24 - 1 transition metals and complexes transition metals complexes and coordination compounds...

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Transition Metals and ComplexesTransition Metals and Complexes

Transition MetalsTransition Metals

Complexes and Coordination CompoundsComplexes and Coordination Compounds

Stereoisomerism of ComplexesStereoisomerism of Complexes

Polydentate Ligands and Chelate Polydentate Ligands and Chelate ComplexesComplexes

Constitutional Isomerism in ComplexesConstitutional Isomerism in Complexes

Nomenclature of ComplexesNomenclature of Complexes

dd Orbitals Orbitals

Bonding in ComplexesBonding in Complexes

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Transition metalsTransition metals

• Not as reactive as Group IA (1), IIA (2) metals and aluminum.

• d-block elements

• Except for palladium, all have either one or two s electrons in the outer shell.

• They only differ in the number of d electrons in n-1 energy level.

• Most have high melting and boiling points, high density, and are hard and strong.

• Multiple oxidation states are common.

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Changes in properties down Changes in properties down groupsgroups

In general, as you move down a group:In general, as you move down a group:• The outer electron configurations are

the same.• Reactivity decreases.

Comparing the three series.Comparing the three series.• The second and third series are usually

more like each other than like the elements of the first transition series.

• Example, zirconium and hafnium always occur together in nature but titanium does not.

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Changes in propertiesChanges in propertiesacross periodsacross periods

Although the first period elements differ from the other two, properties vary in a similar manner as you move across a period.

Lets look at several trends.Lets look at several trends.•Atomic radii•Standard enthalpy of atomization•Melting points•Density•Oxidation numbers.

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Atomic radiiAtomic radii

120

140

160

180 4th period5th period6th period

Rad

ius,

pm

Group number

Radii go through a minimumjust after the center of eachseries. Periods 5 and 6 arenear identical.

Radii go through a minimumjust after the center of eachseries. Periods 5 and 6 arenear identical.

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Standard enthalpy of atomizationStandard enthalpy of atomization

HH This is the energy required to convert an element to individual gaseous atoms.

M(s) M(g)

at 25 oC and one atmosphere.

A maximum is observed near the middle of each period. The same trend is found for melting point, boiling point and density.

ooaa

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Standard enthalpy of atomizationStandard enthalpy of atomization

0

300

600

900

IA IIA IIIB IVB VB

VIBVIIB

VIIIB VIIIB VIIIB IB IIB IIIA IVA

Period 4Period 5Period 6

Hoa,

kJ/m

ol

Group number

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Melting point, Melting point, ooCC

0

1000

2000

3000

4000

IA IIA IIIB

IVB VB VI

BVI

IBVI

IIBVI

IIBVI

IIB IB IIB IIIA

IVA VA


Melt

ing

poin

t, o

C

Group number

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DensityDensity

0

5

10

15

20

25

IA IIA IIIB IVB VB

VIBVIIB

VIIIB VIIIB VIIIB IB IIB IIIA IVA VA


Densi

ty,

g/c

m3

Group number

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Transition metal trendsTransition metal trends

Reason for trends.Reason for trends.

• All show that attractions between atoms are strong.

• Strongest attractions are when the orbitals are half filled.

• Overlap of orbitals that contain one electron results in a covalent bond between atoms.

• Since metals at either end of a period have the fewest unpaired electrons, bonding is not as strong.

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Oxidation states.Oxidation states.

• The number of oxidation states also reaches a maximum near the center of each series.

• They can vary by only one unit whereas nonmetals typically vary by two.

• For metals on the left side, both the nnss and ((nn-1)-1)dd electrons can be involved in reactions.

• Because elements near the middle of each transition series have many oxidation states, much of the chemistry of these elements involves redox reactions.

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Oxidation statesOxidation states

Hg+2+1

Hg+2+1

Cd+2

Cd+2

Zn+2

Zn+2

Au+3+1

Au+3+1

Ag+1

Ag+1

Cu+2+1

Cu+2+1

Hf+4

Hf+4

Zr+4

Zr+4

Ti+4+3+2

Ti+4+3+2

Lu+3

Lu+3

Y+3

Y+3

Sc+3

Sc+3

Pt+4+2

Pt+4+2

Pd+4+2

Pd+4+2

Ni+2

Ni+2

Ir+4+3

Ir+4+3

Rh+4+3+2

Rh+4+3+2

Co+3+2

Co+3+2

Os+8+6+4

Os+8+6+4

Ru+8 +6

+4+3

Ru+8 +6

+4+3

Fe+3+2

Fe+3+2

Re+7+6+4

Re+7+6+4

Tc+7+6+4

Tc+7+6+4

Mn+7 +6+4 +3

+2

Mn+7 +6+4 +3

+2

W+6+4

W+6+4

Mo+6+4+3

Mo+6+4+3

Cr+6+3+2

Cr+6+3+2

Ta+5+4

Ta+5+4

Nb+5+4+2

Nb+5+4+2

V+5 +4

+3+2

V+5 +4

+3+2

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Magnetic propertiesMagnetic properties

Many of the transition metals and their compounds have magnetic properties.

