CHEM1612 - Pharmacy Week 8: Complexes II

Dr. Siegbert SchmidSchool of Chemistry, Rm 223Phone: 9351 4196E-mail: siegbert.schmid@sydney.edu.au

Unless otherwise stated, all images in this file have been reproduced from:

Blackman, Bottle, Schmid, Mocerino and Wille,     Chemistry, John Wiley & Sons Australia, Ltd. 2008

     ISBN: 9 78047081 0866

Lecture 22-3

Complexes Blackman, Bottle, Schmid, Mocerino & Wille Chapters 13,10.4, 11.8

Complex ions Coordination compounds Geometry of complexes Chelates Kstab

Solubility and complexes Nomenclature Isomerism in complexes Biologically important metal-complexes

Co(EDTA)-

Lecture 22-4

Example: AgBr(s) Ag+(aq) + Br-

(aq)

Calculate the solubility of AgBr in:a) waterb) 1.0 M sodium thiosulfate (Na2S2O3)

(Ksp (AgBr)= 5.0·10-13, Kstab ([Ag(S2O3)2]3- )= 4.7·1013 )

Complex Formation and Solubility

AgBr(s) Ag+(aq) + Br-

(aq)

Ksp = x2 = 5.0·10-13 x = 7.1 ·10-7 M

Ksp = [Ag+][Br-]

x x

a) Solubility of AgBr in water

Lecture 22-5

AgBr(s) Ag+(aq) + Br-

(aq)

Koverall = Ksp x Kstab = = 5.0·10-13 x 4.7·1013 = 24

Ag+(aq) + 2S2O3

2-(aq) [Ag(S2O3)2]3-

(aq)

AgBr(s) + 2S2O32-

(aq) [Ag(S2O3)2]3-(aq) + Br-

(aq)

(1)

(2)

(1)+(2)

b) Solubility of AgBr in sodium thiosulfate

[Ag(S2O3)23-][Br-]

[S2O32-]2

Initial Conc.ChangeEquilibrium Conc.

1.0 M-2x

1.0 -2x

0+xx

0+xx

Substitute: Koverall = x2/(1.0 - 2x)2 = 24 x = 0.45

Solubility of AgBr in thiosulfate is 0.45 M (c.f. in water 7.1 x 10-7 M)

1.0 M Na2S2O3

Lecture 22-6

Rules for nomenclature of coordination compounds:

Name cation, then anion, as separate words.Examples:

[Pt(NH3)4Cl2](NO2)2 tetraamminedichloridoplatinum(IV) nitrite

[Pt(NH3)4(NO2)2]Cl2 tetraamminedinitritoplatinum(IV) chloride

Name the ligands then the metal, all in same word.

Number of ligands as Greek prefixes (di-, tri-, tetra-, penta-, hexa-), except ligands that already have numerical prefixes which use Latin prefixes (bis, tris, tetrakis…) e.g. bis(ethylenediamine) for (en)2

Nomenclature I

Lecture 22-7

Nomenclature II Oxidation state in Roman numeral in parentheses after name of metal

e.g. [Ag(NH3)2]NO3 diamminesilver(I) nitrate

Anionic ligands end in '-ido‘:

Neutral ligands named as molecule, except those listed here:

(Please modify accordingly pp.518-519 of your book)

Ligand Name FormulaFluorido F-

Chlorido Cl-Bromido Br-

Iodido I-

Cyanido CN-

Hydroxido OH-

Ligand Name FormulaAmmine NH3

Aqua H2OCarbonylCONitrosyl NO

Lecture 22-8

Ligands named in alphabetical order (but prefixes do not affect the order) e.g. [Co(NH3)5Cl]SO4 pentaamminechloridocobalt(III) sulfate

Anionic complexes end in ‘-ate’ e.g. K3[CrCl6] potassium hexachloridochromate(III)

Some metals in anionic complexes use Latin -ate names:

