assignment 5 complexes

8/12/2019 Assignment 5 Complexes

1/14


2/14

3

Introduction

We observe that many compounds are coloured in Chemistry. There are two origins of

colour:

- Light of a specific frequency (i.e., colour) is emitted in processes such as fluorescence or

phosphorescence.

- Light of a specific frequency is absorbed. If this light is in the visible region of the

Electromagnetic spectrum, the absorption process is accompanied by an electronic

transition. The chemical compound that absorbs light of one frequency or colourwill

exhibit the colour complementary to the absorbed colour.

Complementary colours are:

Green-red

Yellow- violet

Blue-orange

Figure 1.0 Spectrum of visible light

The spectrum of visible light is presented above (Figure 1). Visible wavelengths cover a

range from approximately 400 to 800 nm. A compound is colored if it absorbs part of the visible

light. When a sample absorbs visible light, the color we observe is the sum of the remaining

colors that are reflected or transmitted by the sample and strikes our eyes. Because we are

dealing with absorptions of the complexes in the uv-vis region and the involvement of the

electronic energy levels, the study of the transition of electrons in the range of 190 to 800 nm is

also known to be electronic spectroscopy. Hence, the properties of electrons must be studied first

in order to explain the appearance of peaks in the uv-vis spectra.


3/14

4

UV-Visible Spectroscopy

According to Christian (2004) spectroscopy is based on the absorption of photons by the

analyte.Absorption of energy will induce valence electron from ground state to excited state

which also can be stated as jumping of electron from low energy orbital to a higher energy

orbital (Moore et al., 2008). Electromagnetic spectrums consist of different regions as shown in

figure 2.0.

Figure 2.0: Regions of electromagnetic spectrum (The Royal Society of Chemistry, n.d).

Absorption of radiation by a molecule in ultraviolet or visible region involves energy

(Moore et al., 2008). The ultraviolet region is from 200 to 380 nm; the visible region ranges from

380 to 780nm (Christian, 2004).For ultraviolet-visible analysis, only small portion of absorbing

molecules are needed (The Royal Society of Chemistry, n.d). The UV-Visible spectrometer

functions as shown in figure 3.0.

Figure 3.0: Ultraviolet/Visible spectrometer.


4/14

5

Beer Lambert law is used in calculating the concentration of compounds based on the

absorbance of the compounds.

A=c

Where,

A : Absorbance of the compound containing solution.

: Molar absorption coefficient (mol-1dm3cm-1)

: the path length of the absorbing solution (cm)

c: the concentration of the absorbing species (moldm-3)

Chemical constituent that is responsible for the absorption is defined as chromophore

(Hamid, 2007).

Ultraviolet visible spectra

Ultraviolet visible spectra can provide us three crucial data for structures as below:

1. Distinct cis and trans isomers. This is because trans isomers have the ability to

absorb at a longer wavelength (Moore et al., 2008)

2. Detect presence of some functional group ( aromatic, carbonyl, etc.) as

absorption in UV-Vis depends on the characteristic wavelengths (Moore et al., 2008).

3. Detects the transition of electrons within d orbitals for transition metal

compounds (Moore et al., 2008).

Quantum Numbers

Electrons in an atom reside in shells characterized by a particular value of n, the Principal

Quantum Number. Within each shell, an electron can occupy an orbital which is further

characterized by an Orbital Quantum Number, l, where lcan take all values in the range:

l= 0, 1, 2, 3, ... , (n-1), traditionally termed s, p, d, f, etc. orbitals.

Each orbital has a characteristic shape reflecting the motion of the electron in that

particular orbital. This motion being characterized by an angular momentum which reflects the

angular velocity of the electron moving in its orbital. Magnetic Quantum Number, ml is a

subset of l, where the allowable values are: ml = l, l-1, l-2, ..... 1, 0, -1, ....... , -(l-2), -(l-1), -l.


5/14

6

There are thus (2l +1) values of mlfor each lvalue, i.e. one s orbital (l= 0), three p orbitals (l =

1), five d orbitals (l = 2), etc. There is a fourth quantum number, ms,that identifies the orientation

of the spin of one electron relative to those of other electrons in the system. A single electron in

free space has a fundamental property associated with it called spin, arising from the spinning of

an asymmetrical charge distribution about its own axis. The value of msis either + or - .

In summary then, each electron in an orbital is characterized by four quantum numbers:

Table 1.0 Quantum Numbers

Number of microstates

The electrons may be filled in orbitals by different arrangements since the orbitals have

different mlvalues and electrons may also occupy singly or get paired. Each different type of

electronic arrangement gives rise to a microstate. Thus each electronic configuration will have a

fixed number of microstates.

