Week 10 Crystal Field Theory - Workbook PDF

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Week 10: Crystal Field Theory - Workbook

Site: Monash University Unit: CHM1022 - Chemistry II - S2 2018 Book: Week 10: Crystal Field Theory - Workbook Printed by: Kellie Vanderkruk Date: Wednesday, 24 October 2018, 9:28 AM

Table of contents Week 10: Pre-lectorial Work Isomerism Crystal field theory Are you ready? Pre-Test Checklist Week 10: Pre-lectorial Test Week 10: Lectorial/Session 1 Recap from pre-class work Activity 1: Isomers Activity 1: Answers Activity 2: Isomer Problems Activity 2: Answers Week 10: Lectorial/Session 2 Recap from pre-class work Activity 1: Orbitals Activity 1: Answer and discussion Activity 2: Applying Crystal Field Theory Activity 2: Answers Summary Page

Week 10: Pre-lectorial Work

Introduction This section of the workbook will take you through the required pre-lectorial readings and activities. It is important that you take the time to carefully review the material provided and complete any activities. We recommend that you allocate at least 60 minutes to work through the topics and activities in this section of the workbook, with additional time to complete the prompted readings as necessary (this will vary depending on your reading speed and any prior knowledge you may have). Activity type

Time to complete (approx)

Reading

25 mins

Video

7 mins

Activity

10 mins

Self test

10 mins

Pre-quiz

10 mins

Total

62 mins

Learning Outcomes By the end of this week, you will be able to: Identify and draw different types of structural isomerism Identify types of stereoisomerism Able to determine chirality in coordination complex Able to explain why d orbitals are not degenerate in an octahedral complex Apply Crystal Field Theory to other geometries (square pyramidal, square planar and tetrahedral geometries) Demonstrate the use of Crystal Field Theory to solve problems

What do I need to do? The following activities must be completed prior to attending your first lectorial for this week. Watch a short video Read through the eBook slides and the associated chapters of the textbook. Work through some self-test problems. Finally, complete an activity that will test your understanding of the material covered in this eBook.

Isomerism

The following content is also covered in Blackman’s pages 559-562 and pages 569 (Bonding in Transition metal complexes)

to 573.

Isomerism in Coordination Compounds In the early history of coordination chemistry, the existence of pairs of compounds with the same formula yet different properties proved very perplexing to inorganic chemists. Werner was studying Co and Pt complexes and noticed that some had that some had different colours, yet analysed for the same formula! He was among the first to realise that the different properties represented different structural arrangements (isomers). -

In Figure 1, the NO 2 ligand is bound to the central Co atom in two different ways, one through the N atom and one through one of the O atoms. This approach also correctly enables us to predict that there are two possible forms of [CoCl2 (NH3 )4 ]Cl. One of these has the two Cl ligands next to each other, the other has them opposite each other (Figure 2)

2+

Figure 1: Two isomers of [Co(NH ) (NO )] . The left hand structure has the oxygen bonded to the Co and the right hand structure has the 3 5 2 nitrogen bonded to the Co.

Approach correctly predicts there would be two forms of “CoCl3 ∙4NH3 ” The formula would be written [CoCl2(NH3) 4]Cl

One form has the two chlorides next to each other.

The other has them opposite each other.

Figure 2: The two different structural isomers of + [CoCl (NH ) ] 2

3 4

Alfred Werner - Werner's Insights There are two main types of isomerism in coordination compounds. In structural isomers there are different bonds between the central metal atom and the ligands. In stereoisomerism the bonds between the metal and the ligands are the same but the ligands are arranged differently in space around the metal ion. Figure 3 is a summary of the types of isomers you will see in inorganic molecules.

