Class Note for CHEM 242 at UMass(2)
Class Note for CHEM 242 at UMass(2)
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Date Created: 02/06/15
Understanding the trends in ligandfield splitting is simplified considerably by considering a series of complexes with the same metal in a given oxidation state the only major variable in this case is the ligand identity From a large number of studies it is known that ligands can be arranged in a sequence according to their ability to cause d orbital splitting This series is known as the spectrochemical series halides lt NC 39 lt OHquot lt oxalate lt H20 lt ECS39 lt pyridine lt NH3 lt en lt E02quot lt 2Nquot lt 20 Underlined atom is the one coordinated to the metal Note the general trend shown here halide ligands are weakerfield ligands than sulfur donors which are weaker field than oxygen donor ligands which are weakerfield than nitrogen donors which are weakerfield than multiplybonded ligands The stronger the interaction of the ligand with the metal s dorbitals the stronger the field strength of that ligand The magnitude of ligand field strength increases by a factor of about two as one moves from halide to CN39 in the spectrochemical series The structure of the oxalate ion C2042 is shown below Note the resonance forms of this compound 390 039 00 0 0 0 O 0 O o 0 00 00 Figure 2 Structure of the oxalate ion The objective of this experiment is to quantify A0 for a series of CrIII complexes by electronic absorption spectroscopy CrIII compounds are d3 and their electronic spectral characteristics are reasonably easy to interpret Because of electronic selection rules see lecture the absorption band corresponding to the energy of the crystal eld strength Ao will be the one at the longest wavelength lowest energy in the spectrum and it should be more intense than any other nearby transition Ordering the octahedral CrIII compounds from longest to shortest wavelength will place the ligands in order of increasing crystal eld strength as A DC lAt7 and will allow you to build your own spectrochemical series In mixedligand complexes the Rule of Average Environments states that the observed value of A0 in such complexes is the weighted average of A0 for each of the homoleptic single type of ligand complexes The rst equation below is general the second equation is for the specific example of CrHzO4Clz If the Ads of CrHzO4Clz and CrHzO53 are known then by rearranging the equation you can solve for the A0 of CrC153 You can use this A0 to nd the A0 of other unknown complexes AOMAan 16 nA0MA6 onMB 1 A0CrH204C12 16 4A0CrH2063 2A0CIC16339 2 Syntheses In this experiment the bidentate ligand acetylacetonate acac39 will be generated via the deprotonation of acetylacetone acacH by ammonia The ammonia is generated by hydrolysis of urea Figure 3 subsequently ammonia acts as a base to deprotonate acacH Note the resonance forms of acac O k H20 gt 2NH3 C02 H2N NH2 O O H o o o 0 H30 C CH3 H3O c CH3 H3O c CH3 H2 H H Figure 3 Top Hydrolysis of urea Bottom Deprotonation of acetylacetone forms the bidentate ligand acetylacetonate acac Systematic name is 24pentanedionate Experimental Procedure You will work in pairs in this lab to prepare two compounds Cracac3 and Cren3C13 2H20 One of the pair will synthesize one compound and the other partner will prepare the other compound The procedure for the syntheses for both compounds must be written in your prelab You will then share experimental and spectroscopic data for all the compounds not just for the two that you prepare Another compound CrHzO5N033 3HzO will be provided to you you will spectroscopically analyze both this complex and your starting material CrC13 6HzO this compound is actually CrClzHzO4Cl 2HzO Finally the electronic spectrum of CrNH35Cl C12 will be provided on the class web page for your use in the Data Analysis section of your lab report How to Heat a Reaction with a Sand BathMagnetic Stirrer In this experiment both syntheses require that you heat your reaction mixtures to boiling or to quotre uxquot in a sand bath placed on top of a magnetic stirring plate Getting the reaction mixture to stir well while heating can be problematic so use the following procedure Place an empty beaker containing a stir bar on the magnetic stirrer and turn on the stirrer Check to see that the bar can stir quickly and well If it cannot use a di erent magnetic stirrer Place your sand bath on top of the magnetic stirrer and bury your ask containing the stir bar and