The number of microstates possible for any electronic configuration may be calculated from the

formula,

Number of microstates = n! / r! (n - r)!

where n is the twice the number of orbitals, r is the number of electronsand! is the factorial.

For d configuration, the number of microstates is given by;

Number of microstates = 10! / 2! (102)!= 10 x 9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1

2 x1 (8 x 7 x 6 x 5 x 4 x 3 x 2 x1)

= 45 microstates

Thus, a d2configuration will have 45 microstates.

Quantum Numbers

n Principal Quantum Number - largely governs size of orbital and its energy

l Azimuthal/Orbital Quantum Number - largely determines shape of orbital

ml Magnetic Quantum Number

ms Spin Quantum Number - either + or - for single electron


6/14

7

Russell Saunders coupling

The ways in which the angular momenta associated with the orbital and spin motions in

many-electron-atoms can be combined together are many and varied. In spite of this seeming

complexity, the results are frequently readily determined for simple atom systems and are used to

characterize the electronic states of atoms.

The interactions that can occur are of three types;

spin-spin coupling

orbit-orbit coupling

spin-orbit coupling

There are two principal couplingschemes used:

Russell-Saunders (or L - S) coupling

j - j coupling

In the Russell Saunders scheme it is assumed that;

Spin-spin coupling > orbit-orbit coupling > spin-orbit coupling

This is found to give a good approximation for first row transition series where J coupling is

ignored, however for elements with atomic number greater than thirty, spin orbit coupling

becomes more significant and the j-j coupling scheme is used.

Spin-Spin coupling

S - The resultant spins quantum number for a system of electrons. The overall spin S arises from

adding the individual mstogether and is as a result of coupling of spin quantum numbers for the

separate electrons.

Orbit-Orbit coupling

L - The total orbital angular momentum quantum number defines the energy state for a system ofelectrons. These states or term letters are represented as follows:

Total Orbital Momentum

L 0 1 2 3 4 5

State S P D F G H


7/14

8

Spin-Orbit coupling

Coupling occurs between the resultant spin and orbital momenta of an electron which gives rise

to J, the total angular momentum quantum number. Multiplicity will occur when several levels

are close together and is given by the formula (2S+1).

So, the Russell Saunders term symbol that results from these considerations is given by:

Term symbol =(2S+1)

LJ

The Russell Saunders term symbols for the other free ion configurations are given in the

Table 2.0 below.

Table 2.0 Term symbols for the free ion configurations with its ground terms

Thus, the term symbols for d2configuration are 3P, 3F, 1S, 1D and 1G.

Hund's Rules

The Ground Terms are deduced by using Hund's Rules.

The two rules are:

1) The Ground Term which has the maximum multiplicity, S is the most stable state.

2) If there is more than one term with maximum multiplicity, then the Ground Term which have

the largest value of L is the most stable state.

The overall result shown in the Table 2 above is that:

4 configurations (d1, d

4, d

6, d

9) give rise to D ground terms

4 configurations (d2, d

3, d

7, d

8) give rise to F ground terms

d5configuration gives an S ground term

So, by using Hund's Rule, the ground state term for d2configuration is

3F.This is because the


8/14

9

term3F has the highest S and the largest L values.

The Crystal Field Splitting of Russell-Saunders terms

The effect of a crystal field on the different orbitals (s, p, d, etc.) will result in splitting

into subsets of different energies, depending on whether they are in an octahedral or tetrahedral

environment. The magnitude of the d orbital splitting is generally represented as a fraction of

Doct or 10Dq.

The ground term energies for free ions are also affected by the influence of a crystal field

and an analogy is made between orbitals and ground terms that are related due to the angular

parts of their electron distribution. The effect of a crystal field on different orbitals in an

octahedral field environment will cause the d orbitals to split to give t2gand egsubsetsand the

D ground term states into T2gand Eg,(where upper case is used to denote states and lower case

orbitals).

Mulliken Symbols

Mulliken Symbols is used to describe and identify the terms arising for octahedral and

tetrahedral complexes. According to Mulliken Symbols, the symbol A will be representing an

orbital which is a single state, the symbol E is a doublet and T is a triplet state for the orbitals

respectively.

Terms for free metal ions Terms for an octahedral field Terms for a tetrahedral field

S A1g A1

P T1g T1

D Eg+ T2g E + T2

F A2g+ T1g+ T2g A2+ T1+ T2

G A1g+ Eg+ T1g+ T2g A1+ E + T1+ T2

Table 3.0The Term Symbols for Free Metal ions, Octahedral and Tetrahedral Fields.


