MASSACHUSETTS INSTITUTE OF TECHNOLOGY DEPARTMENT OF CHEMISTRY

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Co H2O 6 NO3 2 the six waters are coordinated to cobalt through an electron. pair on water a Lewis base in an octahedral array around the metal a cationic. Lewis acid Nitrates are relatively weak ligands and are not coordinated to the. metal in this case although they could coordinate in different circumstances. The coordination environment of the metal can be viewed most simply in. electrostatic terms which is the simplest theory of bonding in transition metal. chemistry called Crystal Field Theory CFT The lowest energy orbitals in a. cationic cobalt complex are the 3d orbitals followed by the 4s and 4p orbitals. Cobalt is in group 9 in the periodic table thus Co3 contains six electrons beyond. the last inert gas configuration In an octahedral environment the 3d orbitals are. split into two classes based on their energy The dxz dyz and dxy orbitals fall into a. class that is called t2g while the dx2 y2 and dz2 orbitals fall into a class called eg The. symbols eg and t2g denote a doubly degenerate and triply degenerate set of. orbitals respectively which are gerade i e the sign of their wave function does. not change upon inversion through the origin The orbitals pointing toward the. six ligands are higher in energy because the ligands have electron pairs pointing. toward the metal The relative energies of the two classes of d orbitals are shown. If six electrons are added such that they are paired up in the t2g set of orbitals the. energy difference between the eg and t2g orbitals is large with respect to the. energy required to pair up the electrons in the t2g orbitals the metal is said in. CFT to have the low spin d6 configuration or more accurately t2g 6 eg 0. Many octahedral transition metal complexes can be transformed into others by. ligand substitution reactions which may be either associative or dissociative. SN2 or SN1 respectively in terms learned in organic chemistry or sometimes. complex and subtle variations of these basic reactions The associative reaction. would proceed through a seven coordinate intermediate complex while the. dissociative reaction would proceed through a five coordinate intermediate. complex In general low spin octahedral complexes of Co III are relatively slow. to exchange ligands at the metal because the coordination number is relatively. high six thereby discouraging formation of a seven coordinate species for steric. reasons Also the metal electrons are in orbitals that lie between the axes dxy. dyz dxz thereby minimizing repulsion between them and the electrons on the six. ligands bases In contrast a Co II complex d7 therefore with electrons in the eg. orbitals is usually readily substituted For this reason we begin the synthesis of a. Co III species with Co H2O 6 NO3 2 a high spin d7 Co II species. B IR and Visible spectroscopy, Infrared Spectroscopy is a powerful tool for simply and quickly learning. something about many organic and inorganic species IR spectroscopy measures. the absorption of infrared radiation due to the vibrations of a molecule as a. function of their energy The specific number of vibrations can be determined. through a normal coordinate analysis of the possible vibrations in the molecule. for example a non linear molecule has 3N 6 vibrational modes where N is the. number of atoms in the molecule An IR spectrum can be acquired on a. gaseous liquid or solid sample For inorganic samples where only a qualitative. spectrum is desired it is often easiest to grind the sample in a relatively non IR. absorbing material such as solid KBr crystals which are subsequently placed. under high pressure to form a thin window or an oil such as Nujol a mineral oil. that consists of C20 C30 alkanes or Krytox fluorolube a grease made from. polytetrafluoroethylene to yield a paste which is either applied to a microporous. polymeric film such as polytetrafluoroethylene or polyethylene in a thin layer or. squeezed gently between two solid salt plates typically single crystals of NaCl It. is important that these materials do not, absorb IR radiation in the regions of interest This technique effectively provides. a spectrum of a solid unless the sample happens to dissolve in the grinding. material In the often complex IR spectrum there are frequently strong vibrational. modes that are relatively characteristic of a given type of bond e g the C O. stretch in a ketone or in carbonate CO32 or the N O stretch in nitrate NO3. Visible spectroscopy employs visible light which is higher energy than infra red. light Absorption of visible light by a sample again in the gas solution or solid. phase promotes electronic transitions Absorption of visible light gives rise to the. colors of many transition metal complexes and some organic compounds most. organic compounds are colorless or nearly so According to the Lambert Beer. law or sometimes simply Beer s Law the amount of light transmitted T by an. absorbing sample I Io I intensity is given by,T I I0 e A e cl 3 1. where the absorbance A is proportional to the concentration c in mol L of the. solute the length of the path the light travels through the sample l in cm and. the constant of proportionality called molar absorptivity coefficient units M 1. cm 1 or molar extinction coefficient which is characteristic of the sample and the. electronic transition vibrational if using infra red radiation in question Therefore. A cl Since the absorbance is directly proportional to concentration both IR. and visible spectroscopy can be employed to follow changes in the concentration. of an absorbing species involved in a reaction,C Kinetic studies. In this experiment you will follow the conversion of one compound into. another by, measuring the visible spectrum of the compound being consumed at a specified.
