it ionizes to givea new speies. Such
an ion is called conplex ion [Ni(NH:)6.
Ni (NH] Ch- > Ni(NH:%| +2CH
Coordination compounds are the compounds in which the central metal atom is bound to a
number of anions or neutral molecules by coordinate bonds.
Coordination entity and Coordination sphere:
Coordination entity: A coordination entity constitutes a central metal atom or ion bounded to a
fix number of oppositely charged ions or neutral molecules. For example. [Co Cl; (NH;l3]
Coordination sphere: The central metal atom or ion and the molecules or ions bonded to i are
enciosed in a square bracket and a collectively called the Coordination sphere.
Central atomor ion andLigands:
Central atom or ion: The atom or ion to which a fixed number of neutral molecules or ions are
attachedin the coordination entity is called Centralatom or ion.
Ligands: The neutral molecules or ions bonded to the central atom or ion in thecoordination
entity are called ligands. For example.
Ni(NH;)6]:Central atom = N, ligands = NH: molecules
Types of ligands:
(0) Unidentate or monodentate ligands: When a ligand is bound to a metal ion through
asingle donor atom, as with CT. IH0 or NH. the ligand is said to be lnidentate
ligands.
(i) Didentate or Bidentate ligands: When a ligand can bind through two donor atoms as
in HNCH:CH;NH: (ethane-I. 2-diamine) or C:0: (oxalate). the ligand is said to he
Didentate ligands.
(ii) Polvdentate ligands: Ligands having ore the twodonor atoms present in the
molecule., the ligands are said to be Polvdentate ligands.
Diethvlene triamine acts as tridentate ligand having three donor \ atems.
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thiNNNK is an amrant hevadentate ligand. t can hind
SngN id ohachebate ligand lhN numr ot Nach ligating gnupsis called
the ott a Stwnglexe cala ckelate omnlee tend to more stalble than
ambientate ligand.
E \ a y N ligais tN NO N SC ns. NO ncan coordinate either though
Simiar, S in cn vinate thnugh the sulphur or nitnngen m.
Coondination number: The Nal uniNr ofhands atachd oaentral metal atom or ion is
calldthnatin nn'er otthat meal atomor ion. Fore.
NiNH:LCo-4
IlPACnomenclature of coondination compounds: \Wrie the l|PAC names ofthe ollow ing
([CoCk(en:]CI
(e) Hg {Co (SC))
(t) [Cr(NH:):H:0):)C:
(gl[CotH:NCHCH:NH)3:(SO.):
(a) Dianminechloridonitrito-N-platinum (ll)
tb) Potassium trioxalatochronate (|D
t) Drhkoridobis (ethane-I. -dhamine) cohult (l!D chloride
(dì Pentaamminecarhonatocobalt (lll)chloride
(e) Mercury ()tetrahiocvanato-S-cohaltate (ll)
) Triammineriaquachromium (li!) chloride
(g) Tris (ethane-1.2 diaminelcobalt(ll)sulpihate
(h) Diamninesilver (i) dicyanidoargentate ()
Werner's theory: Werner prosed the coneept ota prinary alence and a secondary alence
for a metal ion. Binary compounds such as Cr:, CoC:or Pdh have primary valenceot3. 2
and respctively.
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series compounds of cobali (|)
of?
In a chloride with
chloride ions could be precipitated
as AgClon adding ammonia. it was found that some of
remained in solution. excess silver nitrate solution the
sOme
Inol CoCl,.6NH, (Yellow) in cold but
gave 3 mol AgCl
Tmol CoCli.5NT:(Purple) gave 2 mol AgCl
Imol CoCh.4NHi(Green) gave Imol AgC|
| mol CoCh.4NII, (Violet) gave I mol AgCI
Werner proposed the term sccondary valencc for
metal ion; in cach of these Cxamples the
the number of
secondary valences are groups
six.
bound directly to the
ormulation ofCobalt (1l) C
Colour Formua Chloride-Ammonia Complexes:
Solution conductivity
Corresponds to
Yellow |Co (NH)6|" 3Cr 1:3
Purple |Co CI(NH3)s 2CI 1:2
Grecn Co Ch (NH)4| C 1:1
Violet |Co Ch (NH:)]' CF I:1
The main postulates are:
1In coordination compounds metals show two types of linkages (valences)-primary and
secondary.
2. The primary valences are normally ionisable and
3. The secondary valences are non ionisable. These are sat isfied by negative ions.
are
ions. The secondary valence is equal to the coordinationsatisfied by neutral molecules or negative
number and is fixed for a metal.
4. The ions/groups bound by the secondary linkages to the metal have
arrangements corresponding to different coordination numbers.
characterist ic spatial
Valence Bond Theory (VBT) for bonding in Coordination Compounds:
The main assumptions of this theory are listed below:
1. The central metal ion in the complex makes available a number of empty orbitals for the
formation of coordination bonds with suitable ligands.