9/14

10

Selection Rules

There are two selection rules which govern the relative intensities of electronic absorption bands:

i) Spin Selection Rule

Transitions between different spin multiplicities are forbidden. For example, transitions

between4A2and

4T1states are spin-allowed (S = 0), but transitions between

4A2 and

2A2

are spin-forbidden (S 0).

ii) Laporte (Parity) Selection Rule

Transitions between states of the same parity (symmetry with respect to inversion) are

forbidden. For example, transitions between d-orbitals are forbidden (dd) but transitions

between p and d orbitals are allowed (pd).

Use of Orgel diagrams

The observation of three peaks in the electronic spectra of d2 spin complexes requires further

treatment involving electron-electron interactions. Using the Russell-Saunders (LS) coupling

scheme, these free ion configurations give rise to F ground states which in octahedral and

tetrahedral fields are split into terms designated by the symbols A2(g), T2(g) and T1(g). Figure 2

below shows an example of an orgel diagram of a d2in an octahedral field.

Figure 4.0Orgel diagram of a d2 in an octahedral field with its spectra


10/14

11

Report of complexes

Complex Types of metal ion

centre

Colour of complex Number of peaks in

UV-VIS spectrum

A d2 Red 3

B d2 Blue 2

C d2 Green 1

D d2 Colourless None

E d5 Colourless None

All the complexes show a d2 configuration for metal ion centre. We can deduce that

all these complexes are transition metal complexes .Generally, transition metal complexes will

have d2configuration for its metal ion centre. All these complexes were detected in the UV-

Visible spectroscopy as all the complexes reflects its respective colours. The visible region that

lies in the range of 380nm to 780nm is a little portion of electromagnetic spectrum that is

visible to the eye which proves that the light is seen as colour (Christian, 2004).Colour of

compounds that is reflected is the unabsorbed range of region (Moore et al., 2008) where it was

further proved by Christian (2004) that the colour is complementary to the absorbed

colours.If we take a look at the table, complex A, B and C shows its own respective colour.

On the other hand, complex D and E is colourless.

Complexes Colour Number of

peaks in

UV-VIS

spectrum

Explanation

A Red 3

Shows that it falls on the visible range which is

400 to 800 nm

The complex absorbed other colours in the region

and does not absorb the red region.

The unabsorbed red region is transmitted as red


11/14

12

colour of the complex.

The spin multiplicity is a triplet

Spectrum of complex A shows three peaks

because all the transition of the complex is in the

visible region. All the transition is in the energy

level rage of visible region.

The transition of complex A is as below:

3T1g3T2g

3T2g3T1g

3T1g3A2g

B Blue 2 The complex absorbs the yellow region and does

not absorb the blue region.

The unabsorbed region is transmitted as blue

colour of the complex.

The complex is a conjugated compound as it

reveals bright colours as transition of electrons in

conjugated compounds occurs in its pi electrons.

There were only two peaks observed in UV-VIS

Because two of the transition of the complex is in

the visible region.

other transition of the complex is in higher energy

level than the energy range of visible region

Hence, the other peaks are not observed.

The transition of complex B is as below:

3TIg 3T2g

3T2g3T1g

Hence , two peaks will be observed

C Green 1 The complex absorbed other colours in the region

and does not absorb the green region.

The unabsorbed green region is transmitted as

green colour of the complex.

The complex is conjugated compound as it

reveals bright colours as transition of electrons in


12/14

13

conjugated compounds occurs in its pi electrons.

Spectrum of complex C shows one peak.

Because only one transition of the complex is in

the visible region.

Other transition of the complex is in higher

energy level than the energy range of visible

region.

. Hence, the other peaks are not observed.

The transition of complex C is as below

3TIg3T2g

D Colourless None Complex D is sigma bonded complex and is

colourless

Sigma bonded complexes does not absorb light in

the visible region.

Spectrum of complex D shows no peaks because

all the transition of the complex is not in the

visible region.

All the transition of the complex is beyond the

energy level rage of visible region.

Hence, no peaks were observed

E Colourless None Complex E is sigma bonded complex and is

colourless.

Sigma bonded complexes does not absorb light in

the visible region.

Spectrum of complex E shows no peaks because

all the transition of the complex is not in the

visible region.

All the transition of the complex is beyond the

energy level rage of visible region.

Hence, no peaks were observed


13/14

14

REFERENCES

Atkins, P. W., Longford, C. H., & Shriver, D. F. (2010).Inorganic Chemistry, 5th

Edition.Oxford University, Press.

Christian, G.D. (2004). Analytical Chemistry. 6th

ed. United States of America: John Wiley &Son, Inc.Pp457-521

Cooke, J.(2005). Spectroscopy in Inorganic Chemistry (Theory). Retrieved fromwww.chem.ualberta.ca/~inorglab/spectheory.pdf[15 April1024].

