14) coordination chemistry.pdf

8/20/2019 14) Coordination Chemistry.pdf

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Coordination Compounds

Coordination compound is a neutral species containing one or

more complex ions.

Complex ion contains a central metal atom or ion bonded to

one or more ions or molecules

Most elements exhibit two types of valence:

i) primary valence - corresponds to the oxidation number

ii) secondary valence - corresponds to coordination number

Werner Coordination Theory

Example: CoCl3.6NH3 or [Co(NH3)6]Cl3

primary valence = +3 sec. valence or coordn # = 6


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The formula [Co(NH3)6]Cl3 indicates that the ammonia

molecules and the cobalt atom form a complex ion; thechloride ions are not part of the complex but are held to it

by ionic forces.

Ligands - the molecules or ions that surround the metal in a

complex ion.

ionic

coordinate or dative

covalent bond


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The interactions between a metal atom and the ligands is kind

of Lewis acid-base reaction.

Ligand acts as Lewis base donating one or more electron

pairs to the metal, the Lewis acid.

Every ligand has at least one unshared pair of valence

electrons.

Examples

The metal-ligand bonds are usually coordinate covalent

bonds.


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Donor atom - The atom in a ligand that is bound directly to the

metal atom.

In [Co(NH3)6]Cl3, N in NH3 is directly bound to Co; N is the

donor atom.

Example:


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Coordination number (CN) - the number of donor atoms

surrounding the central metal atom in a complex ion.

Ag+ in [Ag(NH3)2]+ ; CN = 2 (2 N, one from each NH3)

Examples:

Cu+2

in [Cu(NH3)4]+2

; CN = 4 (4 N, one from each NH3)Fe+3 in [Fe(CN)6]-3 ; CN = 6 (6 C, one from each CN-)

Classification of Ligands based on the number of donor atoms

present per molecule or ion:1. Monodentate - 1 donor atom per molecule

Examples:


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2. Bidentate - 2 donor atoms per molecule of ligand

Example:

e

thylenediammine (en)

The two nitrogen atoms can coordinate with a metal atom

as shown

Simplified structure of

the same complex.

Structure of metal-ethylene-

diamine complex.

Each e th y le n e d iam in e

molecule provides two N

donor atoms and is therefore

a bidentate ligand.


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Bidentate and polydentate ligands are also called chelating

agents because of their ability to hold the metal atom like aclaw (from the Greek chele, meaning “claw”).

ethylenediaminetetraacetate ion (EDTA), (hexadentate)- a polydentate ligand used to treat metal poisoning

Example:


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Six donor atoms enable EDTA to form a very stable complex ion

with lead.

EDTA complex of lead.

In this form it is removed from

the blood and tissues and

excreted from the body.

EDTA is also used to clean up

spills of radioactive metals.


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Oxidation Numbers (ON) of Metals in Coordination Compounds

The net charge of a coordination compound or complex ion is

the sum of the charges on the central metal atom and its

surrounding ligands.

Examples:

[PtCl6]-2 : x + 6(-1) = -2 ; x = +4

[Co(NH3)4Cl2]+ : x + 4(0) + 2(-1) = +1 ; x = +3

[Co(NH3)5Cl]Cl2 : x + 5(0) + 1(-1) +2(-1) = 0 ; x = +3

(NH4)[Ni(NH3)2(C2O4)2] : +1 + x + 2(0) +2(-2) = 0 ; x = +3

Let x = ON of metal


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[Pt(NH3)4][PtCl4]2

[+2+4(0)] + [+2+4(-1)]2 ! 0 ; [Pt(+2)][ Pt (+2)] is incorrect

[+4+4(0)] + [+4+4(-1)]2 ! 0 ; [Pt(+4)][ Pt (+4)] is incorrect

(known OS of Pt: +2, +4)

[Pt

(?)

(NH3)4] [Pt

(?)

Cl4 ]2

[+2+4(0)] + [+4+4(-1)]2! 0 ; [Pt

(+2)

][ Pt

(+4)

] is incorrect[+4+4(0)] + [+2+4(-1)]2 = 0 ; [Pt(+4)][ Pt (+2)] is CORRECT

The sum of oxidation states for a neutral compound is zero.


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Specify the oxidation number of the central metal atom in

each of the following compounds:

(a) [Ru(NH3)5(H2O)]Cl2

(b) [Cr(NH3)6](NO3)3

(c) [Fe(CO)5](d) K4[Fe(CN)6]

Exercises:


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Nomenclature Rules

1. The positive ion (cation) comes first, followed by the

negative ion (anion).

