Week 10 Crystal Field Theory Workbook - Monash S2 2024 PDF

9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 Week 10: Crystal Field Theory - Workbook Site: Monash Moodle1 Printed by: Kaltham Alzaabi...

9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 Week 10: Crystal Field Theory - Workbook Site: Monash Moodle1 Printed by: Kaltham Alzaabi Unit: CHM1022 - Chemistry II - S2 2024 Date: Sunday, 22 September 2024, 12:32 PM Book: Week 10: Crystal Field Theory - Workbook https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 1/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 Table of contents 1. Pre-workshop material 1.1. Isomers in coordination chemistry 1.2. Colour and magnetism 1.3. Crystal field theory 1.4. Videos 2. Summary 3. Preparation quiz 4. Online lectures 5. Workshops https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 2/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 1. Pre-workshop material This week we are going to discover different types of isomers that coordination complexes can adopt. Isomers are complexes that have the same chemical formula but different structural or spacial arrangements. With coordination complexes this is also true, except that due to the variety of ligands and coordination geometries available various sub-categories of isomer types are available to one metal coordination complex. We will also introduce Crystal Field Theory. This will be used to help explain why transition metal complexes are coloured and why some coordination complexes are attracted to a magnetic field. Overall we will explain the ultimate importance of unpaired electrons and their ability to move between d-orbitals of different energies! https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 3/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 1.1. Isomers in coordination chemistry Please read pages 766-770 (Isomerism in transition metal complexes) and pages 779-784 (Bonding in Transition metal complexes) of Chemistry, Blackman et al (4th ed.) 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 NO2- 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) Figure 1: Two isomers of [Co(NH3)5(NO2)]2+. The left hand structure has the oxygen bonded to the Co and the right hand structure has the nitrogen bonded to the Co. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 4/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 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 [CoCl2(NH3)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). https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 5/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 Pentaamminethiocyanato-κS-cobalt(III) [Co(NH3)5(SCN)]2+ vs Pentaamminethiocyanato-κN-cobalt(III) [Co(NH3)5(NCS)]2+ 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 [PtCl2(NH3)4]I2 [PtI2(NH3)4]Cl2 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 and the fac/mer isomers. 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!! https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 6/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 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 Fac/mer isomerism can occur for octahedral complexes with three ligands of one type and three of another. Facial (fac) isomers are when the identical ligands are all adjacent to each other - i.e. they occupy a face of the octahedron defined by the donor atoms around the metal. Meridional (mer) isomers are when the identical ligands lie around a meridian of the complex - i.e. two ligands are on opposite sides of the complex, with the third lying adjacent to the other two (Figure 7). Figure 7: The fac- and mer-isomers of an octahedral complex https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 7/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 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. Figure 8: Two examples of optical isomers: First is cis-[CoCl2(en)2]+ and second is cis-[Co(en)3]3+ (Blackman's Figure 16:11) Explore the 3D representation of the molecular structure of [Ru(en]3)2+ enantiomers illustrated in this ChemTube 3D page (click the link). https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 8/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 1.2. Colour and magnetism Colours and Magnetism Colour - Many transition metal compounds are coloured. Electrons in partially filled d-orbitals can absorb visible light and move to d- orbitals with slightly higher energy. There will be more on this later. (Sc3+, Ti4+ and Zn2+ are exceptions -they have filled or empty d-subshells) 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 Group 1 and 2 ionic compounds (eg Na+, Ca2+) 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 https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 9/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 Figure 10: A list of various colours observed from numerous complexes. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 10/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 1.3. Crystal field theory 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 or (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. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 11/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 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 Explore the 3D representation of the metal d-orbitals in an octahedral crystal field illustrated in this ChemTube 3D page (click the link). Steps to Complex Formation The five d-orbitals increase in energy relative to free Mn+ ion due to greater ligand - d-electrons repulsion. However, they are not equally affected (Figure 13). 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 t2g orbitals. 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 https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 12/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 1.4. Videos 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. Electron Orbitals - s,p & d 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. Crystal Field Theory | Chemistry Animation Energy Video | L… L… Reference: Classteacher Learning Systems https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 13/18 9/22/24, 12:33 PM Week 10: Crystal Field Theory - Workbook | MonashELMS1 2. Summary This week you have learnt that coordination complexes can have different types of structural isomerism which can be sub-categorised into different groups including: structural – coordination and linkage or stereioisomers – geometric and optical. You have also learnt that one coordination complex can have more than one type of isomer dependent on the ligands attached to the metal center. We have also learnt that the five d-orbitals are not degenerate and introduced Crystal Field Theory to help explain the bonding seen in transition metal coordination complexes. We have looked at Crystal Field Theory for octahedral, tetrahedral, square planar and square pyramidal geometries and are able to predict and draw the representative Crystal Field splitting diagrams for each geometry along with an explanation of why they are all different. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780844 14/18