Coordination Chemistry-I

Dr. Debashree MandalAssistant ProfessorDepartment of ChemistryTarakeswar Degree CollegeTarakeswar, Hooghly

Double and complex salts:

Both double salts as well as

complex salt are formed by the

combination of two or more

stable compounds in

stoichiometric ratio.

They differ in the fact that

double salts such as carnallite,

Mohr’s salt, potash alum,

dissociate into simple ions

completely when dissolved in

water. However, complex salt

such as [Fe(CN)6]4- of

K4Fe(CN)6, do not dissociate

into Fe2+ and CN- ions.

Coordination compounds – compoundscomposed of a metal atom or ion andone or more ligands.

[Co(Co(NH3)4(OH2)3]Br6

Ligands usually donate electrons tothe metal

Includes organometallic compounds

Werner’s totally inorganic optically active compound.

Werner’s theory of coordination complexes:

Two types of bonding-Primary – positive charge of the metal ion is balanced by negative ionsin the compound.Secondary – molecules or ion (ligands) are attached directly to themetal ion.

Coordination sphere or complex ion.Look at complex on previous slide (primary and secondary)

He largely studied compounds with four or six ligands.Octahedral and square-planar complexes.

It was illustrated that a theory needed to account for bonds betweenligands and the metal.

The number of bonds was commonly more than accepted at that time.18-electron rule.

New theories arose to describe bonding.Valence bond, crystal field, and ligand field.

coordination numbers: Number of bonds formed between the metal ion and theligands in the complex ion. 6 and 4 (most common) 2 and 8 (least common)

Low Coordination Numbers (C.N.)

• C.N. 1 is rare except in ion pairs in the gas phase.

• C.N. 2 is also rare.

– [Ag(NH3)2]+, Ag is d10 (how?)

– VSEPR predicts a linear structure.

– Large ligands help force a linear or near-linear arrangment.• [Mn(N[SiMePh2]2)2] in Figure 9-22.

• C.N. 3 is more likely with d10 ions.

– Trigonal-planar structure is the most common.

– [Cu(SPPh3)3]+, adopts a low C.N. due to ligand crowding.

Coordination Number 4

• Tetrahedral and square planar complexes are the most common.

– Small ions and/or large ligands prevent high coordination numbers (Mn(VII) or Cr(VI)).

• Many d0 or d10 complexes have tetrahedral structures (only consider bonds).

– MnO4- and [Ni(CO)4]

– Jahn-Teller distortion (Chapter 10)

Coordination Number 4

• Square-planar geometry

– d8 ions (Ni(II), Pd(II), and Pt(III))

• [Pt(NH3)2Cl2]

– The energy difference between square-planar and tetrahedral structures can be quite small.

• Can depend on both the ligand and counterion.

• More in chapter 10.

Coordination Number 5

• Common structures are trigonal bipyramid and square pyramid.– The energy difference between the two is small. In many

measurements, the five ligands appear identical due to fluxional behavior.

– How would you modify the experiment to differentiate between the two structures?

• Five-coordinate compounds are known for the full range of transition metals.– Figure 9-27.

Coordination Number 6

• This is the most common C.N. with the most common structure being octahedral.

– If the d electrons are ignored, this is the predicted shape.

• [Co(en)3]3+

• This C.N. exists for all transition metals (d0 to d10).

Distortions of Complexes Containing C.N. 6

• Elongation and compression (Fig. 9-29).

– Produces a trigonal antiprism structure when the angle between the top and bottom triangular faces is 60.

– Trigonal prism structures are produced when the faces are eclipsed.• Most trigonal prismatic complexes have three bidentate ligands

(Figure 9-30).

• interactions may stabilize some of these structures.

The Jahn-Teller effect (Ch. 10) is useful in predicting observed distortions.

Higher Coordination Numbers

• C.N. 7 is not common

• C.N. 8

– There are many 8-coordinate complexes for large transition elements.

• Square antiprism and dodecahedron

• C.N.’s up to 16 have been observed.

Classification of ligands:

Molecule or ion having a lone electron pair that can be used to form a bond toa 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

(en)

Chelating Ligands

• Chelating ligands (chelates) – ligands that have two or more points of attachment to the metal atom or ion.

– Bidentate, tridentate, tetra.., penta…, hexa… (EDTA).

trisoxalatochromate(III) ion or just [Cr(ox)3]3-

A Hexadentate Ligand, EDTA

• There are six points of attachment to the calcium metal.

