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CHAPTER 3: COORDINATION CHEMISTRY
CHEM210/Chapter 3/2014/01
A coordination compound, sometimes called a coordination complex, contains a central metal atom or ion surrounded by a number of oppositely charged ions or neutral molecules (possessing lone pairs of electrons) which are known as ligands.If a ligand is capable of forming more than one bond with the central metal atom or ion, then ring structures are produced which are known as metal chelates, the ring forming groups are described as chelating agents or polydentate ligands.
The coordination number of the central metal atom or ion is the total number of sites occupied by ligands. Note: a bidentate ligand uses two sites, a tridentate three sites etc.
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Molecular formula
Lewis base/ligand Lewis acid Donor atom Coordination
number
[Zn(CN)4]2- CN- Zn2+ C 4
[PtCl6]2- Cl- Pt4+ Cl 6
[Ni(NH3)6]2+ NH3 Ni2+ N 6
TYPES OF LIGANDS
MONODENTATEWhen ligands donate one pair of electrons to the metal atom.
chloro hydroxo
amine methylamine
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BIDENTATELigands that contain two of more atoms, each of which can simultaneously form two-electron donor bonds to the same metal ion.
These ligands are also called chelate ligands.
ethylenediamine (en) oxalato (ox2-)
MULTIDENTATE
Diethylenetetramine (trien), EDTA, etc.
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ISOMERSIonization isomers
Isomers can produce different ions in solution e.g. [PtCl2(NH3)4]Br2 ⇌ [PtBr2(NH3)4]Cl2
Polymerization isomers
Loose term, “same stoichiometry, different arrangement in space”
Seven compounds with formula Co(NH3)3(NO2)3
Coordination isomers
[Co(NH3)6]3+ [Cr(NH3)6]-3 [Co(CN)6]3+ [Cr(CN)6]-3
Linkage isomers
e.g. Nitro and nitrito, N or O coordination possible.
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Geometric isomers
Formula is the same but the arrangement in 3-D space is different e.g. square planar molecules give cis- and trans- isomers.
For hexacoordinate systems other species, can also occur.
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For M(X)3(Y)3 systems there is facial and meridian isomers.
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Are “stereo” isomers also possible? An analogy to organic chirality, molecules which can rotate light.
EnantiomersNon-superimposable mirror images.
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NOMENCLATURE FOR COORDINATION COMPOUNDSCotton, Wilkinson and Gaus, p 178- 183
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THE STABILITY OF COORDINATION COMPOUNDS
For a given metal and ligand, complexes where the metal oxidation state is +3 are more stable than +2
Stabilities of complexes of the first row of transition metals vary in reverse of their cationic radii (in general):
MnII < FeII < CoII < NiII > CuII > ZnII
Chelate Effect
A complex containing one or more five- or six- membered chelate rings is more stable, i.e. has a higher formation constant than a complex that is as similar as possible but lacks some or all of the chelate rings.
Ni2+(aq) + 6 NH3(aq) = β6 = 108.6NiH3N
H3N
NH3
NH3
NH3
NH32+
(aq)
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NiHN
HN
NH
NH
NH
NH2+
(aq)
HH
H
H
H
HNi2+(aq) + 3 (en)(aq) = β6 = 1018.3