1308 Chapter 24 MGC Tro Lecture Notes WEB

7/22/2019 1308 Chapter 24 MGC Tro Lecture Notes WEB

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Chapter 24

Transition

Metals andCoordination

Compounds

2007, Prentice Hall

Chemistry: A Molecular Approach, 1st Ed.

Nivaldo Tro

Roy Kennedy

Massachusetts Bay Community College

Wellesley Hills, MA


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Tro, Chemistry: A Molecular Approach 2

Coordination Chemistry

Gemstones The colors of rubies and emeralds are both

due to the presence of Cr3+ ions the

difference lies in the crystal hosting the ion

Some Al3+

ions in Al2O3are replaced

by Cr3+

Some Al3+

ions in

Be3Al2(SiO3)6are replaced

by Cr3+


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Complex Ion Color

The observed color is the complimentary color

of the one that is absorbed


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Tro Chemistry: A Molecular Approach

Technetium is a rare element found on earth with isotopes

that can be used in medical testing. Which of the following

is the correct ground state electron configuration of Tc4+?

A. Tc4+: [Kr]5s24d1

B. Tc4+: [Kr]5s04d3

C. Tc4+: [Kr]5s14d2

D. Tc4+: [Ar]4s03d3

E. Tc4+: [Kr]5s24d5

i -Clicker 24-1:


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Properties and Electron

Configuration of Transition Metals The properties of the transition metals are similar to

each other

and very different to the properties of the main group metalshigh melting points, high densities, moderate to very hard,and very good electrical conductors

In general, the transition metals have two valenceelectronswe are filling the dorbitals in the shell

below the valenceGroup 1B and some others have 1 valence electron due to

promotion of an electron into the dsublevel to fill it

form ions by losing the ns electrons first, then the (n1)d


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Coordination Compounds

When a complex ion combines with counter-ions tomake a neutral compound it is called a coordinationcompound

The primary valenceis the oxidation number of themetal

Thesecondary valenceis the number of ligandsbonded to the metal

coordination number

Coordination number range from 2 to 12, with the mostcommon bein

g 6 and 4

CoCl36H2O = [Co(H2O)6]Cl3


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Ligands

3+

What is the charge on the centr al ion?


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Complex Ion Formation Complex ion formation is a type of Lewis acid-

base reaction

A bond that forms when the pair of electrons isdonated by one atom is called a coordinate

covalent bond


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Ligands with Extra Teeth

Some ligands can form more than onecoordinate covalent bond with the metal atomLone pairs on different atoms that are separate

enough so that both can reach the metal

Chelate is a complex ion containing amultidentate ligand

Ligand is called the chelating agent


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EDTA

a Polydentate Ligand


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Complex Ions with

Polydentate Ligands


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Geometries in Complex Ions


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Naming Coordination Compounds

1) Determine the name of the noncomplex ion2) Determine the ligand names and list them in

alphabetical order

3) Determine the name of the metal cation4) Name the complex ion by:

1) Name each ligand alphabetically, adding a prefix in front ofeach ligand to indicate the number found in the complex ion

2) Follow with the name of the metal cation

5) Write the name of the cation followed by the name ofthe anion


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Common Ligands


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Common Metals found in

Anionic Complex Ions


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What is the correct name of the coordination compound[Pt(NH3)5Cl]Br3?

A.pentaamminechloroplatinum(IV) tribromide

B. triamminechloroplatinum(IV) bromideC. pentaamminebromoplatinum(IV) chloride

D.pentaamminechloroplatinum(IV) bromide

E. Chloropentaammineplatinum(IV) bromide

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What is the correct formula for sodiumtetrachloronickelate(I I )

A.Na2[NiCl4]

B. Na2[Ni4Cl4]

C. Na2[NiCl4]2+

D.Na2[NiCl3]

E. Na[Ni4Cl]2+

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Isomers

Structural isomersare molecules that have thesame number and type of atoms, but they are

attached in a different order Stereoisomersare molecules that have the same

number and type of atoms, and that are attached

in the same order, but the atoms or groups ofatoms point in a different spatial direction


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Linkage Isomers


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Geometric Isomers geometric isomers are stereoisomers that differ

in the spatial orientation of ligands

cis-trans isomerism in octahedral complexes MA4

B2

fac-mer isomerism in octahedral complexes MA3

B3

cis-trans isomerism in square-planar complexes MA2

B2


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Ex. 24.5 Draw the structures and label the

type for all isomers of [Co(en)2Cl2]

+

the ethylenediamine ligand (en = H2NCH2CH2NH2) is

bidentate

each Cl ligand is monodentateoctahedral

MA4B2


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Optical Isomers

optical isomers arestereoisomers that are

nonsuperimposable mirrorimages of each other

[Co(en)3]3+


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Ex 24.7 Determine if the cis-trans isomers of

[Co(en)2Cl2]

+

are optically active

draw the mirrorimage of the

given isomerand check to

see if they are

superimposable

trans isomer identical to its mirror image

no optical isomerismcis isomer mirror image is nonsuperimposable

optical isomers


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Bonding in Coordination Compounds

Electrostatic Model

Overly Simplistic but USEFUL! Works well for Main Group metal complexes.

