Coordination Compounds

Coordination compounds contain a central metal atom or ion bonded to a fixed number of ions or molecules called ligands, and they play vital roles in biology, industry and analysis. In this chapter you will study Werner's theory, which explained compounds such as CoCl3.6NH3 and CoCl3.5NH3 using primary and secondary valencies, and key terms such as coordination entity, central atom, ligand (unidentate, didentate, polydentate and ambidentate), coordination number, coordination sphere, coordination polyhedron, oxidation number and homoleptic and heteroleptic complexes. You will learn IUPAC nomenclature and formula writing, and structural and stereoisomerism, including geometrical (cis-trans, fac-mer), optical, linkage, coordination, ionisation and solvate isomerism. The chapter then explains bonding through valence bond theory, using hybridisations such as d2sp3, sp3d2, dsp2 and sp3 to predict shape and magnetic behaviour, and crystal field theory, with splitting of d orbitals in octahedral and tetrahedral fields, the spectrochemical series, high-spin and low-spin complexes, and the origin of colour. Finally, you will learn about bonding in metal carbonyls and the importance of coordination compounds, from chlorophyll and haemoglobin to cisplatin and EDTA titrations.

Detailed Worksheet: Coordination Compounds Section A - Definitions (10 marks) 1. State the main postulates of Werner's theory of coordination compounds. (2 marks) 2. Define ambidentate ligand and chelating ligand with one example of each. (2 marks) 3. What is the coordination number of the central atom? Give its value in [Co(en)3]3+ and [Fe(C2O4)3]3-. (2 marks) 4. Distinguish between a double salt and a coordination compound with one example of each. (2 marks) 5. What is the spectrochemical series? Distinguish between a weak field ligand and a strong field ligand. (2 marks) Section B - Calculations and Applications (15 marks) 6. 0.01 mol of CoCl3.5NH3 is treated with excess AgNO3 solution, and 0.02 mol of AgCl is precipitated. Write the formula of the complex, its IUPAC name and the mass of AgCl formed (AgCl = 143.5). (3 marks) 7. Calculate the spin-only magnetic moments of [Fe(H2O)6]2+ (high spin) and [Fe(CN)6]4- (low spin) using mu = sqrt(n(n+2)) BM. State the hybridisation of each (sqrt(24) = 4.90). (3 marks) 8. Calculate the crystal field stabilisation energy (in terms of Delta_o and P) for a d4 ion in (i) a weak octahedral field and (ii) a strong octahedral field. Write the electronic configuration in each case. (3 marks) 9. [Ti(H2O)6]3+ absorbs light of wavelength 498 nm. Calculate the energy of one photon and the crystal field splitting energy per mole in kJ mol^-1 (h = 6.626 x 10^-34 J s, c = 3 x 10^8 m/s, NA = 6.022 x 10^23 mol^-1). Explain why the complex appears violet. (3 marks) 10. Write the IUPAC names of (i) [Co(NH3)4(H2O)Cl]Cl2 (ii) K3[Fe(CN)6] (iii) [Pt(NH3)2Cl2] (iv) [Ni(CO)4] (v) K2[Zn(OH)4] (vi) [Cr(en)3]Cl3. (3 marks) Section C - Diagrams (10 marks) 11. Draw the splitting of the d orbitals in an octahedral crystal field and in a tetrahedral crystal field, labelling t2g, eg, Delta_o and Delta_t, and state the relation between Delta_t and Delta_o. (4 marks) 12. Draw the cis and trans isomers of [Pt(NH3)2Cl2] and the fac and mer isomers of [Co(NH3)3(NO2)3]. (3 marks) 13. Draw the two optical isomers (enantiomers) of [Co(en)3]3+ and of cis-[PtCl2(en)2]2+. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Using valence bond theory, explain the hybridisation, shape and magnetic behaviour of (i) [Co(NH3)6]3+ (ii) [CoF6]3- (iii) [Ni(CN)4]2- (iv) [NiCl4]2-. Draw the orbital box diagrams. (5 marks) 15. Explain why [Fe(CN)6]3- is weakly paramagnetic while [Fe(H2O)6]3+ is strongly paramagnetic, using crystal field theory. Analyse the limitations of valence bond theory that crystal field theory overcomes. (5 marks) 16. Explain the synergic bonding in metal carbonyls and why it strengthens the metal-carbon bond. Describe three applications of coordination compounds in biology, medicine and analytical chemistry, with the complexes involved. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw structures and orbital diagrams neatly. Use the given constants for calculations.