I G. R. A. Hunt Glamorgan Polytechnic South Wales. U.K.
The Design of Integrated Inorganic Experiments An example from organo-transition-element chemistry
As indicated by recent articles in this Journal (1, 2) there is increasing interest in the use of "integrated exercises" in undergraduate practical chemistry. Though concepts of what constitutes an integrated experiment differ, the underlying reason for the use of this type of practical work is the conviction that a set of related syntheses, reactions, and characterization techniques sums to a greater learnine exoerience than an eaual number of unrelated experiments. he present author agrees with this reasoning hut feels that a number of factors require a closer examination of the criteria used for the planning of these exercises. Thouah the subsequent remarks are directed to the oarticular area of inorganic chemistry they should have application in other areas. The principal difficulty in designing any current inorganic course is well recognized to he the size of the field to he covered ( 3 , 4 ) .Where a semi-research project occupies a considerable portion of the time available for laboratory work, the problem of choice of the remaining practical material can become acute. Thus if integrated exercises are used i t becomes particularly important that they are well planned. Besides the usual elements of good exoerimental desian such as considerations of cost, safety, time, and the necessitv to illustrate sound experimental techniaue, the followine criteria are proposed fog use in planning integrated exercises. Design Criteria 1) The experimental subject or area should he an important one included in the descriptive chemistry of the lecture course. 2) The compounds chosen should be amenable to synthesis by the student and to further illustrative reactions. 3) The exercise should illustrate the best use of physical methods (such as ir or nmr soeetroscoov) in showine" how different methods give different or complementary information, or in showing that the results of different methods refer to the same phenomena. 4) The exercise should attempt to place the student in a real scientific situation by demanding (a) interpretation of results, (b) evaluation of the completeness of the data obtained and conclusions reached, ic) reference to original literature, (dl suggestions of suitable further experimentation. 5) It should preferably form part of an area of current research interest. 6) It should suggest the formulation of semi-research projects.
The Chemistry of [(h5-C5H5)2Fe(C0)2]21
The following series of experiments, recently introduced into the author's own teaching program a t Glamorgan Polytechnic, are proposed as an illustration of the way the above criteria could he applied in the field of organo-transition-metal chemistry. It was pointed out by Abel and Stone (5) that interaction between the two fields of carhonyl and organometallic chemistry has been very marked since the discovery of ferrocene (1951, and independently in 1952) and dihenzene chromium (1955). Certainly it would he difficult to envisage a course on organo-transition-metal compounds without previous coverage of the carhonyl and other P-acceptor ligands. A group of experiments set around the cyclopentadienyl carbonyl of iron (dimer) neatly bridges these fields. The following sections indicate the experiments involved
in the integrated exercise and outline the features of chemical interest which make them worthwhile. Full exnerimental details and safety precautions for the syntheses and reactions can be obtained from the references quoted. Synthesis The compound is prepared (6, 7) by refluxing under nitrogen the uncracked cyclopentadiene dimer (C5H,J2 with iron carhonyl Fe(C0)5 a t 135-140°C for 8 hr.2
If a heating mantle is used, only occasional attention is required so that the students can perform subsequent sections simultaneously provided they are given some previouslv oreoared startine material. The ourole-black crvs. . tals obtained are air s t a 6 e for several months and so make an ideal intermediate for orocedures which have to be carried out over several laboratory periods. Reactions Reduction with sodium amalgam in T H F under nitrogen (6,8) produces the carhonylate anion
-
[(h'-C,H,)FeiCO)ili+ ?Na/Hg 2[(h3-C,H,)Fe(CO),]- 2 ~ a '
+
+ 2Rg
This anion being strongly nucleophilic displaces halide ions from alkyl halides
+
[ih'-C,H:,)FetCO)Il, CH,,I-
THF N,
+
(hi-C,H,)Fe(CO)ICHt I-
The methyl compound is an orange-red solid (mp 7882'C). It is rather unstable in air but decomposition is not significant in the short time required to prepare a solution for infrared spectroscopy. Reaction with iodine in refluxing chloroform (9, 10) also causes fission of the dimer to give the cyclopentadienyl iron dicarhonyl iodide
Physical Methods lor the Study of Structure and Bonding
Magnetism Use of the Gouy balance (11) readily demonstrates that the dimer [(h5-C5H&Fe(CO)2]2is diamagnetic. This uhservation indicates the presence of an Fe- - - -Fe bond and oossible structures as shown in Fieure 1. This can he related to areas of study such as the mode of bonding of the carhonvl lieand. carhonvls - . the structure of ~olvnuclear . . . (5. . . 12). ~. and'the use of the "18-electron rule" in organo-metal reactions and homogeneous catalysis (13). 0