Nomenclature of Ligands

Not IronateNot CopperateNot LeadateNot SilverateNot GoldateNot Tinnate

Lecture 22-9

Nomenclature - Exercises [Co(H2O)6]CO3

hexaaquacobalt(II) carbonate

[Cu(NH3)4]SO4

tetraamminecopper(II) sulfate

(NH4)3[FeF6]ammonium hexafluoridoferrate(III)

K4[Mn(CN)6]potassium hexacyanidomanganate(II)

Lecture 22-10

Example 1:Find O.N. of Co in : [Co(NH3)5Cl]SO4 pentaamminechloridocobalt(?) sulfate

[Co(NH3)5Cl]2+ ammine is neutral, chloride is -1

O.N. -1 = +2 (sum of O.N.s = overall charge)

O.N. = +3

Assigning oxidation numbers

Example 2:Find O.N. of Mn in :K4[Mn(CN)6] potassium hexacyanidomanganate(?)

[Mn(CN)6]4- (CN) is -1 overall

O.N. + 6x(-1) = -4 (sum of O.N.s = overall charge)

ON = +2

Lecture 22-11

Isomerism in ComplexesComplexes can have several types of isomers:

Structural Isomers: different atom connectivities

1. Coordination sphere isomerism2. Linkage isomerism

Stereoisomers: same atom connectivities but different arrangement of atoms in space

3. Geometric isomerism4. Optical isomerism

Lecture 22-12

Coordination Isomers Ligands and counter-ions exchange place:Example:

[Pt(NH3)4Cl2](NO2)2 tetraamminedichloridoplatinum(IV) nitrite

[Pt(NH3)4(NO2)2]Cl2 tetraamminedinitritoplatinum(IV) chloride

Two sets of ligands are reversed:[Cr(NH3)6][Co(CN)6] NH3 is a ligand for Cr3+

[Co(NH3)6][Cr(CN)6] NH3 is a ligand for Co3+

ligands counterions

Lecture 22-13

Linkage isomers Occur when a ligand has two alternative donor atoms.

NCSthiocyanate ion

H3N Co

NH3

NH3

NNH3

H3NC S

2+

H3N Co

NH3

NH3

SNH3

H3NC N

2+

and

cyanate ion NCOcyanato NCO:→ isocyanato OCN:→

thiocyanato NCS:→ isothiocyanato SCN:→

Pentaammineisothiocyanatocobalt(III) pentaamminethiocyanatocobalt (III)

Lecture 22-14

Square planar complex. Four coordinate: cis- and trans-[Pt(NH3)2Cl2]

Stereoisomers: Geometric Isomers

No

anti-tumour

effect

cisplatin –

highly effective

anti-tumour agent

Figure from S

ilberberg, “Chem

istry”, McG

raw H

ill, 2006.

Lecture 22-15

Stereoisomers: Geometric Isomers

2 Cl next to each other

Octahedral complex. Six coordinate: cis- and trans- [Co(NH3)4Cl2]+

violet

green2 Cl axial to each other

Lecture 22-16

[NiClBrFI]2-

Stereoisomers: Optical Isomers

When a molecule is non-superimposable with its mirror image. Example: four different substituents about tetrahedral centre. Same physical properties, except direction in which they rotate the

plane of polarized light.

Lecture 22-17

cis-[Co(NH3)4Cl2]+ cis-[Co(en)2Cl2]+

ClCo

NH3

NH3H3N

Cl

NH3

ClCo

NH2

NH2H2N

Cl

NH2

+ +Has no optical

isomers

Has optical

isomers

Stereoisomers: Optical isomers Metal atoms with tetrahedral or octahedral geometries (but not

square planar) may be chiral due to having different ligands. For the octahedral case, several cases are possible, e.g.

1. Complex with four ligands of two types.

Lecture 22-18

[M(en)3]n+ complexes have optical isomers:

Notsuperimposable

H2N CoNH2

NH2H2N

NH2

NH2

NH2Co

NH2

NH2NH2

H2N

H2N

3+ 3+

Mirrorplane

Stereoisomers: Optical isomers

2. Having three bidentate ligands of only one type - gives a propeller-type structure.

www.pt-boat.com

Lecture 22-19

Octahedral complex - stereoisomerism

rotation of I by 180° gives III ≠ II

Mirror

image

Cis-

Dichlorido

Bis(ethylendiamine)cobalt(III) ion

Figure from S

ilberberg, “Chem

istry”, McG

raw H

ill, 2006.