Isomerism in Coordination Compounds

Figure 3. A summary of the types of isomers in inorganic complexes. Note the similarities with isomerism in organic compounds

Isomerism: Structural Isomers Linkage isomerism – where a single ligand has two or more donor atoms and can attach in more than one way (Figure 4). 2+

[Co(NH 3) 5(SCN)] vs 2+ [Co(NH3) 5(NCS)]

Pentaamminethiocyanato-κS-cobalt(III) Pentaamminethiocyanato-κN-cobalt(III)

Figure 4. Example of ligands that can form linkage isomers

Hydration and Ionisation isomers Exchange of ligands in coordination complex with counter-ions and water molecules. Hydration isomers involve water swapping with a ligand [CrCl2 (OH2 )4]Cl▪2H2O

[CrCl(OH2 )5]Cl2▪H2O

Ionisation isomers involve exchange of anionic ligands with counter anions [PtCl (NH ) ]I 2

[PtI (NH ) ]Cl

3 4 2

2

3 4

2

Coordination isomerism Can occur when both a complex cation and anion are present. This allows for multiple combinations of ligands coordinated to the two metal centres. [Co(bpy)3][Fe(CN)6]

[Co(bpy)2(CN)2][Fe(bpy)(CN)4]

[Fe(bpy)3][Co(CN)6]

Isomerism: Stereoisomers In stereoisomers we see parallels with isomerism in organic chemistry for example cis/trans and optical isomerism. Remember stereoisomers are molecules with the same number of ligands bonded to the metal, but in different arrangements in space.

Geometric Isomers The geometric isomers you will need to know are the cis/trans isomers (there are other geometric isomers which are not assessed in CHM1022). cis-isomers are when the identical ligands are adjacent to each other. Trans-isomers are when they are on opposite sites of the metal centre (Figure 6). Example (Figure 5): Cisplatin in a highly effective and widely used anticancer drug with formula cis-[PtCl2 (NH3 )2 ]. Especially effective in ovarian and testicular cancers. Interestingly trans-[PtCl2 (NH3 )2 ] is not active and is toxic!!

Robbie Gray - cancer survivor Figure 5: The cis- and trans-isomers of platin [PtCl2 (NH3 )2] complex.

Figure 6: The cis- and trans-isomers of square planar and octahedral complexes

Optical isomers Occur most commonly when there is more than one bidentate ligand in the complex. Just as in organic chemistry, optical isomers are non-superimposable mirror images of each other. Enantiomers are also stereo isomers i.e. mirror images that are non-superimposable.

+

3+

Figure 7: Two examples of optical isomers: First is cis-[CoCl (en) ] and second is cis-[Co(en) ] 2

2

3

(Blackman's Figure 16:11)

Something to do To explore the these concepts further see the following link from The University of LiverpoolChemTube 3D: http://www.chemtube3d.com/TM-Ru(en)3enantiomers.html

Crystal field theory

Colours and Magnetism Colour - Many transition metal compounds are coloured. Electrons in partially filled d-orbitals can absorb visible light & move to d-orbitals with slightly higher energy. There will be more on this later. 3+

4+

2+

(Sc , Ti & Zn are exceptions -filled or empty d-subshell)

Figure 8. Examples of coloured and non-coloured complexes

Magnetism - typically transition metals have a partially filled set of d-orbitals and some of the d-electrons may be unpaired. These unpaired electrons give rise to the complex being paramagnetic. Paramagnetic compounds are attracted to a magnetic field. These unpaired electrons can be randomly aligned. If they are aligned in the same direction then we observe ferromagnetism. If all electrons are paired this gives rise to diamagnetism. Diamagnetic compounds are repelled by a magnetic field.

Diamagnetic - all electron spins paired;

Paramagnetic - unpaired spins. Magnetic fields are randomly arranged, unless placed in an external

no net magnetic moment.

magnetic field - Ferromagnetic

Figure 9: An example of a diamagnetic and paramagnetic molecule's electrons +

2+

Group 1 and 2 ionic compounds (eg Na , Ca ) are colourless and diamagnetic whereas many coordination complexes are coloured (Figure 10) and exhibit magnetic properties. But, how is this possible? Aren’t the 3d orbitals equal in energy? We need a model of bonding that explains colour and magnetism. Transition Metals and Colour

Figure 10: A list of various colours observed from numerous complexes.

Crystal Field Theory Valence Bond Theory You were introduced to valence bond theory in the organic section, where hybrid orbitals are formed to explain the geometry of the molecule's bonds (Figure 11).

Complex formation is explained by donation of electron pair(s) (by a Lewis Base) to a metal ion (Lewis acid) to form a coordination bond. Hybrid orbitals of equal energy provide no explanation for the colour and magnetic properties of transition metal complexes. So valence bond theory fails In this regard we use a more sophisticated model called Crystal Field Theory.

Figure 11: Possible hybrid orbitals for geometrical shapes found in complexes.

