Week 8: Compounds and complexes - workbook PDF

9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 Week 8: Compounds and complexes - workbook Site: Monash Moodle1 Printed by: Kaltham Alzaabi...

9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 Week 8: Compounds and complexes - workbook Site: Monash Moodle1 Printed by: Kaltham Alzaabi Unit: CHM1022 - Chemistry II - S2 2024 Date: Monday, 9 September 2024, 8:27 AM Book: Week 8: Compounds and complexes - workbook https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 1/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 Table of contents 1. Pre-workshop material 1.1. Chemistry of Group 6 Elements 1.2. Chromium Oxoanions 1.3. Activity 1.4. Alfred Werner and Coordination Chemistry 1.5. Coordination Compounds and Complexes 1.6. Activity 2. Summary 3. Preparation quiz 4. Online lectures 5. Workshops https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 2/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 1. Pre-workshop material The chemistry of the d-block metals is diverse but there are some clear trends which we should be familiar with. All the d-block elements are metals, they form compounds in a variety of oxidation states, form coloured compounds and exhibit magnetic properties. We will explore all these properties in the unit. For now we will study aspects of the chemistry of the Group 6 elements: chromium, molybdenum and tungsten, to illustrate some features of d- block chemistry. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 3/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 1.1. Chemistry of Group 6 Elements Let's consider some of the properties of Group 6 which includes the metals chromium (Cr), molybdenum (Mo) and tungsten (W). Atomic Radius (pm) From Figure 1 below, we can see there is an increase in atomic radius down the group from Cr to Mo but the radii of Mo and W are the same. This is due to lanthanoid contraction, which we discussed last week. The result of this contraction is that the properties of the first element in the group, Cr, differ from those of the 2nd and 3rd members of the group, Mo and W. Figure 1: Properties of Chromium, Molybdenum and Tungsten. Ionisation Energy (kJ/mol) The first ionisation energy increase down the group is due to the combination of an increase in the effective nuclear charge with a similar metallic radius (see Figure 2). However, subsequent removal of electrons from the 2nd and 3rd row transition metals becomes easier due to shielding by the innermost electrons. This leads to a much more gradual increase in the second and subsequent ionisation energies. Consequently the 2nd and 3rd row elements become more stable in higher oxidation states and are much more easily oxidised in low oxidation states (+2 and +3) than the corresponding first-row transition metals. Figure 2: First Ionisation energy of Cr, Mo and W Remember: OIL RIG Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons) Chemistry of Chromium We will now focus on the chemistry of chromium. Chromium gets its name from the Greek for colour - chroma The ground state configuration 4s1 3d5. Note the influence of a stable half-filled shell of electrons. The common oxidation states of chromium are +2, +3 and +6. Compounds of chromium have several important uses. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 4/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 Compounds of chromium have several important uses Stainless steel contains up to 18% chromium. The gemstone ruby is an aluminium oxide doped with traces of chromium. Copper chromium arsenate is used as a wood preservative Figure 3. Where to find chromium amongst common items. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 5/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 1.2. Chromium Oxoanions Please read chromium (Page 804-805) of Chemistry, Blackman et al. (4th ed.) Oxoanions Charged metal ions do not exist in isolation. They are surrounded by neutral or anionic species. Oxoanions are negatively charged ions containing two or more elements, one of which is oxygen. Oxoanions are very common for Group 6 elements, where the negatively charged ion is formed from oxygen and either Cr, Mn or W. Figure 4 shows an oxoanion of Cr6+, CrO42-. Cr = 6+ 4 x O2- = 8- Overall = 2- Figure 4: The Cr6+ atom is in a tetrahedral geometry surrounded by four O2- anions. The overall charge of the chromium oxoanion is therefore 2-. Chromium oxidation states H2CrO4 acts as an acid as it reacts with the OH- anions to produce CrO42- (yellow) and water (condensation reaction). The product CrO42- reacts under acidic conditions to produce the dichromate Cr2O72- (orange) and water (condensation reaction). Notice how the bridging oxygen atom links the two Cr6+ ions. The (-ate) ending in dichromate is due to the overall negative charge of the oxoanion (More on this in week 9). See Figure 5 for structure of dichromate. The colour change in this example is due to the acid-base reaction causing a structural change. Figure 5: Reaction of chromate CrO42- to dichromate Cr2O72-. The dichromate Cr2O72- oxoanion, which also has an oxidation state Cr6+, acts as a strong oxidiser (itself gets reduced) in acid to form Cr3+. The Cr3+ oxidation state is the most stable. Other oxidation states of Chromium Chromium also exists in low oxidation states - Cr0 and Cr2+. They are strong reducing agents and easily oxidise to Cr3+. Figure 6 shows the colour difference of Cr2+ and Cr3+. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 6/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 Figure 6: A solution of blue Cr2+ and green Cr3+ ions. As the oxidation state of the chromium increases (Cr2+ to Cr3+ to Cr6+), its tendency to remove more electrons decreases (ionisation energy increases) making it more electronegative, less metallic and more acidic (more able to accept an electron from a donor compound cf. Lewis acid). Note: Cr2O3 is amphoteric, which means it can behave as an acid or a base. Summary of Chromium Oxides Chromium trioxide video CrO3 is a strong oxidising agent (Cr6+ is highly oxidised), see this in action in the video here. Chromium Trioxide (FAIL) - Periodic Table of Videos https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 7/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 1.3. Activity Read the following news article from the link below on the oxidation states of chromium titled: “Research raises concerns over long-term use chromium diet pills” http://newsroom.unsw.edu.au/news/health/research-raises-concerns-over-long-term-use-chromium-diet-pills Identify three important points that you gained from reading this article. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 8/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 1.4. Alfred Werner and Coordination Chemistry Alfred Werner (1866-1919) ‘Father of Coordination Chemistry’ Awarded Nobel Prize for Chemistry in 1913. (Further reading: https://www.nobelprize.org/prizes/chemistry/1913/Werner/facts/) Introduced the concept of coordination number, and explained the existence of optically active metal complex isomers. Made hundreds of new compounds and studied their configurations - over 2500 compounds in his collection are still held at the University of Zurich Please read Sections 13.1-13.3 about complexes and ligands (page 751, 752) of Chemistry, Blackman et al. (4th ed.) Coordination Complexes Metal ions do not exist as isolated ions – they are surround by other molecules/anions. A coordination complex consists of a central cation (metal ion) bonded to molecules or anions (called ligands) Figure 7 demonstrates a typical coordination complex. Figure 7: Example of a coordination complex. Co is the metal and ammonia are the ligands. The square brackets around the structure tells us that it is a complex. Coordination complexes form because the positive metal ion is a Lewis acid (an electron pair acceptor) and the ligands surrounding it are Lewis bases (electron pair donors). Figure 8 highlights the bond formation of the Lewis acid and Lewis base. Figure 8: diagram showing the coordination bond forming between a metal cation and ligand. The bond formed is known as a coordinate bond, a donor covalent bond or a dative bond. Coordination Compounds Counter ions may be present to maintain charge balance in a coordination compound (Figure 9). A coordination compound is a salt (cation + anion) in which one or both of the ions is a coordination complex. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 9/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 Figure 9: Example of a coordination compound. Note the chloride ion outside the square brackets is the counter ion and does not form a coordination bond with the metal A compound may dissociate into anions and cations in solution - can behave as an electrolyte. Ligands remain attached to the metal ion. Figure 10: There are four different complexes in the first column. As the overall charge of the complex is not neutral, there are counter ions ("Free" Cl- ions per unit formula column) to create the compounds in the last column. Taken from Blackman et al. 3rd edition. From Figure 10, the compounds [Co(NH3)6]Cl3 dissociates into [Co(NH3)6]3+ and 3 Cl- ions. These ionic solutions conduct electricity. Alfred Werner – Werner’s Insights Realised “CoCl3(NH3)6” was not a simple ‘ionic’ salt but a ‘complex’ cation, [Co(NH3)6]Cl3 Complex was ionic Oxidation state was Co3+ Molecule was octahedral 6 NH3 ammonia ligands (Coordination number = 6) Cl- not coordinated but ionic ‘counter ions’ Figure 11a: [Co(NH3)6]3+ complex https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 10/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 Figure 11b: [Co(NH3)4Cl2]+ complex showing the cis structure 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. Figure11c: [Co(NH3)4Cl2]+ complex showing the cis and trans structure https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 11/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 1.5. Coordination Compounds and Complexes Geometries of Coordination Complexes Kepert theory: Complex shapes depend on repulsion of ligands Coordination Number (C.N.) is number of donor atoms around metal. Donor atoms: The atom that forms the coordination bond with the metal. eg. For NH3 ligand, the nitrogen forms the bond with the metal, thus nitrogen is the donor atom. Geometry - depends on coordination number and nature of ligands Linear – 2 donor atoms Trigonal planar – 3 donor atoms Tetrahedral – 4 donor atoms (Exception for some 4-coordinate complexes which are square planar) Trigonal bipyramidal or square-based pyramidal - 5 donor atoms Octahedral - 6 donor atoms Components of Coordination Complexes Coordination Number (C.N.) is 6 Components of Coordination Compounds https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 12/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 Metal oxidation state can be calculated... Determine the charge of the complex. For example: Nitrate (counter ion) has a charge of -1 so complex must have +1 charge. Determine the charges of the ligands. For example: Cl- is -1 and NH3 is neutral. Oxidation state of metal = Complex charge – sum of ligand charges Here, oxidation state of metal = +1 – (2 x -1) = +3 https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 13/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 1.6. Activity Consider the following compounds: Identify the: 1. Metals 2. Ligands 3. Counter-ions What is the oxidation state of the metal in each complex? https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 14/19 9/9/24, 8:28 AM Week 8: Compounds and complexes - workbook | MonashELMS1 2. Summary This week you have learnt about some of the physical and chemical properties of the transition metals in group 6, with a focus on chromium. Chromium has both very toxic consequences and potentially beneficial biological applications that is highly dependent on the oxidation state. You have learnt the importance of identifying the oxidation state of an element because of the key role it plays in determining the physical and chemical properties. You have been shown how to draw coordination complexes, which consist of a central metal cation bonded to ligands. https://learning.monash.edu/mod/book/tool/print/index.php?id=2780838 15/19