– Octahedral-type geometryethylene diamine tetraacetic acid

(EDTA)

ethylenediaminetetraacetatocalcium ion or just [Ca(EDTA)]2-

IUPAC Nomenclature

• Ligands are named in alphabetical order (name of ligand, not prefix)– [Co(NH3)4Cl2]+ and [Pt(NH3)BrCl(CH3NH2)]+2

• Anionic ligands are given an ‘o’ suffix. Neutral ligands retain the usual name. – Coordianted water is called ‘aqua’.

– Chloro, Cl-

– Sulfato, SO42-

• The calculated oxidation number of the metal ion is placed as a Roman numeral in parentheses after the name of the coordination sphere.– [Pt(NH3)4]+2 and [Pt(Cl)4]-2

– A suffix ‘ate’ is added to the metal ion if the charge is negative.

• The prefixes cis- and trans- designate adjacent and opposite geometric location, respectively.– trans-diamminedichloroplatinum(III) and cis-

tetraamminedichlorocobalt(III)

Q. Name the following coordination compounds.

(a) [Co(H2O)6]Br3 (b) Na2[PtCl4]

(a) Hexaaquacobalt(III) bromide

(b) Sodium tetrachloroplatinate(II)

Name the following coordination

compounds.

(a) [Co(H2O)6]Br3 (b) Na2[PtCl4]

(a) Hexaaquacobalt(III) bromide

(b) Sodium tetrachloroplatinate(II)

Nomenclature

• Bridging ligands between two metal ions have the prefix ‘’.

– -amido--hydroxobis(tetraamminecobalt)(IV)

There is an error in this picture. What is it?

Isomerism in coordination compounds:

20_441

Isomers

(same formula but different properties)

Stereoisomers

(same bonds, different

spatial arrangements)

Structural

isomers

(different bonds)

Optical

isomerism

Geometric

(cis-trans)

isomerism

Linkage

isomerism

Coordination

isomerism

Ionisationisomer

Hydrationisomer

Polymerizationisomer

Polymerization Isomerism:This is not true isomerism because it occurs between compounds having the same empirical formula, but different molecular weights.[Pt(NH3)2Cl2],[Pt(NH3)4][PtCl4][Pt(NH3)4] [Pt(NH3)Cl3]2.

Ionization Isomerism:This type of isomerism is due to the exchange of groups between the complex ion and the ions outside it. [Co(NH3)5Br]SO4 is red – violet. An aqueous solution gives a white precipitate of BaSO4 with BaCl2 solution, thus confirming the presence of free SO4

2- ions. In contrast [Co(NH3)5SO4]Br is red. A solution of this complex does not give a positive sulphate test with BaCl2. It does give a cream – coloured precipitate of AgBr with AgNO3, the confirming the presence of free Br– ions.

Hydrate Isomerism:Three isomers of CrCl3.6H2O are known. From conductivity measurements and quantitative precipitation of the ionized chlorine, they have been given the following formuale:[Cr(H2O)6]Cl3 : violet (three ionic chlorines)[Cr(H2O)5Cl]Cl2.H2O : green (two ionic chlorines)[Cr(H2O)4Cl2].Cl.2H2O : dark green (one ionic chlorine)

Coordination isomerism: [Cr(NH3)5SO4]Br and [Cr(NH3)5Br]SO4

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

Linkage Isomers:

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

chloride

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

chloride

Geometric isomerism (cis-trans): Atoms or groups arranged differently spatially relative to metal ion Pt(NH3)2Cl2

20_444

H3N

Co

H3N

NH3

NH3

Cl

Cl

H3N

Co

H3N

NH3

Cl

Cl

NH3

Cl

Cl

Co

Cl

Cl

Co

(a) (b)

Optical isomerism: Have opposite effects on plane-polarized light(no superimposable mirror images)

20_448

Left handRight hand

Mirror image

of right hand

20_449

N

N

N

N

N

NCo

N

N

N

N

N

NCo

Mirror image

of Isomer I

Isomer I Isomer II

N

N

N

N

N

NCo

20_450

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 be

superimposed exactly

on isomer I. They are

not identical structures.

The trans isomer and

its mirror image are

identical. They are not

isomers of each other.

Isomer II has the same

structure as the mirror

image of isomer I.(b)(a)

Concept Check

Does [Co(en)2Cl2]Cl exhibit

geometrical isomerism?

Yes

Does it exhibit optical isomerism?

Trans form – No

Cis form – Yes

Explain.

The end