temperature as a function of time The reaction rate can be measured as the. change in concentration of a reactant x per unit of time t The change in. concentration of a reactant or alternatively a product is followed Therefore. As a differential the rate would be written as d x dt Therefore in a reaction in. which reagent x is consumed we might find that the rate depends upon the. concentration of x as shown in equation 3 3 where k is the observed rate. constant for the reaction in question under the conditions employed In general. however the rate might depend in a more complex fashion upon x and also. upon the concentration of other species such as a catalyst y which is not. consumed in the reaction equation 3 4 If a 1 then the reaction is said to be. first order in x, If b 1 2 then the reaction is inverse 1 2 order in y and so forth The observed. rate law will be consistent with the mechanism of the reaction i e a series of. elementary reactions An example of an elementary reaction is a simple collision. between two species It is often possible to think of several elementary reactions. each with its own absolute rate constant that taken together would yield the. observed rate law with the observed rate constant The units of the rate of a. reaction in solution are mol l 1 s 1 or M s 1 The rate of a reaction generally is. limited by a relatively slow elementary reaction Relatively fast elementary. reactions that occur before or after a relatively slow reaction are not observable. i e are not part of the observed law In a given solvent e g water generally it. cannot be determined to what extent the solvent itself is involved in the reaction. since its concentration is constant Therefore a rate constant is given for a. specific solvent at a specific temperature The Nobel Prize in chemistry in 1903. was awarded to Svante Arrhenius who proposed that the rate constant for a. reaction depended upon temperature according to equation 3 5. the Arrhenius equation where k is the rate constant A is the pre exponential. factor R is the universal gas constant 1 99 cal deg 1 mol 1 or 8 31 joules deg 1. mol 1 T is the temperature in Kelvin and Ea in kcal mol 1 or kJ mol 1 is the. activation energy for the reaction units must fully agree to cancel The activation. energy is the energy required by molecules reacting with one another to pass. through a transition state and yield a product or products If a reaction is well. behaved in a given temperature range a plot of ln k versus 1 T will produce a. straight line with slope Ea R and intercept ln A Rate constants and therefore. reaction rates usually are found to increase by a factor of 2 3 with every 10. degree increase in temperature Later and more advanced treatments in physical. chemistry courses will reveal more details about A but the simple treatment. shown in equation 3 5 is still correct and will suffice for now. IV General References, General Spectroscopy Skoog D A et al Fundamentals of Analytical. Chemistry 8th Ed Part V Spectrochemical Methods Chapter 24 Introduction to. Spectrochemical Methods and Chapter 25 Instruments for Optical Spectrometry. UV VIS Spectroscopy Skoog D A et al Fundamentals of Analytical Chemistry. 8th Ed Part V Spectrochemical Methods Chapter 26 Molecular Absorption. Spectrometry, IR Spectroscopy Mohrig J R et al Techniques in Organic Chemistry 2nd Ed. Technique 18, Kinetics Skoog D A et al Fundamentals of Analytical Chemistry 8th Ed. Chapter 29 Kinetic Methods of Analysis See also your general chemistry. textbook s section on kinetics, Coordination Chemistry See your general chemistry textbook s section on.