the solution to be heated deep into the sand Turn on the magnetic stirrer very slowly and try to get the stir bar to stir vigorously There is often a problem in accomplishing this because the stir bar is too far away from the magnetic stirrer to respond well to it To correct this take sand out of your sand bath place it in a beaker and RETURN IT to the sand bath when you are finished and move your ask down towards the bottom of the bath until it almost touches the metal bottom of the sand bath The closer the bottom of the ask comes to the bottom of the sand bath the better the stir bar will stir Do not however let the bottom of your ask come into direct contact with the metal of the sand bath the ask may melt Again turn on the magnetic stirrer very slowly and keep removing sand and moving the ask down into the bath until the bar is stirring vigorously and well Now heap the extra sand back into the bath underneath and around the shoulders of the ask to help transfer heat and encourage quick heating of the solution When your reaction mixture is vigorously stirring and the extra sand has been replaced around the shoulders of the ask turn on the sand bath and heat your reaction mixture Make sure if possible that your reaction mixture is stirring vigorously throughout the heating period If the stirring stops repeat the above procedure until you can get efficient stirring again A Cracac3 the preparation of T139is24 pentanedionatechr0miumIII Dissolve 260 mg of CrC13 6HzO in 40 mL of distilled water in a 25 mL roundbottomed ask and then add 1 g of urea to the ask Measure out 08 mL of acetylacetone using the lmL graduated pipette provided and add the acetylacetone to the ask also Next place a re ux condenser on the ask after first applying a thin layer of stopcock grease to the male ground glass joint of the re ux condenser Re ux your reaction with stirring for one hour using a sand bath As the urea releases ammonia and the solution becomes basic deep maroon crystals begin to form After one hour cool the ask thoroughly in an ice bath Very thorough cooling is needed If no crystals form carefully make a small scratch on the bottom of the ask using a glass rod or metal spatula and cool the ask again Collect the crystals by suction ltration and wash them with three 03 mL portions of distilled water Dry the crystals as much as possible on the lter using the aspirator and then collect them and spread them on a piece of lter paper to air dry Determine the percentage yield and transfer the Cracac3 to a labeled vial B Cren3C132H20 the preparation of TI39isethylenediaminechr0miumIII Weigh out 100 mg of mossy zinc into a 25 mL roundbottomed ask Remove the surface layer of ZnO and generate a clean and active Zn0 surface by washing the Zn with HCl immediately prior to use To accomplish this add enough 6M HCl to cover the mossy zinc swirl the ask brie y the mossy zinc may completely dissolve upon prolonged contact with HCl and pipette off the acid Then wash the mossy zinc three times with 10 mL portions of methanol to remove as much acid as possible Weigh out 266 mg of CrC13 6H20 and add it and 1 mL of methanol to the roundbottomed ask In the hood add 1 mL of ethylenediamine Next place a re ux condenser on the ask after rst applying a thin layer of stopcock grease to the male ground glass joint of the re ux condenser Re ux your reaction with stirring for one hour using a sand bath Take care not to turn up the sand bath power too high methanol re uxes at a lower temperature 65 0C than water 100 0C Cool the purple solution in an ice bath Collect the yellow crystalline product it will appear purple because it is wet with the solution by suction filtration using a Hirsch funnel Remove any unreacted zinc with tweezers Wash the filtered product with 05 mL portions of 10 ethylenediamine in methanol until the purple solution is gone the product is yellow and the washings are colorless Follow this with a 05 mL rinse with ether to rinse away the methanol and help dry the product Dry the crystals as much as possible on the filter using the aspirator and then collect them and spread them on a piece of lter paper to air dry Determine the percentage yield and transfer the Cren3C13 2HzO to a labeled vial C Spectroscopy of the CrIII Complexes Prepare aqueous solutions of Cren3C13 2HzO and CrHzO5N033 3HzO by dissolving approximately 5 mg of each complex in about 5 ml of distilled water Because the analysis of A0 the objective of this experiment requires