COORDINATION CHEMISTRY

V.SANTHANAMDEPARTMENT OF CHEMISTRY

SCSVMV

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COLOR AND MAGNETISM

e- in partially filled d sublevel absorbs visible light moves to slightly higher energy d orbital

Magnetic properties due to unpaired electrons

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COORDINATION COMPOUND

Consist of a complex ion and necessary counter ions

[Co(NH3)5Cl]Cl2

Complex ion: [Co(NH3)5Cl]2+

Co3+ + 5 NH3 + Cl- 1(3+) + 5 (0) + 1(1-) 2+

Counter ions: 2 Cl-

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COORDINATION CHEMISTRY Transition metals act as Lewis acids

Form complexes/complex ionsFe3+(aq) + 6CN-(aq) Fe(CN)6

3-(aq)

Ni2+(aq) + 6NH3(aq) Ni(NH3)62+(aq)

Lewis acid Lewis base

Complex ion

Lewis acid Lewis base

Complex ion

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Complex contains central metal ion bonded to one or more molecules or anions

Lewis acid = metal = center of coordination

Lewis base = ligand = molecules/ions covalently bonded to metal in complex

V. SANTHANAMComplex ion remains intact upon dissolution in water

[Co(NH3)6]Cl3 [Pt(NH3)4]Br2

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COMPLEX ION Species where transition metal ion is

surrounded by a certain number of ligands.

Transition metal ion: Lewis acid Ligands: Lewis bases

Co(NH3)63+

Pt(NH3)3Br+

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LIGANDS Molecule or ion having a lone electron pair

that can be used to form a bond to a metal ion

(Lewis base). coordinate covalent bond: metal-ligand

bond

monodentate: one bond to metal ion bidentate: two bond to metal ion polydentate: more than two bonds

to a metal ion possible

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FORMULA OF COORDINATION COMPOUNDS

1. Cation then anion 2. Total charges must balance to zero 3. Complex ion in brackets

K2[Co(NH3)2Cl4]

[Co(NH3)4Cl2]Cl

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NAMES OF COORDINATION COMPOUNDS

1. Cation then anion 2. Ligands in alphabetical order before

metal ion neutral: molecule name* anionic: -ide -o prefix indicates number of each 3. Oxidation state of metal ion in ()

only if more than one possible 4. If complex ion = anion, metal ending

-ate

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EXAMPLES K2[Co(NH3)2Cl4]

potassium diamminetetrachlorocobaltate(II)

[Co(NH3)4Cl2]Cl

tetraamminedichlorocobalt(III) chloride

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Isomers(same formula but different properties)

Stereoisomers(same bonds, differentspatial arrangements)

Structuralisomers

(different bonds)

Opticalisomerism

Geometric(cis-trans)isomerism

Linkageisomerism

Coordinationisomerism

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STRUCTURAL ISOMERISM 1 Coordination isomerism Composition of the complex ion varies.

[Cr(NH3)5SO4]Br and [Cr(NH3)5Br]SO4

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STRUCTURAL ISOMERISM 2 Ligand isomerism: Same complex ion structure but point of

attachment of at least one of the ligands differs.

[Co(NH3)4(NO2)Cl]Cl and [Co(NH3)4(ONO)Cl]Cl

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LINKAGE ISOMERS

[Co(NH3)5(NO2)]Cl2Pentaamminenitrocobalt(III)

chloride

[Co(NH3)5(ONO)]Cl2Pentaamminenitritocobalt(III)

chloride

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STEREOISOMERISM 1 Geometric isomerism (cis-trans):

Atoms or groups arranged differently spatially relative to metal ion

[Pt(NH3)2Cl2]

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H3N

Co

H3N

NH3

NH3

Cl

Cl

H3N

Co

H3N

NH3

Cl

Cl

NH3

Cl

Cl

Co

Cl

ClCo

(a) (b)

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STEREOISOMERISM 2 Optical isomerism:

Have opposite effects on plane-polarized light

(no superimposable mirror images)20_446

Unpolarizedlight

Polarizingfilter

Polarizedlight

Tubecontainingsample

Rotatedpolarized light

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Left hand Right hand

Mirror imageof right hand

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N

N

N

N

N

NCo

N

N

N

N

N

NCo

Mirror imageof Isomer I

Isomer I Isomer II

N

N

N

N

N

NCo

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Cl

Cl

N

N

N

NCo

Cl

Cl

N

N

N

NCo

Cl

Cl

N

N

N

NCo

Cl

Cl

N

N

N

NCo

Cl

Cl

N

N

N

NCo

Isomer IIIsomer I

cistrans

Isomer II cannot besuperimposed exactlyon isomer I. They arenot identical structures.

The trans isomer andits mirror image areidentical. They are notisomers of each other.

Isomer II has the samestructure as the mirrorimage of isomer I.(b)(a)

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BASIS FOR BONDING THEORIES Models for the bonding in

transition metal complexes must be consistent with observed behavior.

Specific data used include stability (or formation) constants, magnetic susceptibility, and the electronic (UV/Vis) spectra of the complexes.

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BONDING IN COMPLEXES Werner’s Theory Sidgwick’s Theory Crystal Field Theory Ligand Field Theory

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WERNER’S THEORY

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BONDING APPROACHES Valence Bond theory provides

the hybridization for octahedral complexes.

For the first row transition metals, the hybridization can be: d2sp3 (using the 3d, 4s and 4p orbitals), or sp3d2 (using the 4s, 4p and 4d orbitals).

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LIMITATIONS OF VBT The valence bond approach isn’t used

because it fails to explain the electronic spectra and magnetic moments of most complexes