Lecture 22-20

Octahedral complex - stereoisomerism

rotation of I by 270° gives III = II

Mirror

image

Trans-

Dichlorido

Bis(ethylendiamine)cobalt(III) ion

Figure from S

ilberberg, “Chem

istry”, McG

raw H

ill, 2006.

Lecture 22-21

Biologically Important Complexes Many biomolecules contain metal ions that act as Lewis acids.

Give some examples of naturally occurring complexes.

Heme

Chlorophyll

Vitamin B12

Enzyme Carbonic anhydrase

Lecture 22-22

Heme

Heme is a square planar complex of Fe2+ and the tetradentate ring ligand porphyrin (bonds to 4 donor N atoms). Present in hemoglobin, which carries oxygen in blood, and myoglobin, which stores oxygen in muscle.

Porphyrin ring

O2 bound to Fe2+

Myoglobin protein

Blackman Figure 13.37

Lecture 22-23

Chlorophyll

Chlorophyll is a photosynthetic pigment, that gives leaves the characteristic green colour.It is a complex of Mg2+ and a porphyrin ring system (four N atoms are the chelae).

Figure from S

ilberberg, “Chem

istry”, McG

raw H

ill, 2006.

Lecture 22-24

Dorothy Crowfoot HodgkinThe Nobel Prize in Chemistry 1964

Nobelprize.org

Vitamin B12

Image download from Wikipedia

Lecture 22-25

CO2(g) + 2H2O(l) H3O+(aq) + HCO3

- (aq)

Carbonic anhydraseTetrahedral complex of Zn2+.

Catalyses reaction between water and carbon dioxide during respiration. Coordinated to 3 N, fourth site left free to interact with molecule whose reaction is being catalysed (here with water).

By withdrawing electron density, makes water acidic to lose proton and OH- attacks partial positive C of CO2 much more vigorously. Cd2+ is toxic because it competes with zinc for this spot.

Figure downloaded from Wikipedia

Lecture 22-26

Summary

Concepts: Complex formation Stability constant and stepwise stability constant Acidity of some metal ions in solution Coordination compounds and geometry Nomenclature of coordination compounds Isomerism in Complexes

Calculations Complex Formation Equilibria in solution: complex formation + solubility

Lecture 22-27

Question Does the square planar complex ion [Pt(NH3)(N3)BrCl]- have optical

isomers?

BrPt

N=N=N

NH3

Cl

Br

Pt

NH3

ClN=N=N

This complex has no optical isomers because it can be superimposed

on its mirror image.

Lecture 22-28

Metal complex formation can influence the solubility of a compound.

e.g. AgCl(s) + 2 NH3 [Ag(NH3)2]+ + Cl-

This occurs in 2 stages:

AgCl(s) Ag+ + Cl- (1)Ag+ + 2 NH3 [Ag(NH3)2]+ (2)

Complex formation removes the free Ag+ from solution and so drives the dissolution of AgCl forward.

Complex Formation and solubility

Lecture 22-29

AgBr(s) Ag+(aq) + Br-

(aq)

Koverall = Ksp x Kstab = = 5.0·10-13 x 1.7·107 = 8.5·10-6

1.0 M NH3

Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+

(aq)

AgBr(s) + 2NH3(aq) [AgNH3]+(aq) + Br-

(aq)

(1)

(2)

(1)+(2)

Solubility of AgBr in Ammonia

[Ag(NH3)2+][Br-]

[NH3]

Initial Conc.ChangeEquilibrium Conc.

1.0 M-2x

1.0 - 2x

0+xx

0+xx

Substitute: Koverall = x2/(1.0-2x)2 = 8.5·10-6 x = 2.9·10-3 M

Solubility of AgBr in NH3 is 2.9·10-3 M (c.f. in thiosulfate 0.45 M)

Kstab(Ag(NH3)2+)= 1.7·107)