• Iron and to a lesser extent, cobalt and nickel are ferromagnetic.ferromagnetic. –Can be permanently magnetized.

• For most transition metals, at least one oxidation state is paramagnetic.paramagnetic. –Attracted to a magnetic field.

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Magnetic propertiesMagnetic properties

• Paramagnetic species have unpaired electrons.

• The magnitude of the effect depends on the number of unpaired electrons.

ExampleExample• Paramagnetic Cr [Ar]

• Diamagnetic Pd [Kr]

3 d 4s

3 d 4s

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Formation of complexesFormation of complexesand coordination compoundsand coordination compounds

Formation of complexes is a characteristic property of transition metals.

•The most easily observed property of transition metal complexes is color.

•The color is dependent on the identity of the central atom, its oxidation state and the type of ligand.

Coordination compounds.Coordination compounds.Those that include one or more complexes.

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ComplexesComplexes

• Alfred Werner studied the formation of coordination compounds of platinum.

• He found that one mole of platinum(IV) chloride would combine with 2, 3, 4, 5 or 6 moles of ammonia.

PtCl4.2NH3

PtCl4.3NH3

PtCl4.4NH3

PtCl4.5NH3

PtCl4.6NH3

This format was used to show the proportions of PtCl4 and NH3 that had combined.

This format was used to show the proportions of PtCl4 and NH3 that had combined.

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ComplexesComplexes

• Werner observed that the reactivities of the five coordination compounds differed.

• For example, addition of silver nitrate resulted in differing amounts of AgCl being formed.

PtCl4.2NH3 (aq) + excess Ag+ no reaction

PtCl4.3NH3 (aq) + excess Ag+ 1 AgCl

PtCl4.6NH3 (aq) + excess Ag+ 4 AgCl

This indicated that some of the chlorides must be bound to Pt.

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ComplexesComplexes

Werner defined the coordination number as the number of atoms or groups that are firmly bound to the central atom.

Empirical Number Number Number of Formula of ions of Cl-

nonionic Cl

PtCl4.6NH3 5 4 0

PtCl4.5NH3 4 3 1

PtCl4.4NH3 3 2 2

PtCl4.3NH3 2 1 3

PtCl4.2NH3 0 0 4

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ComplexesComplexes

To indicate bound verses free ions, an alternate format for the formula was developed.

[Pt(NH3)6]Cl4

[PtCl(NH3)5]Cl3

[PtCl2(NH3)4]Cl2

[PtCl3(NH3)3]Cl

[PtCl4(NH3)2]

Coordinated atoms and groups are placed inside square braces. Free ions are placed on the outside.

The symbol for the central atom is placed first. Ionic then neutral ligands are then listed in that order.

Coordinated atoms and groups are placed inside square braces. Free ions are placed on the outside.

The symbol for the central atom is placed first. Ionic then neutral ligands are then listed in that order.

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Stereoisomerism in complexesStereoisomerism in complexes

• Werner also evaluated the arrangement of coordinated groups around the central atom.

• He found that for one of the platinum complex, [PtCl2(NH3)2], two different isomers were observed.

• This could only be explained if the geometry was square planer.

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Two geometric isomers were observed for [PtCl2(NH3)2].

cis-[PtCl2(NH3)2] trans-[PtCl2(NH3)2]

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Some complexes exist in enantiomeric forms.

• To determine the number of forms that can exist, do the following.

• Draw or use a model kit to develop different geometric forms for your complex.

• Make a mirror image for each geometric isomer.

• If the model and its mirror image can be superimposed, they represent the same compound

• If they can’t be superimposed, they represent different compounds.

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Example 1Example 1

The left and right forms can be superimposed.They represent the same compound.

Original model Mirror image

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ExampleExample

Original model Mirror image

The left and right forms can’t be superimposed.They represent the different compound.

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Polydentate ligands Polydentate ligands and chelate complexesand chelate complexes

Ligands can be classified by dentate number - number of bonds/ligand

MonodentateMonodentate1 bond/ligand - ammonia

BidentateBidentate2 bonds/ligand - ethylene diamine

MultidentateMultidentatevariable number based on needEDTA

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Monodentate ligandsMonodentate ligands

Possess only one accessible donor group.

