University of Puget Sound Department of Chemistry Chem 230 EXP. 10 SPECTROSCOPY OF TRANSITION METAL COMPLEXES LABORATORY OBJECTIVES AND ASSESSMENTS 1. Understand how the color of transition metal complexes depends on the strength of the ligand. a. Use visible spectra to determine qualitative differences in ligand strength. b. Determine a spectrochemical series of ligands. 2. Understand how absorbance is related to energy attributed to Crystal Field Splitting in metal-ligand complexes. a. Measure absorption spectra of metal-ligand complexes to determine wavelengths associated with ligand strength. b. Calculate the energy associated with d-d electronic transitions (Δ o ). c. Determine a semiquantitative spectrochemical series of ligands. 3. Understand ligand binding in terms of Lewis structures. a. Draw the Lewis structure of a metal ion ligand b. Draw the 3D structure of an octahedral transition metal complex. c. Identify Lewis base/ligand groups. d. Recognize mono/multi-dentate ligands. INTRODUCTION Read Gilbert, Chapter. 17-8, pages 868-878. This experiment was adapted for this lab from Inorganic Chemistry, Gary Wulfsberg, University Science Books, 2000, pp 938-939. Many transition metal complexes are colored. The color arises due to the fact that these complexes absorb in the visible region of the electromagnetic spectrum. The energy of the absorbed light can be attributed to Crystal Field Splitting. In this lab, we will focus on transition metal complexes in which the metal adopts an octahedral geometry (recall VSEPR). Due to the presence of the ligands in this geometry, the d orbitals on the metal are split into two groups; 3 orbitals of lower energy (called t 2g ) and 2 orbitals of higher energy (called e g ). The color observed for the metal complexes can be attributed to the excitation of an electron from one of the t 2g orbitals to an e g orbital. The energy gap between the two sets of orbitals (Δ o ) can be calculated from the wavelength of absorption. In this lab, you will examine a series of complexes for a given transition metal ion and correlate the ligand with the splitting energy (Δ o ). eg dz2 dx2-y2 Δ t2g 3d orbitals in free ion Average potential energy of 3d orbitals raised in octahedral ligand field dxy dyz dxz 3d orbitals splitting in octahedral ligand field Figure 1. Splitting of d-orbital energies by an octahedral field of ligands. Exp. 10 Spectroscopy of Transition Metal Complexes 1
EXPERIMENTAL PROCEDURE WORK WITH PARTNER FOR EACH NUMBERED STEP CAREFULLY RECORD ALL OBSERVATIONS IN YOUR NOTEBOOK PART A. PREPARATION OF COMPLEXES AND THEIR ORDERING BY VISUAL INSPECTION 1. You and your partner will be assigned one of the following metal ions: Ni 2+, Co 2+, or Cu 2+. 2. Clean seven small test tubes and give them a final rinse with deionized water. Place the test tubes in a rack. To each test tube add 2 ml of 0.1 M solution of your metal ion. In turn, add 2 ml of one of the following solutions to each of the test tubes: 1) deionized water; 2) 1.0 M sodium glycinate; 3) saturated K 2 C 2 O 4 ; 4) 1.0 M NH 3, buffered; 5) 1.0 M pyridine; 6) 1.0 M EDTA; 7) 1.0 M buffered 1,2- diaminoethane (commonly named ethylenediamine or en, for short). Carefully stir each test tube, by holding in one hand and tapping the bottom of the test tube with the tip of index finger of other hand, to mix its contents. 3. Clearly label each test tube and record the color of each test tube. 4. Arrange the seven test tubes, by color, in order of the visible spectrum (beginning with the lowest energy color). In Table 1 record the resulting order of ligands. 5. Find the other groups in your lab that are forming the same metal complexes and discuss the following; Does the ordering of the ligands with your metal ion agree with the observations made by the other groups? If not, try to obtain some consensus before going on. 6. Now as a whole lab section, determine if the ligand ordering for Ni 2+ complexes is reasonably consistent with the ordering of the ligands for complexes of the Co 2+ ion and of the Cu 2+ ion. Try to make slight rearrangements in the ligand ordering such that they are more nearly metal-independent. Record this metalindependent listing of ligands in order. This is a qualitative spectrochemical series of ligands. PART B. COLLECTION AND ANALYSIS OF THE SPECTRA 7. There is uncertainty associated with the qualitative ordering of ligands due to the limited ability of our eyes to discriminate similar colors. Hence, the visual ordering will be checked by collecting the spectra of these complexes with a spectrophotometer. Each group assigned to the same metal ion will collect the UV-visible data for each of the seven ligands (solutions in test tubes). Please sure to record the wavelength of each peak. Note: Since many of the cobalt complexes undergo air oxidation, the Co 2+ group will have to remake their seven solutions and immediately collect the spectral data. Each member of each metal ion group will have 7 spectra, please make sure they are clearly labeled and that wavelengths of peaks are recorded. 8. DISPOSE ALL THE METAL SOLUTIONS IN THE APPROPRIATE LABELED WASTE CONTAINERS. 9. Convert the wavelength measurements to energy units of reciprocal centimeters using the equation: Exp. 10 Spectroscopy of Transition Metal Complexes 2