New Theory Needed: Crystal Field (CF) Theory This model explains stability, colour and magnetism but not the nature of metal-ligand bonding. It describes how the energies of the d-orbitals on the metal ion are affected as the ligands approach. Important Assumption Complexes result from electrostatic attractions between metal cation and negative charges or lone pair of electrons on ligands. (i.e. they are not covalent in character)

Important Question: What happens to the energy of the metal d-orbitals when 6 ligands approach? Consider that the ligands (balls of negative charge) can approach the metal centre (uniform positive charge) in two ways: (i) Directly along the x, y, z, axes (ii) Between the axes If we consider the 6 point charges are directed at the metal along the three axes, we will observe greater ligand – d electron repulsion from those electrons on the x-, y- and z-axes (i.e. in dx2 -y2 and dz2 orbitals), rather than those between axes (dxy , dyz , dxz ) - see Figure 12. In other words, higher energy is required for some d-orbitals as the complex forms bonds with the ligands. This is due to the two sets of negatively charged electrons (1 set from the lone pair on the ligand and the other from the d-orbitals) overcoming their repulsion of each other to form that complex bond.

Figure 12. The change in energy of the d-orbitals when ligands approach along the x-, y- and z-axes, as for an octahedral complex

To explore the these concepts further see the following link from The University of Liverpool- ChemTube 3D:

http://www.chemtube3d.com/SALC-CFOh.htm

Steps to Complex Formation

n+

n+

The five d-orbitals increase in energy relative to free M affected (Figure 13).

ion due to greater ligand - d-electrons repulsion. However, they are not equally

Figure 13: The shape of all 5 d-orbitals and how their energy levels are affected by octahedral ligands.

Direct head-to-head repulsion leads to a higher energy (more unstable) situation for dz2 or dx2-y2 orbitals – collectively referred to as the eg orbitals. Tangential repulsion (more side-on) leads to a relatively lower energy situation for dxy , dxz and dyz orbitals – collectively referred to as the t orbitals. 2g

The energy difference between eg and t2g is Δoct (or Δo) - Figure 14.

Figure 14: The energy levels of the 5 d-orbitals after an octahedral complex is formed

Something to Watch: Electron Orbitals This video is designed to help you visualise the s-, p- and d-orbitals. Notice how the five d-orbitals fit together in 3 dimensions. Duration: 1:36 mins

Something to Watch: Crystal Field Theory This video demonstrates Crystal Field Theory. In particular, the splitting of d orbitals that occurs when transition metal ions are placed in an octahedral or tetrahedral field. Duration: 5:20 mins

Reference: Classteacher Learning Systems

Something to do The following questions in the textbook are relevant to this weeks topics. Page 562, Q 13.4 Page 595, Q 13.19-13.26 Remember to use other textbooks in the library with similar questions to test yourself further.

Are you ready? Pre-Test Checklist

Before the test and the lectorial, check your knowledge: Lectorial 1: Identify and draw different types of structural isomerism Identify types of stereoisomerism Able to determine chirality in coordination complex Lectorial 2: Able to explain why d orbitals are not degenerate in an octahedral complex Apply Crystal Field Theory to other geometries (square pyramidal, square planar and tetrahedral geometries) Demonstrate the use of Crystal Field Theory to solve problems Activities: Check that you have completed all your pre-class activities

Week 10: Pre-lectorial Test

Introduction Now that you have completed your Pre-lectorial work, you now need to complete your assessed pre-lectorial test. Ensure that you check the due date and time for the test as it may be due several days before your first lectorial for the week.

What do I need to do? Go to the following Pre-lectorial Test and attempt the quiz activity. Take note of the instructions provided for the test. Pre-lectorial Test for Week 10

Week 10: Lectorial/Session 1

Introduction This section is a guide to support you through the topics and activities covered during the lectorial. Ensure that you have completed any prior pre-learning activities in the previous book.

Learning Outcomes By the end of this session, you will be able to: Identify and draw different types of structural isomerism Identify types of stereoisomerism Able to determine chirality in coordination complex

Recap from pre-class work

Isomerism in Coordination Compounds Further clarification You will have seen this diagram from your pre-class readings on Isomerism in Coordination Compounds. More information around this diagram will be given and a chance to ask any questions if you need more clarification.