Examples: diamminesilver(1) chloride, [Ag(NH3)2]Clpotassium hexacyanoferrate(lII), K3[Fe(CN)6]

2. Within the coordination sphere, the ligands are named

before the metal, but in formulas, the metal ion is written

first.

Examples: tetraamminecopper (II) sulfate, [Cu(NH3)4]SO4 hexaamminecobalt(III) chloride, [Co(NH3)6]Cl3


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3. The number of ligands of one kind is given by the following

prefixes. If the ligand name includes these prefixes or iscomplicated, it is set off in parentheses and the second set

of prefixes is used.

2 di bis3 tri tris

4 tetra tetrakis

5 penta pentakis

6 hexa hexakis

7 hepta heptakis8 octa octakis

9 nona nonakis

10 deca decakis


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Examples: dichlorobis(ethylenediamine)cobalt(III),

[Co(NH2CH2CH2NH2)2CI2]+

tris(bipyridine)iron(II), [Fe(NH4C5-C5H4N)3]2+

4. Ligands are named in alphabetical order (according to the

name of the ligand, not the prefix), although exceptions tothis rule are common. An earlier rule gave anionic ligands

first, then neutral ligands, each listed alphabetically.

Examples: tetraamminedichlorocobalt(III), [Co(NH3)4CI2]+

(tetraammine is alphabetized by a anddichloro by c, not by the prefixes)

amminebromochloromethylamineplatinum(II),

[Pt(NH3)BrCl(CH3NH2)]


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5. Anionic ligands are given an o suffix. Neutral ligands retain

their usual name. Coordinated water is called aqua and

coordinated ammonia is called ammine.

Examples: chloro, Cl- methylamine, CH3NH2 bromo, Br - ammine, NH3 *

sulfato, SO42- aqua, H2O

*(the double m distinguishes NH3 from alkyl amines)

6. Two systems exist for designating charge or oxidation number:

a. The Stock system puts the calculated oxidation number of

the metal ion as a Roman numeral in parentheses after the

name of the metal. (commonly used)


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b. The Ewing-Bassett system puts the charge on the

coordination sphere in parentheses after the name of themetal. (used by Chemical Abstracts)

In either case, if the charge is negative, the suffix -ate is added

to the name of the coordination sphere.

Examples:

[PtCl4]2-

tetrachloroplatinate(II) or tetrachloroplatinate(2-)

[Pt(NH3)4]2+

tetraammineplatinum(II) or tetraammineplatinum(2+)

[PtCl6]-2

hexachloroplatinate(IV) or hexachloroplatinate(2-)


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7. The prefixes cis- and trans- designate adjacent and opposite

geometric locations.

Examples:

cis- and trans-diamminedichloroplatinum(II), [PtCl2(NH3)2]

cis- and trans-tetraamminedichlorocobalt(III), [CoCl2(NH3)4]+

cis trans


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8. Bridging ligands between two metal ions as in have the

prefix !-.

Example:

!-amido-!-hydroxobis(tetramminecobalt)(4+),

[(NH3)4Co(OH)(NH2)Co(NH3)4]4+


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!"

tris(tetraammine-!-dihydroxocobalt)cobalt(6+) bromide [Co(Co(NH3)4(OH2)3]Br 6


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!#

9. When the complex is negatively charged, the names for the

following metals are derived from the sources of their

symbols, rather than from their English names:

iron (Fe) : ferrate lead (Pb): plumbate

silver (Ag) : argentate tin(Sn): stannate

gold (Au): aurate

Examples:

tetrachloroferrate(III) or tetrachloroferrate(1-), [FeCl4]-dicyanoaurate(I) or dicyanoaurate(1-), [Au(CN)2]-


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!$

Exercises:

Name the following coordination complexes:

1. [Cr(NH3)3Cl3]

2. [Pt(en)Cl2]

3. [Pt(ox)2]2-

4. [Cr(H2O)5Br]2+

5. [Cu(en)Cl4]2-

6. [Fe(OH)4]-


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!%

6. Ammonium tetrachlororuthenate(1-)

Exercises:

Give the structures of the following coordination complexes:

1. Tris(acetylacetonato) iron(III)

2. Hexabromoplatinate(2-)

3. Potassium diamminetetrabromocobaltate(III)

4. Tris(ethylenediamine)copper(II) sulfate

5. Hexacarbonylmanganese(I) perchlorate

14) coordination chemistry.pdf

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