Wavenumber in cm -1 = 10 7 /wavelength in nanometers (1) 10. In the Cu 2+ complexes, only one (possibly asymmetrical) peak due to a d-d electronic transition will be observed. The energy of this transition in reciprocal centimeters is the octahedral ligand field splitting, Δ o. More than one d-d transition will be observed with the other metal complexes; however, the lowest energy peak in these complexes equals Δ o. Record Δ o for each the seven complexes. 11. The energy of Δ o can be approximately factored into two components, an f factor due to the ligand and a g factor due to the metal ion: Δ o = f ligand x g ion (2) By convention, an f factor of 1.000 is assigned to the water molecule as a ligand. From the Δ o value for the hydrated metal ion, compute the g factor for your metal ion, and record it. 12. Using the experimental g factor you just calculated, calculate the f factors for the other six ligands. 13. The f factors finally allow us to make the spectrochemical series semiquantitative. In Table 1 list the seven ligands in increasing order of their f values, indicating small and large differences between corresponding ligands with < and <<, respectively. 14. A ligand binds to a metal ion by donating a pair of electrons to the metal. Ligands that have more than one atom with a pair of electrons for donation can bind in a poly- or multi-dentate geometry. WHAT TO DO TO BE TURNED IN IN LECTURE ON MONDAY APRIL 30 THE FORMAL REPORT FOR Exp. 9- Evaluation Methods for Detecting NH 4 + : This is a formal lab report including a brief introductory paragraph stating the purpose of this lab in a few sentences as well as Results, and Discussion. Treat the report as an analysis of the two ammonium detection methods. You should include all pertinent information needed in determining which method to use for what level of ammonium. You should include a discussion on the relative pros and cons of each method such as sensitivity and interferences. See the Lab Info Moodle page or the handout for Exp. 9-Second Week for Details. For Exp. 10 Before Lab: Read this experiment and read Gilbert, Chapter 17-8, pages 868-878. Set up your lab notebook appropriately for this experiment including a title bar. You should also include the following information: A reagent table with the following reagents: Nickel(II) nitrate, Ni(NO 3 ) 2 6H 2 O Copper(II) sulfate, CuSO 4 5H 2 O Cobalt(II) chloride, CoCl 2 6H 2 O Sodium glycinate, Na + NH 2 CH 2 CO 2 - Potassium oxalate, K 2 C 2 O 4 Exp. 10 Spectroscopy of Transition Metal Complexes 3
Pyridine, C 5 H 5 N 1,2-diaminoethane, H 2 NCH 2 CH 2 NH 2 sodium ethylenediaminetetraacetate, (Na + ) 4 [( - O 2 CCH 2 ) 2 NCH 2 CH 2 N(CH 2 CO 2 - ) 2 ] ammonia, NH 3 BE SURE TO NOTE THE DISPOSAL PROCEDURES IN YOUR TABLE. Prepare the following table in your notebook before coming to lab. This table must be one full page, landscape-format, in your notebook. A Printout of the Report Sheet, see below. (Print back-to-back.) Table 1: Summary of data for the Transition Metal complexes of Color of TM complex Ligands Ordered by E (Visual) Low to High Wavelength(s) λ, nm Energy, cm -1 Δ o f factor Ligands Ordered by E (f-factor) Low to High Ligand H 2 O NH 2 CH 2 CO 2 - C 2 O 4 2- NH 3 C 5 H 5 N EDTA H 2 NCH 2 CH 2 NH 2 During the Lab: Carry out the procedure as written. Enter all relevant observations and data directly into your laboratory notebook. Complete your calculations and provide at least one set of sample calculations. Clearly and completely answer each question. Complete the class data table and prepare for the lab discussion. To be Turned In at the end of class: Staple in a neat packet the following; Report Sheet, and the copy pages containing 1) the completed summary table, 2) a sample set of calculations, and 3) observations associated with this lab Exp. 10 Spectroscopy of Transition Metal Complexes 4
REPORT SHEET -EXP. 11 Spectroscopy of Transition Metal Complexes Name Lab Section Q.1. Draw the line (Lewis Dot) structure representations for each ligand H 2 O, NH 2 CH 2 CO 2 -, C 2 O 4 2-, NH 3, C 5 H 5 N, [( - O 2 CCH 2 ) 2 NCH 2 CH 2 N(CH 2 CO 2 - ) 2 ], H 2 NCH 2 CH 2 NH 2. Include lone pairs on atoms that have them. Label each ligand as monodentate, bidentate, tridentate, hexadentate, etc... Q.2. a) What kind of acid-base interaction describes the type of bonding exhibited by the metal complexes? b) Which is acting as the acid (the ligand or the metal ion)? Which is acting as the base? Q.3. Each of the metal complexes formed were octahedral. Sketch each of the complexes using the following templates: Exp. 10 Spectroscopy of Transition Metal Complexes 5
Q.4. Comment on the periodicity of the semiquantitative spectrochemical series. Can you note any grouping of nitrogen- and/or oxygen ligands in the spectrochemical series? Which ligand atoms generally give the greatest Δ o? Q.5. Compare the order of your two spectrochemical series. Do the ligands come in the same order in the qualitative spectrochemical series you obtained by inspection in Part A as the f factors in Part B? Note the any differences. Exp. 10 Spectroscopy of Transition Metal Complexes 6