I6.1 EXPERIMENT I6 SUBSTITUTION REACTIONS OF METAL CARBONYLS Among the stable, isolated, organotransition metal compounds which have been studied, metal carbonyls are well represented. INFRARED SPECTROSCOPY Infrared spectroscopy of metal carbonyls has proved to be a valuable and convenient source of information concerning both structure and bonding. The high intensity of the bands attributable to the carbon-oxygen stretching mode in a metal carbonyl makes the I.R. technique particularly convenient. It is often possible to infer the symmetry of the arrangement of the CO groups from the number of CO stretching bands observed in their spectrum. The procedure consists of first determining from symmetry requirements how many CO bands ought to appear in their spectrum for each possible structure. The experimental observations are then compared to predictions, and thus structures which disagree are discarded. In favourable cases only one structure remains. This procedure is reasonably reliable, however, due regard must be given to the possibilities that bands may be weak or superimposed. A consideration of the expected CO stretching vibrations for cis and trans-ML2(CO)4 is shown below. O C cis L CO L C O L not I.R. L C O L O C CO C OC C O L C O CO L I.R. C O L CO CO L O C O CO I.R. OC C O L L C O L CO CO O C O CO I.R. OC C L L C O L trans C O C O OC C O not I.R. I.R. (same frequency) I.R. O C CO L I6.2 For the cis-isomer, all four are distinct and can absorb infrared radiation. For the trans-isomer, two are equivalent and have the same frequency, forming a degenerate vibration, only this one can absorb infrared radiation. Thus a simple measurement of the number of CO stretching frequencies allows a decision concerning the degree of substitution and whether a complex is the cis or trans isomer. Data for octahedral complexes are listed below in Table 1 and representative spectra are shown at the end of this experiment. TABLE 1 Octahedral Complex No.of CO Bands I.R. Raman M(CO)6 1 2 M(CO)3L 3 4 cis-M(CO)4L2 4 4 trans-M(CO)4L2 1 2 fac-M(CO)3L3 2 2 mer-M(CO)3L3 3 3 cis-M(CO)2L4 2 2 trans-M(CO)2L4 1 1 In addition, the position of the CO stretching frequency also provides information about the bonding modes and the extent of "back-bonding" in a metal carbonyl. Terminal carbonyls of neutral or cationic complexes exhibit bands in the region 21251850 cm1 while bridging groups absorb in the range 1700-1850 cm1. For example, the infrared spectrum of Fe2(CO)9 exhibits CO stretching frequencies at 2090, 2020, 1830 cm1. From this information alone it may be inferred that the structure must contain both terminal and bridging carbonyls. In using the positions of the CO bands to indicate the presence of bridging CO groups it is important to keep conditions in mind: the frequencies of terminal CO groups can be quite low if (a) there are a number of other ligands present in the complex that are good donors but poor acceptors or (b) the complex bears a net negative charge. Both of these effects tend to increase the extent of back donation from metal to carbonyl (i.e., increase the metal-carbon bond order) thus decreasing the carbon-oxygen bond order and CO stretching frequency. I6.3 A series of isolectronic species illustrating this trend (with their I.R. active CO stretching frequency) is: Ni(CO)4 (2060 cm1); [Co(CO)4] (1890 cm1); [Fe(CO)4]2 (179 cm1). EXPERIMENTAL You will be required to prepare two substituted molybdenum carbonyl compounds (consult your demonstrator to determine which of the experiments listed below must be performed). As some of the experiments involve working under nitrogen you will be introduced to the technique of synthesis under inert gas conditions. The experiments are performed with the microscale kits. I6.4 A. PREPARATION OF Mo(CO)4(PPh3)2 Dissolve Mo(CO)6 (0.1 g), NaBH4 (0.05 g) and triphenylphosphine (0.24 g) in ethanol (5 mL, heat up or add more ethanol if reactants do not dissolve completely) contained in a round bottom flask connected to a reflux condenser. Reflux this solution in air for 5 minutes. A yellow precipitate will separate from the solution. Cool the flask in a salt-ice bath for several minutes. Filter in air and wash the precipitate copiously with water. Then wash with small portions of ice-cold ethanol or ether (1-15 mL total) and dry on the water pump. Record your yields. Record the I.R. spectrum in chloroform (or nujol) (2100 - 1700 cm1 region). B. PREPARATION OF Mo(CO)4 (DIPYRIDYL) The 2,2'-dipyridyl is a bidentate ligand. Set up the apparatus as shown below. Degas toluene (5 mL) in a long necked round bottom flask with nitrogen. Consult your demonstrator about this task. Add Mo(CO)6 (0.075 g) and dipyridyl (0.045 g) to the flask and a boiling chip and reflux in the presence of nitrogen for 60 minutes. Allow the solution to cool and increase the flow of nitrogen during this process. (Why?) Filter the product in air. Wash with petroleum ether (5 mL) and dry. Record the yield and the I.R. spectrum ion nujol (2100 - 700 cm1 region). I6.5 REPORT Yields and interpretation of I.R. spectra must be reported. QUESTIONS 1. What are the molecular structures of the compounds you have prepared? 2. Why should one normally work under an inert atmosphere when preparing metal carbonyl derivatives? 3. List all possible geometrical isomers for a complex Mo(CO)3(A)(B)(C). 4. Draw structures for [(5-C5H5)Mo(CO)3]2, [(5-C5H5)Fe(CO)2]2, Fe3(CO)12 and Os3(CO)12. (Consult the library for information.) Are isomers possible for the first complexes? Explain. REFERENCES 1. R.J. Angelici, 'Synthesis and Technique in Inorganic Chemistry'. 2nd Edition, W.B. Saunders Company, Philadelphia, 1977, p 144. 2. D.M Adams, 'Metal-Ligands and Related Vibrations', Edward Arnold Publishers, London, 1967. 3. F.A. Cotton and G. Wilkinson, 'Advanced Inorganic Chemistry', 3rd Edition, Interscience Publishers, New York, 1972.