Activity 1: Isomers

Getting Setup In this activity, we will be drawing geometrical isomers. Please consider your pre-learning in answering the questions. If you need to do research go ahead! This activity (including discussion) should take approximately 15 minutes.

Something to think about 1. Decide ligands and metal ion geometry 2. Decide if ligands can be placed around metal in more than one way 3. Decide whether the mirror images(s) of the complex is/are superimposable

Question 1 -

-

-

Draw all the possible geometrical isomers for the complex [CoBrCl(OH ) (ox)] (ox = oxalate = O CCO ) 2 2

What is the geometry of the complex? How many isomers have you got? What type of isomerism do you observe?

If you have printed this workbook, write your answer here.

MirrorOp Follow the prompts on the screen to log into MirrorOp The correct answers will be shown on the large screens using MirrorOp. Feedback and clarification will be given around the answers.

2

2

Question 2 Students are to draw 2D diagrams of the following three formula noting the following question. What types of isomerism are possible (and what are the isometric forms) for complexes or compounds with the following general formulas. +

1. [Pt(NH3 )3(SCN)] 2. CoBr(NH ) (NO ) 3 5 2 3. FeCl2 (H2O)6 Hint: Think about how many ligands could be attached to the central metal atom, as we have not used square brackets in all these examples. Possible isomers Ionisation Linkage Hydration Coordination isomer Geometric isomer Optical isomer

If you have printed this workbook, write your answer here.

MirrorOp Follow the prompts on the screen to log into MirrorOp The correct answers will be shown on the large screens using MirrorOp. Feedback and clarification will be given to students.

Activity 1: Answers

-

-

-

Question: Draw all the possible geometrical isomers for the complex [CoBrCl(OH ) (ox)] , (ox = oxalate = O2 CCO2 ) 2 2

PLAN: (i) Decide ligands and metal ion geometry. (ii) Decide if ligands can be placed around metal in more than one way. (iii) decide whether the mirror image(s) of the complex is/are superimposable.

Question: Draw all the possible geometrical isomers for the complex [Co(OH ) (ox)BrCl] , (ox = oxalate = O CCO ) 2 2

2

2

ANSWER

Question: What types of isomerism are possible (and what are the isomeric forms) for complexes or compounds with the following general formulas: +

1. [Pt(NH ) (SCN)] 3 3 2. CoBr(NH 3) 5(NO 2) 3. FeCl 2(H 2O) 6 ANSWER

Activity 2: Isomer Problems

Getting Setup In this activity, we will be playing a Card Game and then drawing the isomer structure, long name and chemical formula. Please consider your pre-learning when completing these activities. This activity (including discussion) should take approximately 30 minutes. Form groups of 5-6 students. The following activity is to be completed in groups during class. Please follow the prompts and instructions given by your educator.

Something to do You will be given a set of 18 cards in a plastic bag. The challenge of the game is to match the cards into five groups. Matching the following into each group: Chemical formula Chemical long name 2D chemical representation

Sample only (not correct representation)

You will then be asked to identify the possible isomer(s) in each group and write down the: • • •

Chemical formula Chemical long name 2D chemical representation

If you have printed this workbook, write your answer here.

MirrorOp Follow the prompts on the screen to log into MirrorOp The correct answers will be shown on the large screens using MirrorOp. Feedback and clarification will be given to students.

Activity 2: Answers

Group 1

Group 2

Group 3

Group 4

Group 5

Week 10: Lectorial/Session 2

Introduction This section is a guide to support you through the topics and activities covered during the lectorial. Ensure that you have completed any prior pre-learning activities in the previous book.

Learning Outcomes By the end of this session, you will be able to: Able to explain why d orbitals are not degenerate in an octahedral complex Apply Crystal Field Theory to other geometries (square pyramidal, square planar and tetrahedral geometries) Demonstrate the use of Crystal Field Theory to solve problems

Recap from pre-class work

Crystal Field (CF) Theory This model explains stability, colour and magnetism but not the nature of metal-ligand bonding. It describes how the energies of the dorbitals on the metal ion are affected as the ligands approach. Important Assumption: Complexes result from electrostatic attractions between metal cation and negative charges or dipoles on ligands. (i.e. they are not covalent in character)

Important Question What happens to the energy of the metal d-orbitals when 6 ligands approach? Consider that the...