2. The number of empty orbitals made available for this purpose is equal to coordination
number of the central metal ion. For example, if coordination number is 6, six empty
orbitals aremade available and if coordination number is 4, four empty orbitals are made
available in the central metal ion.
3. The appropriate atomic orbitals (s. p andd) of the metal hybridize to give aset of
equivalent orbitals of de finitegeometry such as square planar, tetrahedral and octahedral
and so on.
4. The d-orbitals involved in the hybridization may be cither inner d-orbitals ie. (n-l) dor
Outer d-orbitals i.e. nd. For example, in case of octahedral hybridizat ion. The orbitals may
be two 3d, one 4s and three 4p (d'sp) or one 4s, three 4p and two 4d (sp'd')
. Each ligand has at least one orbital (of donor atom))containing alone pair of electrons.
O. Ihe empty hybrid orbitals of metal ion overlap the filled orbitals of the ligand to form
metal-igand coordinate covalent bonds.
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Number of Orbitaks, Types of Hybridizations and Magnetie Properties of Coordination
Compounds:
Coordination Type of Complexv/in Geometry No. of Magnetic
number hybridization unpaired Character
electrons
4
sp |NiCLP Tetrahedral Paramagnetic
4
dsp [Ni(CN) Square planar Diamagnetic
6
|CoF] Octahedral 4 Paramagnetic
d'sp' |Co(NH3)6Octalhedral Diamagnetic
CRYSTAL FIELD THEORY (CFT): The crystal field theory (CFT) is an electrostatic model
which considers the metal-ligand bond to be ionic arising purely from clectrostatic interactions
between the metal ion and the ligand. Ligands are treated as point charges in case of anions or
point dipoles in case of neutral molecules. The five dorbitals in an isolated gaseous metal
atom ion have same energy, i.e., they are degenerate. This degeneracy is maintained if a
spherically symmetrical field ofnegative charges surrounds the metal atom/ion. However, when
this negative field is due to ligands (either anions or the negative ends of dipo lar molecules like
NH;and H;0) in a complex, it becomes asymmetrical and the degeneracy of the d orbitals is
lifted. It results in splitting of the d orbitals. The patternof splitting depends upon the nature of
the crystal field.
The conversion of five degenerate d-orbitals of the metal ion into different sets of orbitals having
different energies in the presence of electric field of ligand is called crystal field splitting.
(a) Crystal field splitting in octahedral coordination entities:
Metol
d ortbitals
uf he dorbtals tm
Splitng of d orottala
in wtthedral
Fre* tital io1:
rystal fickd
d orbital splitting in an octahedral crystal field (fig. 9.8
ncert)
Spectrochemical series: ligands can be arranged inaseries in the order of increasing lield
strength as given below:
I<Br <SCN < CI <$<F <OH <C0, <H0< NCS <edtat <NH,<en < CN <CO
For d ions, two possible patterns of electron
(i) The fourth electron could either enter the todistribution arise:
level and nair with an existing clectron, or
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) ltQoud avodpaying the price ofthe pairing energy by occupying the e, level. Which of
thess psibilitiesoccurs, depends on the relative magnitude of the crystal field splitting. Ao and
the airing energy. P(Prepresents the energy required tor electron pairing in a single orbital),
lhe wooptivns ar:
tAoP, the tourth electron enters one of the egorbitals giving the configuration1 e
liganis orwhich Ao <Pare kInOwIn as weak field ligands and form high spin complexes.
ap.
(i0 lfAo>P, it becones more energetically favourable for the fourth electron to occupy
orbal with contiguration e". Ligands which produce this effect are known as strong field
ligznis and fòm low spin complexes.
(b) Crstul field spliting in tetralhedral coordination entities:
Energy
A
Asrage netgy of the Splting ofd orbitals
dorbials in spèerca! i tetrabedrai ery stai
ncert)
d orbital splitting in atetrahedral crystalfield (fig.9,9
orbital splitting (Fig. 9.9) is inverted and is
In tetrahedralcoordination entity formation. the d For the same metal. the same ligands and
splitting.
smaller as compared to the octahedral field
metal-ligand distances, it can be shown that A, =
(4/9) Ao.
crystal field theory attributes the colour of the
Colour in Coordination Compounds: The
coordination compounds to d-d transition of
the electron.
in some
between the Wavelength of Light absorbed and the Colour observed
Relationship
Coordination Entities: Colour of coordination
Coordinationentity Wavelength of light Colour of light
absorbed (nm) absorbed entity
Pale Yellow
310 Violet
[CoCN)] Blue Yellow Orange
[CoNH;)] 475
Blue Green Red
[CoNH;)s(H;0)j* 500
510
Blue Green Purple
[TiH;0).J Yellow Violet
Co CINH:):| 535 Blue
600 Red
[Cu(H:0):*
Isomerism in Coordination Compounds:
atoms
same molecular formula but different arrangement of
Iwo or more compounds having the
phenomenon is called Lsomerism
are called isomers and the isomerism (b) Stereoisomerism
IWo types of isomerismare: (a)Structural
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