H2O is a good example since all metal ions exist as aqua complexes in water.

Although two e- pairs areavailable, only one isaccessible.

The other will always pointthe wrong way

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Some aqua complexesSome aqua complexes

Ag(H2O)2+

Cu(H2O)42+

Fe(H2O)63+

The charge and coordination number are NOT related.

Fe(H2O)62+ also exists.

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Common monodentate ligandsCommon monodentate ligands

anionic neutral

X- OH- H2OSCN- RCOO- NH3

CN- S2- RNH2

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Bidentate ligandsBidentate ligands

• Form two bonds to central species.• A good example is ethylene diamine.

•NH2CH2CH2NH2 - (en)•The amine groups are far enough

apart to permit both to interact.

Zn2+ + 2 en ZnCN

CN

C N

C N

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Other common bidentate ligands.Other common bidentate ligands.

8 - hydroxyquinoline8 - hydroxyquinoline

O-

NZn2+ + 2

O

NZn

2

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Other common bidentate ligands.Other common bidentate ligands.

Dimethylglyoxime - dmgDimethylglyoxime - dmg

C N

O

H

H3C

CH3C

N

O

Ni

N

O

C

CN

OH

CH3

CH3

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Ni (dmg)2

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1,10 phenanthroline1,10 phenanthroline

N

N

Fe(II)

3

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Iron(II) 1,10-phenanthroline complexIron(II) 1,10-phenanthroline complex

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EDTAEDTA

Ethylenediamine tetraacetic acidEthylenediamine tetraacetic acid

• A commonly used ligand.

• Forms 1:1 complexes with most metals except the Group IA (1).

• Forms stable, water soluble complexes

• High formation constants.

• A primary standard material.

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EDTAEDTA

EDTA is typically used as the disodium salt to increase solubility.

H2Y2-

HOOC

N

Na+ -OOC

C C N

COO- Na+

COOH

H|

H|

|H

|H

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EDTAEDTA

The molecule contains 6 donor groups.

Regardless of the coordination number of the central species, the molecule will adapt to the number needed.

Mg2+ + H2Y2- MgY2- + 2H+

Fe3+ + H2Y2- FeY- + 2H+

HOOC

N

Na+ -OOC

C C N

COO- Na+

COOH

H|

H|

|H

|H

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EDTAEDTA

FeFe3+ 3+ - EDTA complex- EDTA complex

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EDTAEDTA

Formation constants for some metal - EDTA Formation constants for some metal - EDTA complexes.complexes.

Ion logK Ion logK Ion logK

Fe3+ 25.1 Pb2+ 18.0 La3+ 15.4Th4+ 23.2 Cd2+ 16.5 Mn2+ 14.0Cr3+ 23.0 Zn2+ 16.5 Ca2+ 10.7Bi3+ 22.8 Co2+ 16.3 Mg2+ 8.7Cu2+ 18.8 Al3+ 16.1 Sr2+ 8.6Ni2+ 18.6 Ce3+ 16.0 Ba2+ 7.8

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Constitutional isomerismConstitutional isomerism

StereoisomersStereoisomers• Differ only in the spatial arrangement of

the ligands.• Geometric and mirror-image isomers.

Constitutional isomersConstitutional isomers• LinkageLinkage - differ in how the ligands are

attached to the central atom• CompositionalCompositional - different quantities of

solvent or ligand are present in the complex.

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Linkage isomersLinkage isomers

Some ligands can attach to the central metal ion by either of two different atoms

yellow red

2+

H3NCo

H3N NH3

NH3

H3N

N

O O

2+

H3NCo

H3N NH3

NH3

H3N

O

NO

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Compositional isomersCompositional isomers

Different quantities of the solvent can be present in the complex.

Example.Example. CrCl3.(H2O)6

Complex Color of aqueous solution

[Cr(H2O)6]Cl3 violet

[CrCl(H2O)5]Cl2 pale blue-green

[CrCl2(H2O)4]Cl dark green

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Nomenclature of complexesNomenclature of complexes

• In the name of a complex, the ligands are named first in alphabetical order.

• Prefixes are used to indicate the presence of multiple ligands of the same type

• The metal is then given followed by its oxidation state.

• For anionic complexes, the metal name is modified with an -ate-ate ending. Latin names are used in some cases.

• There are no spaces between the various parts of the name.