Gerken, M.(n.d).Chemistry 3830/3841 Lecture Notes. Retrieved from

http://classes.uleth.ca/200901/chem3840a/3830_3840%20lecture%20notes%20part10_2009_spectrosc.pdf.[17 April 2014]

Hamid, H.(2007).Pharmaceutical Analysis.Ultraviolet and visible Spectrophotometry. Retrievedfrom

http://nsdl.niscair.res.in/bitstream/123456789/772/1/revised+Ultraviolet+and+Visible+

Spectrophotometry.pdf[15 April 2014]

Harris, D., &Bertolucci, M. (1989).Symmetry and Spectroscopy. An Introduction to

Vibrational and Electronic Spectroscopy.Dover Publications, Inc., New York.

Moore, J.W., Stanitski.,&Jurs, P.C. (2008).Chemistry. The Molecular Science.3th ed. Volume 1.

United States of America: Thomson Learning, Inc.Pp 406-407.

Schonherr, T., & Jorgensen, A.C.K.2004. Optical Spectra and Chemical Bonding in TransitionMetal Complexes.Germany: Springer.Pp 145-147

The Royal Society of Chemistry.(n.d).Modern Chemical Techniques.Ultraviloet/visiblespectroscopy. Retrieved from

http://media.rsc.org/Modern%20chemical%20techniques/MCT4%20UV%20and%20v

isible%20spec.pdf.[18 April 2014]

Yeow , Y.G.(n.d). Electronic Spectra. Retrieved from

http://web.usm.my/chem/LECTURER/ktt212/electronic_spectra.pdf [15 April 2014]

Images were obtained from http://www.chem.iitb.ac.in/~rmv/ch102/ic3.pdf
http://classes.uleth.ca/200901/chem3840a/3830_3840%20lecture%20notes%20part10_2009_spectrosc.pdfhttp://classes.uleth.ca/200901/chem3840a/3830_3840%20lecture%20notes%20part10_2009_spectrosc.pdfhttp://classes.uleth.ca/200901/chem3840a/3830_3840%20lecture%20notes%20part10_2009_spectrosc.pdfhttp://nsdl.niscair.res.in/bitstream/123456789/772/1/revised+Ultraviolet+and+Visible+Spectrophotometry.pdfhttp://nsdl.niscair.res.in/bitstream/123456789/772/1/revised+Ultraviolet+and+Visible+Spectrophotometry.pdfhttp://nsdl.niscair.res.in/bitstream/123456789/772/1/revised+Ultraviolet+and+Visible+Spectrophotometry.pdfhttp://media.rsc.org/Modern%20chemical%20techniques/MCT4%20UV%20and%20visible%20spec.pdf.%20%20%5b18http://media.rsc.org/Modern%20chemical%20techniques/MCT4%20UV%20and%20visible%20spec.pdf.%20%20%5b18http://media.rsc.org/Modern%20chemical%20techniques/MCT4%20UV%20and%20visible%20spec.pdf.%20%20%5b18http://web.usm.my/chem/LECTURER/ktt212/electronic_spectra.pdf%20%5b10http://web.usm.my/chem/LECTURER/ktt212/electronic_spectra.pdf%20%5b10http://web.usm.my/chem/LECTURER/ktt212/electronic_spectra.pdf%20%5b10http://media.rsc.org/Modern%20chemical%20techniques/MCT4%20UV%20and%20visible%20spec.pdf.%20%20%5b18http://media.rsc.org/Modern%20chemical%20techniques/MCT4%20UV%20and%20visible%20spec.pdf.%20%20%5b18http://nsdl.niscair.res.in/bitstream/123456789/772/1/revised+Ultraviolet+and+Visible+Spectrophotometry.pdfhttp://nsdl.niscair.res.in/bitstream/123456789/772/1/revised+Ultraviolet+and+Visible+Spectrophotometry.pdfhttp://classes.uleth.ca/200901/chem3840a/3830_3840%20lecture%20notes%20part10_2009_spectrosc.pdfhttp://classes.uleth.ca/200901/chem3840a/3830_3840%20lecture%20notes%20part10_2009_spectrosc.pdf


14/14

15

SKT 6014 ADVANCED INORGANIC CHEMISTRY

Assignment 5

UV-VISIBLE SPECTROMETRY

GROUP MEMBERS

NorazilawatiBtAzmi M20122001519

Chew KeatGeork M20122001523

NurulAinBtDnyal M 20131000601

Kamily A/P Balan M 20131000599

assignment 5 complexes

Documents