Coordination Chemistry 3 Part 2 Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Experiments Index ChemLab Home		Goals In this experiment, you will study the reaction between your cobalt complex and the nitrite ion, NO2-. The nitrite ion can exchange with the water ligand of [Co(NH3)5(H2O)]3+ and the mechanism of this ligand exchange will be investigated. The reaction rate will be measured by following the change in solution absorbance with time and the reaction order and reaction rate constant will be calculated. Finally, the rate of isomerization between two different forms of the nitro ligand will be studied. To Learn or Review Transition Metal Complexes Oxtoby, Gillis, and Nachtrieb pp. 665-672 Rate Laws Oxtoby, Gillis, and Nachtrieb pp. 450-458 Reaction Mechanisms Oxtoby, Gillis, and Nachtrieb pp. 458-467 Light and Color Oxtoby, Gillis, and Nachtrieb pp. 525-526; pp. 615-616 Web Resources The Techniques section contains pages showing how to use the analytical balance, a pipet, a volumetric flask, and the spectrometer that you will use for kinetics measurements this week. Pages for this week's experiment show photos of the procedure. Review the use of these instruments and equipment, before coming to lab. Introduction The complex ion [Co(NH3)5(H2O)]3+ will react in a solution of sodium nitrite (NaNO2) and nitrous acid (HNO2) to substitute a nitrite anion for the water ligand. This may sound like a simple ligand switch, but the mechanism of the reaction is more complex. In this week's preliminary experiments, you will follow the ligand substitution reaction to elucidate the mechanism in a qualitative way. Next week, you will quantify this understanding of the reaction by determining the reaction orders and rate constants for the two steps of the reaction. The nitrite ion, NO2-, has two different resonance structures, each with the negative charge on a different oxygen atom: In coordination complexes, the nitrite ion, NO2-, is an ambidentate ligand, because it can bind to a metal ion in two different ways. In the nitro form, this ligand binds through the lone pair of the central nitrogen atom. In the nitrito form, the bond is made by donation of an oxygen lone pair. The structure of these two isomers is shown below:       Nitro linkage           Nitrito linkage These two linkage isomers of the NO2- ligand introduce a complexity to the mechanism that you will explore this week. Your preliminary experiments will distinguish between two possible mechanisms for the ligand substitution reaction: aqua nitrito nitro (1) aqua nitro nitrito (2) The goal of your kinetics measurements this week is to distinguish between mechanism (1), where the nitrito isomer forms first and then converts to the nitro isomer and mechanism (2), where the nitro isomer forms initially and then converts to the nitrito. The nitro and nitrito complex ions can be distinguished from each other by color. The visible absorption spectra of the aqua, nitro, and nitrito substituted pentaammine cobalt complexes are shown in the Figure. Note that the wavelength of maximum absorbance differs for the nitro and nitrito isomers. For the nitro and nitrito complexes, this means that the light transmitted by solutions of these two isomers will also differ, and they will be two different colors. The different absorption spectra of the two isomers and the aqua complex can be used to investigate the kinetics of the ligand exchange reaction by monitoring absorbance vs. time at a particular wavelength. For example, if the absorbance was monitored at 500 nm as a function of time, mechanisms (1) and (2) could be distinguished. Figure: Visible absorption spectra of two linkage isomers, [Co(NH3)5(ONO)]2+, and [Co(NH3)5(NO2)]2+ and the starting complex ion, [Co(NH3)5(H2O)]3+. Concentrations of all solutions are 6.7 x 10-3 M and path length is 1.0 cm.
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