Ch. 15: Let’s Get Coordinated!!! Lewis Acids and Bases and Coordination Complexes Lewis Acids/Bases I. Remember: A. Lewis acids are lone pair acceptors. B. Lewis bases are lone pair donors. II. Quick points: A. The larger the charge of the cation, the stronger acid it is. B. This only applies to metals. III. Predicting Lewis Acid/Base Reactions A. This should be a “last resort” type of prediction. If the reaction that you see doesn’t fit any that you have seen before, it’s probably this one. B. One of the reactants must have a lone pair. C. The other reactants must be able to accept a lone pair. IV. Examples: A. Boron trichloride gas and ammonia are mixed. 1. BCl3 + NH3 --> Cl3BNH3 B. Phosphine (phosphorus trihydride) gas is bubbled into liquid boron trifluoride. 1. PH3 + BF3 --> H3PBF3 Coordination Complexes I. Re: Transition Metals A. This applies to all 4th row transition metals (TM) and aluminum, silver, gold. B. In the presence of a ligand, a Lewis acid/base reaction will occur to form a complex ion. 1. Ligand must fit two criteria a. Must have a neutral or negative charge b. Must have at least one lone pair to donate 2. This complex ion, along with with another ion, form coordination compounds. 3. Common ligands: H2O, NH3, CN-(cyanide), OH-, SCN- (thiocyanate) C. Examples: 1. Fe(CN)632. Mn(H2O)42+ II. Predicting complex ions A. Analyze your reactants: 1. One reactant must be a 4th row TM, Al, Ag, Au 2. The other reactant must be a ligand. B. Determine the charge of the metal. C. The number of ligands that bond to the metal is double the charge of the metal. D. Calculate the charge of the overall ion by adding the charges of the constituent ions. E. Examples: 1. Fe3+ + 6CN- --> [Fe(CN)6]3+ 2. Mn2+ + 4H2O --> [Mn(H2O)4]2+ III. Naming coordination compounds. A. Name cation before anion
Ch. 15: Let’s Get Coordinated!!! Lewis Acids and Bases and Coordination Complexes B. When naming a complex ion: 1. Name the ligand first a. Replace the ending of the ligand with an “-o” if the ligand is negative b. If the ligand is neutral, name the ligand as is. c. Exceptions: H2O=aqua; NH3=ammine; NO=nitrosyl; CO=carbonyl 2. Use mono, di, tri, etc. to communicate the # of ligands. 3. Then look at the charge of the ion a. If positive, name the metal ion as usual (don’t forget Roman numerals!) b. If the complex ion has a negative charge, replace the ending with “-ate” i. exceptions: iron=ferrate; copper=cuprate; silver=argentate; gold=aurate 4. Name the rest of the compound as usual. C. Examples: 1. [Co(NH3)6]Cl3 2. K3[Fe(CN)6] D. Two things to look out for: 1. You cannot have more than six ligands bonded. 2. The oxalate ion, C2O42-, counts as two ligands. IV. Writing coordination complex formulas: A. Watch out for the exceptions! Be AWARE of them... B. Put brackets around the complex ion. C. Examples: 1. Tetraaquaplatinum(II) chloride 2. Potassium hexafluorocobaltate(III) V. Predicting products A. If you see the following ligands as a reactant, a complex ion probably forms. 1. Ammonia, cyanide, hydroxide or thiocyanate 2. Examples: a. An excess of potassium hydroxide solution is added to a solution of aluminum sulfate. i. 6 OH- + Al3+ --> [Al(OH)6]3+ b. Aqueous ammonia is added to solid silver chloride i. 2 NH3 + Ag+ --> [Ag(NH3)2]+ c. A solution of ammonium thiocyanate is added to a solution of iron (III) chloride. i. Fe3+ + SCN- --> [Fe(SCN)6]3d. Solid aluminum nitrate is dissolved in water i. Al(NO3)3 + 6H2O --> [Al(H2O)6]3+ + NO3B. When a strong acid is added to a coordination complex, the coordination complex will dissociate. 1. Excess dilute nitric acid is added to a solution containing tetraamminecadmium(II) ion. a. 4H+ + [Cd(NH3)4]2+ --> Cd2+ + 4NH4+ 2. Dilute hydrochloric acid is added to a solution of diamminesilver(I) nitrate. a. 2H+ + [Al(NH3)2]+ --> Al3+ + 2NH4+ C. If a transition metal hydroxide and a strong base hydroxide is added, a complex ion with hydroxide will form. 1. Excess concentrated potassium hydroxide solution is added to a precipitate of zinc hydroxide. a. 2OH- + Zn(OH)2 --> [Zn(OH)4]2-
Ch. 15: Let’s Get Coordinated!!! Lewis Acids and Bases and Coordination Complexes 2. Sodium hydroxide solution is added to a precipitate of aluminum hydroxide in water. a. 3OH- + Al(OH)3 --> [Al(OH)6]3VI. Colors you will need to memorize A. Transition metals for colored solutions 1. Memorize the ion colors that are on your color sheet 2. Focus on Fe, Cr, Mn, Co, Cu 3. Memorize the two redox color changes as well. B. Know the precipitate colors