Assumes a purely ionic attraction between thecentral metal ion and charged or partially

charged polar ligands.

It predicts ligand-metal bonds will increase instrength with increasing charge density on thecentral metal atom. Al(H2O)6

3+ > Mg(H2O)62+


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Electrostatic Model

It predicts ligand-metal bonds will increase instrength with increasing ligand polarity.

Al(H2O)63+ > Al(NH3)6

3+

It Fails to adequately explain 1) Color;

2) Stability; 3)Paramagnatism; 4) Square

planar geometries, in transition metal

complexes.


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i -Clicker 24-4:

A.Al(H2O)63+

B. Al(NH3)63+

C. Mg(H2O)62+

Which forms the strongest l igand-metal ion bond?


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Valence Bond Theory

Bonding take place when the filled atomic

orbital on the ligand overlaps an empty atomicorbital on the metal ion

Explain geometries well, but doesnt explain

color or magnetic properties


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Valence Bond Theory

A covalent bond forms when the orbital of

one atom overlaps with the orbital of

another atom

In transition metal complexes, covalent

bonds are formed through the overlap of a

filled ligand orbital with an empty transition

metal orbital


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Valence Bond Theory

Hybridization determines the geometry of

the complex

If the geometry is know the hybrid orbitals

used in bonding are known


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Crystal Field Theory (CFT) More sophisticated model than the electrostatic model. More accurate than the Valence Bond Theory.

Still simple enough for general utility. Assumes that ligands retain their electrons and centralions retain their electrons.

Assumes that the d-electrons in the metal atoms areinfluenced (repelled) by the electron pairs in theligands.


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Crystal Field Theory (CFT) Bonds form due to the attraction of the electrons on the

ligand for the charge on the metal cation

Electrons on the ligands repel electrons in theunhybridized dorbitals of the metal ion

The result is the energies of orbitals the dsublevel aresplit

The difference in energy depends the complex andkinds of ligandsCrystal field splitting energy

Strong field splitting and weak field splitting


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Splitting ofd-Orbital Energies due to

Ligands in a Octahedral Complex


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Complex Ion Color and

Crystal Field Strength The colors of complex ions are due to electronic

transitions between the split d-sublevel orbitals

The wavelength of maximum absorbance can beused to determine the size of the energy gap

between the split d-sublevel orbitals

Ephoton = hn = h(c/l) = D


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Ligand and

Crystal Field Strength The strength of the crystal field depends in large

part on the ligands

Strong field ligands include: CN> NO2> en >NH3

Weak field ligands include H2O > OH> F> Cl>

Br> I

Crystal field strength increases as the charge onthe metal cation increases


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Magnetic Properties and

Crystal Field Strength The electron configuration of the metal ion with split d

orbitals depends on the strength of the crystal field

The 4th and 5th electrons will go into the higher energydx2-y2 and dz2 if the field is weak and the energy gap issmallleading to unpaired electrons and a

paramagnetic complex

The 4th thru 6th electrons will pair the electrons in thedxy, dyzand dxzif the field is strong and the energy gapis largeleading to paired electrons and a diamagneticcomplex


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Strong and Weak Field Splitting


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Low Spin & High Spin Complexes

paramagnetic

high-spin complex

diamagnetic

low-spin complex

Only electron configurations d4, d5, d6, ord7 can have low or high spin

T h d l G d


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Tetrahedral Geometry and

Crystal Field Splitting

because the ligand approach interacts morestrongly with the planar orbitals in the

tetrahedral geometry, their energies are raised

most high-spin complexes

S Pl G d


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Square Planar Geometry and

Crystal Field Splitting

d8 metals the most complex splitting pattern

most are low-spin complexes


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Complex Ion Color

The observed color is the complimentary colorof the one that is absorbed


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The Visible Spectrum


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The Visible Spectrum


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Colours of Transition Metals

What happens when visible light is

absorbed by an atom, ion or molecule?