coordination compounds transition metal chemistry For more advanced. treatments of coordination chemistry see Miessler Gary and Tarr Donald. Inorganic Chemistry 2nd Edition,b TA Demonstrations. How to use balances,How to use a volumetric flask,FT IR spectrometer. UV Vis spectrometer,c Digital Laboratory Techniques Manual. 1 Volumetric Techniques,7 Filtration,11 Balances,V Procedure. You will first prepare two cobalt compounds Co NH3 4 CO3 NO3 and. Co NH3 5Cl Cl2 These reactions may be carried out in air However they should. be performed in a fume hood Each member of a pair of researchers will. individually synthesize Co NH3 4 CO3 NO3 As part of the pre lab calculate the. molar quantities of each reactant required for the two syntheses For each. synthesis specify the limiting reagent,Day 1 Synthesis of Co NH3 4 CO3 NO3.
In a 50 or 100mL beaker containing a half inch magnetic stir bar set on a. magnetic stir hot plate in a fume hood dissolve 4 0 g of Co H2O 6 NO3 2 in 10 mL. of water Helpful hint note how this volume appears in the beaker see below. In a small beaker dissolve 5 g of NH4 2CO3 in 10 mL of water and add 15 mL of. concentrated ammonia in water Add the NH4 2CO3 solution slowly with stirring to. the cobalt solution Observe the color change While stirring add 2 mL of 30. H2O2 dropwise over a period of about 1 minute, While heating and stirring concentrate the solution by allowing the temperature. solution measure it with a thermometer to reach but go no higher than 75 80 C. Add a total of 1 g of NH4 2CO3 in several small portions during the process of. reducing the volume to 10 mL A small final volume ensures that more of the. product will crystallize out Let stand undisturbed for 15 minutes then place the. beaker in an ice bath for 15 minutes Disturbing the solution will cause rapid. formation of fine crystals that are not as easy to filter off. Filter off the purple pink crystals using a 30 50 mL medium glass frit and wash. once with 3 mL of ice water then twice with 3 mL of ethanol Helpful hint turn off. the vacuum to add the wash solution and then turn the vacuum back on Dry in. vacuo if possible by placing a rubber stopper over the top of the funnel and. pulling a vacuum through the end of the funnel Release by shutting off the. vacuum holding the stopper and removing the tube to the vacuum Record the. weight repeating the drying process until the weight no longer changes Helpful. hint use two cork rings stacked to support the glass frit on the balance Store. the product in your desiccators Helpful hint be sure the desiccant is dry i e. free flowing Alternatively you may simply let the product air dry until the next. class period Calculate your theoretical yield based on the actual quantity of. limiting reagent used and your percent yield which should be in the range 35. 65 Clean the glass frit by running dilute hydrochloric acid followed by water. through the frit, Obtain IR spectra following the procedure demonstrated by your TA of the. product Co NH3 4 CO3 NO3 CaCO3 chalk and Co H2O 6 NO3 2 Be sure to. also obtain spectra of the materials used to prepare the sample for analysis. Compare the spectra obtained Identify the absorptions above 1300 cm 1 that can. be assigned to ammonia carbonate water and nitrate. Question You can readily identify the peak due to free or uncoordinated. carbonate in the, CaCO3 spectrum Without consulting the literature how can you determine which. peak is due to nitrate s NO stretch and which are due to carbonate s CO. stretches in Co NH3 4 CO3 NO3 Justify you answer,Day 2 Synthesis of Co NH3 5Cl Cl2. Partners should combine their products in order to perform the second synthesis. In a 100 mL Erlenmeyer on a stir hot plate dissolve 2 g of Co NH3 4 CO3 NO3 in. 15 mL of water and add 3 mL of concentrated HCl dropwise until no more CO2 is. expelled Neutralize with concentrated aqueous ammonia use pH paper to. indicate neutrality to avoid contamination of the cobalt solution use a glas. MASSACHUSETTS INSTITUTE OF TECHNOLOGY DEPARTMENT OF CHEMISTRY URIECA Chemistry 5 35 Module 2 Synthesis of Coordination Compounds and Kinetics 1 I Purpose of the Experiment This experiment is designed to introduce you to the following

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