Complexes A few slides to jog memories What is a complex? Transition metal Dative coordinate bonds from ligands Ligands are lone pair donors (anions or neutral) Complexes have more bonds than the oxidation number of the TM How can we describe the metal ion in terms of acidity? Lone pair acceptor – thus it is a Lewis acid. The ligands are Lewis bases. NAMING COMPLEXES 2, di 4, tetra 6, hexa H2O aqua NH3 ammine Cl- chloro CN- cyano OH- hydroxo Transition metal name Positive charge English name Negative charge Latin and “ate”. Diethane-1,2-diaminedichlorocobalt 3+ ion Eg [Cu(NH3)4]2+ tetraamminecopper II ion [Fe(CN)6]4- hexacyanoferrate II ion Shapes of complexes Octahedral complexes with six-fold coordination are the most common hexaaquairon III ion Shapes of complexes Linear complexes are mainly formed by: Silver I Copper I diamminesilver I ion Shapes of complexes tetrahedral complexes are mainly formed by: Large bonding ligands that cannot pack around the central atom as in a octahedral complex, like TMs in 2+ oxidation states with chlorine, or all oxidation states with bromine Questions tend to focus on the oxidation states of the metal ion and chlorine or bromine ligand and packing issues. diamminedichlorocopper II ion Shapes of complexes Square planar complexes are much rarer but you should be aware of: Cis-platin cis-diamminedichloroplatinum II Cis-platin can exchange the chlorine ligands for nitrogen in DNA causing it to distort and stop working. It is an effective anti- cancer treatment for hormone related cancers like breast and testicular cancer. Remember that cyanide ligands form square planar complexes Don’t always assume that chlorine is too large to be in octahedral complexes. the central ion size also plays a part. cis and trans tetraamminedichlorocobalt III ion [Co(NH3)4Cl2]+ Rotational asymmetry What ligand property is also shown in this picture? Chelation Chelation using EDTA. How many binding points has EDTA? 6 What is the charge on EDTA? 4- What would you call this, as a ligand? Hexadentate ligand Remember that EDTA has 6 binding sites (lone pair donors) but not all the sites are identical. EDTA cobaltate III ion Haemoglobin Overall OCTAHEDRAL Square planar porphyrin ring around the Fe II ion (tetradentate and neutral) Large globin molecules dominate the size of the structure binding at one site Either water or oxygen binds to the sixth octahedral site (concentration dependent) 4 haemoglobin complexes Silver chemistry – what two complexes should you be aware of? Tollen’s reagent: diammine silver I ion [Ag(NH3)2]+ reacts with warm aldehyde. The aldehyde is oxidised to an acid and the silver is reduced to silver metal and forms a silver “mirror”. In the test for silver halides. AgCl and AgBr will dissolve in dilute ammonia forming the [Ag(NH3)2]+ complex. AgI will not. Photography. Silver is deposited from silver bromide in B&W photographic paper. This process continues in light until the film is “fixed”. The “fixer” is sodium thiosulphate that complexes silver and stops further deposition. AgBr + 2S2O32- [Ag(S2O3)2]3- + Br- dithiosulphateargentate I ion Exam Questions from June 2003 The ion C2O42- can act as a bidentate ligand. What is the meaning of bidentate ligand. Sketch the structure of the octahedral complex ion formed by Fe3+ ions which contains C2O42- as the only ligand. Include the overall charge for the complex ion. 5 marks Explain the meaning of the “chelate” effect. 2 marks The chloride ion can act as a monodentate ligand. Deduce the formula of the linear complex formed when an excess of concentrated hydrochloric acid is added to silver chloride. Explain why metal II ions do not usually form octahedral complexes when chloride ions are the only ligands. 2 marks This question then went further to consider catalytic uses. Exam Questions from June 2004 State what is meant by each of the following terms: Ligand; Complex ion; Coordination number 3 marks Using complex ions formed by Co2+ with ligands selected from H2O, NH3, Cl-, C2O42-, and EDTA4-, give an equation for each of the following: A ligand substitution reaction which occurs with no change in either the coordination number or the charge on the central ion. A ligand substitution reaction which occurs with both a change in the co-ordination number and the charge on the complex ion. A ligand substitution reaction which occurs with no change in the co-ordination number but a change in the charge on the complex ion. A ligand substitution reaction in which there is a large change in entropy. 8 marks Exam Questions from June 2004 An aqueous solution of iron II sulphate is a pale green colour. When aqueous sodium hydroxide is added to this solution a green precipitate is formed. On standing in air, the green precipitate slowly turns brown. Give the formula of the complex ion responsible for the pale green colour. Give the formula of the green precipitate. Suggest an explanation for the change in the colour of the precipitate. 4 marks Exam Questions from June 2005 Identify and state the shape of the copper- containing species present in solution in A. Identify the pale blue precipitate B and write an equation, or equations, to show how B is formed from the copper-containing species present in A. In what way does NH3 behave as a Bronsted- Lowry base? Exam Questions from June 2005 Identify the copper-containing species present in solution C. state the colour of this copper- containing species and write an equation for its formation from precipitate B. In what way does the NH3 behave as a Lewis base. Identify the copper-containing species present in solution D. state the colour and shape of this copper-containing species. The oxidation state of copper in [CuCl4]3- is +1. Give the electron arrangement of a Cu+ ion. Deduce the role of copper metal in the formation of [CuCl4]3- from the copper-containing species in solution D.
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