The energy of light is used to raise the

electron(s) to a higher energy level from a

lower energy level

From a lower energy orbital to a higher

energy orbital


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Colours of Transition Metals

The light energy absorbed corresponds to

the energy difference between the two

orbitals

Because the energy levels are quantized,

light is absorbed only if its energy exactly

matched the energy difference between

the two orbitals


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Electron Jumping

For transition metal complexes, the

splitting between the d orbital energies in

the crystal field corresponds to the

energies of visible light

Absorption of light raises an electron from

a lower energy d orbital to a higher energy

d orbital in the crystal field


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Crystal Field Theory

__ __ __ __

__ __ __ __ __ __

White

Light

Red LightAbsorbed

Green-BlueObserved


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Colour of Transition Metals

Transition metal complexes absorb

energies of visible light corresponding to

the crystal field splitting energies

There we see colour

The absorbed energy is used to promote

and electron from a lower energy orbital to

a higher energy one

C l f T iti M t l


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Colour of Transition Metal

Complexes

Compounds that have no d electrons or

have 10 d electrons can not absorb visible

light because no electrons can be moved

from lower energy orbitals to higherenergy ones

These complexes are colourless!


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Titanium Dioxide

TiO2 is the white pigment used in paper

and paint

Why is it white?

Ti4+ has no d-electrons


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More Peculiarities

Different complexes can exhibit different

colours even when the central transition

metal is in the same oxidation state

How does this happen?



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Complexes

The colour of a transition metal complex

depends on the magnitude of

The magnitude of depends on the nature of

the ligand(s) bonded to the metal The larger, the higher the energy of

absorbed light

The smaller, the lower the energy ofabsorbed light



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Complexes

Some ligands induce a larger splitting of

the d orbitals than others

Thus the colour of transition metals depends

on the nature of the ligands


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Spectrochemical Series

Ligands can be arranged according to their

ability to split the d orbitals

That is according to This is referred to as the spectrochemical

series

Electronic Configurations of


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Electronic Configurations of

Transition Metal Complexes

Electrons tend to fill orbitals according to

the following tendencies

Electrons spread out to minimize electronic

repulsions Electrons tend to occupy the lowest energy

levels first

These tendencies are NOT alwaysfollowed by transition metal complexes

Electronic Configuration of


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Electronic Configuration of

Transition Metal Complexes

For transition metal complexes, the orderthat electrons fill orbitals depends uponboth and the electron pairing energy

If > P, is large strong ligand fieldelectrons pair up in the lower energyorbitals

If < P is small, weak ligand fieldelectrons spread out across the d orbitalsbefore they pair up


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Orbital Diagrams

The situation is more complex for d4d7

complexes


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Orbital Levels in Tetrahedral


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Orbital Levels in Tetrahedral

Complexes

Could you predict the higher and lower

energy d orbitals?

Remember the splitting is due to the repulsive

forces between electrons Which orbitals are directed along the

tetrahedral axis? These will be higher energy


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Square Planar Complexes


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Crystal Field Stabilization Energy

A measure of the stabilization of the

complex

Free MetalIon


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Problem

1. Calculate the CFSE for both high spin

and low spin octahedral complexes of

Co(gly)63-. Which is preferred?

= 21, 476 cm-1, P = 23,625 cm-1,


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Problem 2

What is t if the compound absorbs at

690 nm?

Solution:

= 690 nm (1m/109 nm) = 6.90 x 107 m

= hc/ = (6.626 x 1034 Js)(2.998 x 108 m/s)

6.90 x 107 m

= 2.88 X 10-19 J

Applications of


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Applications of


extraction of metals from oressilver and gold as cyanide complexes

nickel as Ni(CO)4(g)

use of chelating agents in heavy metal poisoningEDTA for Pb poisoning

chemical analysis

qualitative analysis for metal ionsblue = CoSCN+

red = FeSCN2+


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Applications of

Coordination Compounds commercial coloring agentsprussian blue = mixture of hexacyanoFe(II) and Fe(III)

inks, blueprinting, cosmetics, paints

biomoleculesporphyrin ring

cytochrome C

hemoglobin

chlorphyll

chlorophyll


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Applications of

Coordination Compounds carbonic anhydrasecatalyzes the reaction between water and CO2

contains tetrahedrally complexed Zn2+


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Applications of

Coordination Compounds Drugs and Therapeutic Agentscisplatin

anticancer drug

1308 Chapter 24 MGC Tro Lecture Notes WEB

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