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Ligand NameF- FluoroCl- ChloroBr- BromoI- IodoOH- HydroxoCN- CyanoH2O Aqua

NH3 Ammine

CO Carbonyl

Ligand NameF- FluoroCl- ChloroBr- BromoI- IodoOH- HydroxoCN- CyanoH2O Aqua

NH3 Ammine

CO Carbonyl

Metal Anionic nameCobalt CobaltateCopper CuprateGold AurateIron FerrateLead PlumbateMercury MercurateNickel NickelateTin StannateZinc Zincate

Metal Anionic nameCobalt CobaltateCopper CuprateGold AurateIron FerrateLead PlumbateMercury MercurateNickel NickelateTin StannateZinc Zincate

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ExamplesExamples

[CoCl(NH3)5]Cl2pentaamminechlorocobalt(III) chloride

K2[Cd(CN)4]Potassium tetracyanocadmate(II)

cis-diamminedichloroplatinum(II)

H3NPt

Cl Cl

NH3

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dd orbitals orbitals

Bonding in transition-metal complexes involves the d orbitals.

The are 5 d orbitals xy - lies in the xy plane.xz - lies in the xz plane.yz - lies in the yz plane.

x2-y2 - aligned with the x and y axes.z2 - aligned with the z axis.

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Representative Representative dd orbitals orbitals

z2 xy

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A complete set of A complete set of dd orbitals orbitals

Red orbitalslie along the axes.

Blue orbitals liebetween axes.

Note how sphericala complete setof d orbitals is.

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Bonding in complexesBonding in complexes

Crystal field theory.Crystal field theory.• Explains both the colors and magnetic

properties of complexes.• It was originally applied to metal ions in

crystalline solids, which accounts for the name.

According to the theory -According to the theory -Electrostatic attractions between the positive central metal ion and the ligands bonds them together, lowering the energy of the whole system.

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Octahedral complexesOctahedral complexes

• d orbitals are degenerate for isolated gaseous metal ions - have the same energy.

• In complexes, this is not always the case.

• For octahedral, the ligands can be viewed as approaching the central species along the x-, y- and z- axes.

• The ligands will then interact with the d orbitals.

• Since the x2-y2 and z2 orbitals are closer to incoming ligands, they are affected more.

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Ligands willapproach along the x- , y- and z- axes.

This affects the x2-y2 and z2 orbitals most.

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The d orbitals are split into a high and low energy set of orbitals.

The difference in energy is called thecrystal field splittingand is represented by .Since this is for anoctahedral complex,use o

Five degenerated orbitals

o

x2-y2 z2

xy xz yz

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Crystal field splitting can be confirmed by looking at the paramagnetism of a complex.

Electron pairingElectron pairing

• If the split is large, electrons will pair at the lower level before populating the upper level - strong field or low spinstrong field or low spin.

• If the split is small, they will fill all of the orbitals before attempting to pair up- weak field or high spinweak field or high spin.

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Example system with 4 d electrons.

Strong fieldLow spin

Weak fieldHigh spin

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Tetrahedral complexesTetrahedral complexes

With this type of complex, the ligands approach closest to the dxy, dxz, and dzy orbitals.

The splitting is exactly the oppositeof that observedfor octahedral complexes.

Five degenerated orbitals

T

x2-y2 z2

xy xz yz

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Square planar complexesSquare planar complexes

These are a bit more complicated. These are a bit more complicated. • One must imagine that the four ligands

approach the central species along the x- and y- axes.

• The energy of the x2-y2 orbital will be raised the most because it lies along the x and y axes.

• The xy orbital will also also be raised but not as much.

• The z2 orbital can’t be predicted.• Overall, we end up with 4 energy levels.

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Energy levelsEnergy levels

octahedral tetrahedral square planar

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So why are manySo why are manycomplexes colored?complexes colored?

• For most transition metals, we have partially populated d orbitals.

• For a free atom, the energy required to move an electron from one electronic level to another is very large. It corresponds to the vacuum UV region.

• The energy of the crystal field split is much smaller and often corresponds to the energy of visible light.

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UV/Vis absorptionUV/Vis absorption

This ‘splitting’ of the d orbitals results in a d->d transition that is in the UV/Vis

range.

Chromium(III) examplesChromium(III) examples

Ligand max

Cl- 736 H2O 573 NH3 462 CN- 380

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Charge transfer complexesCharge transfer complexesA complex where one species is an electron donor and the other is an electron acceptor.

The resulting complex can be described as a resonance hybrid.

These species tend to show very large absorbtivities (max> 10,000) so many analytical methods are based on forming this type of complex.

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Charge transfer complexesCharge transfer complexes

N

N

3 + Fe2+Fe2+

N

N

3

1,10-phenanthroline ferroin

24 - 1 transition metals and complexes transition metals complexes and coordination compounds...

Documents

transition series

oa slide

kjmol group number slide

o c group number slide

number of oxidation

density density

number of d electrons

period elements