only that the longestwavelength km of each complex be identi ed you do not need to know the exact concentration of the solution as you do when calculating extinction coef cients using Beer39s law Also prepare a similar ethanol solution of Cracac3 Make a similar solution of CrC13 6HzO in water but note that this complex consisting of CrClzHzO4 ions in water slowly substitutes to the hexaaquo species CrHzO53 so it should be analyzed immediately after its preparation Do not prepare this solution until your name is called for use of the UVvis instrument At that point quickly add alreadyprepared portions of CrC13 6HzO and water to a cuvette mix the solution a few times by drawing it up in a Pasteur pipette and then acquire the spectrum of this solution rst Obtain the electronic absorption spectrum of each complex Record the wavelengths and absorbance values for all absorbances for each sample in your notebook Determine and record the longest wavelength of the absorbance peaks for each complex in units of nanometers This is the wavelength you will use to calculate the Ads of the complexes in the Data Analysis section of your lab report Data Analysis 1 3 points Draw the crystal eld energy levels and electron occupancies for octahedral Cr3 ions Indicate the energy gap that corresponds to the transition that is being investigated in this lab 2 10 points Prepare a table of your data including columns for max in nm and A0 in cm39l eV and kJmol a Convert the wavelengths which correspond to A0 into wavenumbers cm39l using the following relationship A0 lA nm 1 x 107 cm391 b Other energy units for A0 may be obtained using the following conversion factors lcm391 124 x104 eV 001196 kJmol c Arrange the entries in order of increasing gap energy Show your complete set of calculations for one of the complexes For the rest of the complexes just record the calculated values in the table 3 12 points List the five ligands in order of increasing ligand field strength Use the Rule of Average Environments to calculate the true ligand field strengths of two of the ligands to be included in the list above Show your complete set of calculations Discussion 1 10 points Discuss the order of the ligand field strengths you obtained comparing them with the spectrochemical series given in the introduction Does the order of ligands obtained by this experiment correspond to the established order of the spectrochemical series Explain any deviations 2 5 points List the five ligands in order of the increasing strength of their interaction with Cr and give the reasoning behind your answer 3 5 points The ligand acac39 is not included in the spectrochemical series given in the Introduction Examining the chemical structure of acac39 and of those of related ligands shown in the spectrochemical series explain where you would expect to find acac39 in the spectrochemical series Does your calculated ligand eld strength for acac39 agree with this prediction Conclusions 10 points Discuss what knowledge has been gained by this experiment Discuss the importance to inorganic chemistry of being able to ascertain the crystal eld energy gap of complexes and the ligand eld strength of ligands What information can be obtained or what advantages in other experiments can be gained about inorganic complexes using these two quantities Questions 1 5 points What is a signi cant difference that is seen between the the UVVisible spectrum of Cracac3 and the spectra of the other complexes What is the reason for this difference 2 5 points Highspin MnII and FeIII complexes are much less intensely colored than those of CrIII Why are they so weakly colored Exp enmem 4 Smdying Lhe Spectmchemiczl Series Crystal Fields nf cyan lnnuduninn Camdnnmm campamds afkanshmmn s are mth calmed Th2 calm results rm ahsmphm ufhgut e epeeene wavelenghs afvmble hgmt ssaemeawem decuamc kmhmswxhnthz mums Thus Lhasa tidkmsuans 9v mmymeummem mns39hax ehmaeme calm 2g cabaltblue Th 2 mm afameul mnm an Dashednl cysts eldslxmmdedbyan acuhz al may ufhgnnds are 99mm mg mugye set and alawex emxgyt 521Fxgure 1 Thnsxs an m 212mm clauds namaeeehngmaa dembmzmg chase dmhmuhathe slang me x v andZ axes The mugy exence between me we andlwwex emrgylevelsxs daaguaud 5 An pmmuneeaae em a 1an Figure 1 timbtals sphtbyan acuhednl ayml em The aegee af sphmug uf me 1 mm and heme me magnum uf AD depends an several facmxs bunhz mammpamm are we charge an me metal and me xdennw uf me hgmd
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