chemical structure: chemical bonding. properties of coordination compounds

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Properties of Coordination Compounds

University of Lincoln presentation

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Coordination Compounds

What is their main characteristic property?

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A clue…A clue…

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Nearly all coordination compounds are COLOURED

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Breathalyzers

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Presumptive tests for drugs

e.g. the Duquenois test for marijuana

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Remember! Coordination compounds are the

compounds of the transition metals

(d block elements)Why are TM compounds coloured?

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We need to look at the electronic configuration of the transition metals, to

answer this question

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Sc Ti V Cr Mn Fe Co Ni Cu Zn

d1 d2 d3 d4 d5 d6 d7 d8 d9 d10

[Ar] 4s23dn

H

BeLi

Na

K

Rb

Cs

Fr

Mg

Ca

Sr

Ba

Ra

Sc

Y

La

Ac

Ti V Cr Mn Fe Co Ni Cu Zn

Zr

Hf Ta W Re Os Ir Pt Au Hg Tl

Nb Mo Tc Ru Rh Pd Ag Cd In Sn

Pb Bi Po At Rn

Xe

Kr

Ar

Ne

Sb Te I

Ga

Al

Ge

Si P S Cl

As Se Br

Ce Pr Nd Pm Sm

Eu Gd Tb Dy Ho Er Tm Yb Lu

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

He

B C N O F

LanthanoidsActinoids

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There are 5 d-orbitalsz

y

x

z

y

x

z

y

x

z

y

x

z

x

y

dyz dxy dxz

dz2 dx

2 y2

Note change of axis

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Energ

y

1s

2s

3s

2p

3p3d

N = 1

N = 2

N = 3

Each orbital will hold 2 electrons

d-orbitals can hold from 1 – 10

electrons

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We get a clue as to how their colour arises, by considering

zinc

Zn = d10

(completely FULL d-orbitals)

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Zinc (d10) compounds are WHITE (not coloured!)

When d-orbitals are FULL there is no colour

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COLOUR must have something to do with

partially filled d-orbitals

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Crystal Field Theory

This theory explains why TM compounds are coloured

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Crystal Field theory says…

““In the ELEMENT, In the ELEMENT,

the d-orbitals are the d-orbitals are DEGENERATE (of DEGENERATE (of the same energy)the same energy)

Each orbital will hold 2 electrons

Energ

y

3d

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……But, in a COORDINATION But, in a COORDINATION COMPOUND,COMPOUND,

NOT all of the orbitals have the NOT all of the orbitals have the same energy”same energy”

For example, in an octahedral For example, in an octahedral coordination compound, the d-coordination compound, the d-

orbitals are split as follows:orbitals are split as follows:Energ

y

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How does this help us to explain COLOUR?

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Consider the Fe2+ ion (d6)

If this ion makes an octahedral complex, its 6 d-electrons will sit in the split d-orbitals, as shown:

Energ

y

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If we shine light on the Fe2+ complex…

An electron could absorb enough energy (=) to move from the bottom orbitals to the top orbitals:

Energ

y Energ

y

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Note: we haven’t changed the number of PAIRED

electrons

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ONE ONE pair of pair of

electronelectronss

ONE ONE pair of pair of

electronelectronss

Energ

y Energ

y

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When an electron is promoted from a low energy

level to a higher energy level, the process is called an

ELECTRONIC TRANSITION

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How do electronic transitions make compounds

COLOURED?

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If the electron is going to jump from the lower level to the higher level, it has to ABSORB energy from visible light

It needs to absorb an amount of energy =

Energ

y

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Electronic Spectrum – Visible light

LOW HIGH

Energy

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Whatever energy is absorbed, the remainder is

TRANSMITTEDIt is the TRANSMITTED light that

gives the compound its colour

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For Example

TRANSMITTED LIGHTTRANSMITTED LIGHT

COLOUR of compound would be a mixture of

these

ABSORBED ABSORBED LIGHTLIGHT

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is largeis large

High energy is High energy is needed to needed to

promote electron:promote electron:

Blue end is Blue end is absorbedabsorbed

RedRed end is end is transmittedtransmitted

is smallis small

Low energy is Low energy is needed to needed to

promote electron:promote electron:

Red end is Red end is absorbedabsorbed

BlueBlue end is end is transmittedtransmitted

Energ

y

Energ

y

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So, why are Zinc compounds white?

Because the orbitals are

completely filled, there is no room

for electronic transitions to take

place

NO COLOUR NO COLOUR (WHITE)(WHITE)

Energ

y

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What happens if is so big, that electrons prefer to pair

up in the lower level, and not jump up to the higher level?

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There comes a point, when is so big, that it is easier for electrons to pair up in the lower level, rather than staying unpaired, by jumping up to the higher level

Energ

y

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Consider the Fe2+ octahedral complex, again

SMALL

VERY LARGE

Energ

y

Energ

y

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How does this affect the COLOUR?

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Extended Electronic Spectrum

ULTRA VIOLETINFRARED

When is very large, the amount of energy required to promote an electron from the lower to the higher level is outside the visible range – hence the compound will appear WHITE

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What other characteristic properties do the TM compounds display?

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Look again at the Fe2+ octahedral complex

The MAGNETIC properties of these two Fe2+ compounds are very

different

Energ

y

Energ

y

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PARAMAGNETIPARAMAGNETICC

DIAMAGNETICDIAMAGNETIC

Energ

y

Energ

y

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This dual magnetic behaviour is another characteristic property of coordination

compounds

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SUMMARY

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What you need to know…

• Two characteristic properties of coordination compounds are:– Colour– Dual magnetic behaviour

•E.g. Some iron(II) compounds are paramagnetic, whilst others are diamagnetic

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Crystal Field Theory

In a coordination compound the d-orbitals are not all the same energy

Colour arises from electronic transitions within the d-orbitals

Dual magnetic behaviour arises due to different values of

There are two reasons for coordination compounds to be white:

• Electronic transitions cannot occur (e.g. if the d-orbitals are full)

is so large that the absorbed energy in an electronic transition is in the UV region and not the visible region of the electronic spectrum

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Acknowledgements

• JISC• HEA• Centre for Educational Research and

Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org