"l'hr nomrtdatur~I I S here ~ ts dui- 11. F. A. Cd intensity, i the individual carbonyl dipole vector and R the resultant vector. (See Fig. 3 (b) and (el.) Substituting 4 = 7 O gives I,,,/I = 66.32. Thus the band C. which was oreviouslv assiened to the cis isomer above, is due almostcompletely touthe symmetric stretch vibration. Band T then can be assieoed to the asvmmetric CEO stretch of the trans isomer. This assignment is confirmed hv the chanee in the relative intensities with the polarity o i t h e solvent. In polar solvents the equilibrium will he expected to shift in the direction of the-cis isomer, which has a dipole moment. This shift is indicated by the increase in intensities of hand C going from CS2 to the more polar relative to hand CHClj (Fig. 2). Since the immediate nei~hhorinz to each termi" lieands " nal carbonyl are the same in hoth cis and trans isomers, and since their stretching frequencies are close (indicating similar force constants), it is reasonable to assume that the dipole vectors i. have similar magnitude in both isomers. It then follows (See Fig. 3 (b) and ( d l ) that
.,,
on
Volume 53,Number
1. January 1976 / 55
where t is the nronortion of trans isomer in the eauilihrium mixture. ~ e a s u r e k e n tof the hand heights in the spectra then allows t to be calculated for the particular solvent. Proton NMR Spectroscopy
Variable-temperature nuclear magnetic resonance (22) has been used extensively in the investigation of fluxional organometallic compounds (23, 24). The proton nmr solution spectrum of [(h5-CsHs)2Fe(C0)2]~ was originally investigated (15) using a mixed C6D5CD3:CS2 solvent system. We have found that good spectra can be obtained without the use of expensive deuterated solvents (Fig. 4). The spectrum in CSv solution at room temperature consists of a single resonance absorption peak, 4.64 ppm downfield from TMS. This singlet arises since the interconversion of the cis and trans isomers occurs a t a rate which is fast compared to the difference in resonance frequency of the cis and trans isomer protons. The singlet then appears at the weighted averaee uosition of the two individual resonances. The contrast bekveen the separate hands observed in the ir spectra and the singlet in the nmr serves to emphasize the difference in time scales of the vibrational and nuclear transitions. As the temperature is lowered the rate of exchange slows and at - 7 5 T two distinct peaks are observed (Fig. 4 I). These signals continue to separate as the temperature is lowered-though crystallization occurs before a slow-ex-
change, low-temperature limit to the peak separation can be established. It can be seen from the spectra (Fig. 4 I, 11, 111) that the low field peak C decreases in area relative to the hieh-field neak T as the temuerature rises. Therefore a t rook temperature peak C shoild correspond to the isomer havine the lower concentration. The ir results (Fig. 2) indicate &at in CSz solution this is the cis isomer. This correlation can he confirmed, as in the ir exueriments, by adding polar solvent and so increasing the dielectric constant (c) of the solvent. Addition of CD7C17 9 at 25'C) . .( r and (CD3)2C0 (e u 21 at 25°C) causes a dramatic change in the relative intensities of the two peaks in the nmr spectrum a t -80°C (Fig. 4 IV, V, VI). As expected from the ir results, the lower-field peak, C, is confirmed to be that due to the cis isomer. A plot of 1JTemperature against the log of the ratio of the peak areas is linear and can be extrapolated back to room temperature, so giving an estimate of the ratio of cis:trans forms. The rather narrow range of temperature (about 40°C) available between peak separation and crvstallization of the solute limits the accuracy of the estimatk by the nmr method. In the author's lahoratory, the ir and nmr results converge on a value of cis:trans ratio = 0.8 in CS2 solution at room temperature. This agreement is important in that i t establishes that both methods are relating to the same fluxional system. Addition of the shift reagent Eu(fodh3 (-0.1 M in the CS2 solution) shifts the singlet obtained a t room temperature further downfield from TMS. This suggests that coordination occurs between the Eu(~"'complex and the carhonyl (25). In contrast, the methyl compound, (h"C5H5)Fe(C0)2CH3,gives no shift in ring proton resonance position, indicating that the E u ( " ~coordinates ~ through the bridging carbonyls of the dimer. This is an interesting example of the application of Pearson's Hard Soft Acid-Base (HSAB) theory (26). The oxygen end of the CO ligand being hard would, according to the HSAB principle preferably coordinate to hard acids such as Eul"') and Al("'). Perhaps not surprisingly, therefore, the first examples of oxygen-bonded metal carbonyls were the complexes prepared by Shriver and his group, of the type [(h.'CsH5)2Fe(CO)2]22AIEt3(27). X-Ray analysis of this compound (28) showed also that only the bridging carbonyls were basic enough to coordinate to the All1"'.
--
Use of the Literature
The interested student who follows up the literature references on this comnound will note that the soectrosco~ic data is as yet far from fully explained in a satisfactory way. For examnle., one of the few references in the literature to the Raman spectrum of [(bS-C5H5)2Fe(C0)z]z(29) would reauire band T (see Fie. - 2) to be assigned - to the cis isomer, since there is a coincidence in the Raman and ir spectra a t 1958 cm-I! Also the assignment of the hands in the nmr to cis and trans forms seems to be incorrectly made (15).The situation is thus one which confronts the student with the usual pattern of real scientific progress, namely a partial conflict of available evidence which in turn suggests the line of further experimentation. The exercise above thus has the flexibility needed to allow students with a range of abilities to show how well they are able to assess the data presently available and to suggest other experiments.
.
Proiects
Figure 4. Proton nmr spectra of [(hS-CsHsFe(C0)2]2in CS2 solution at low temperatures: I, 11. Ill show the effect of lowering the temperature below -75'C. IV at -80% V at -80% plus 5 % CD2C12: VI at -80'C plus 3 % (CDd2CO.
56 1 Journal of Chemical Education
The further experimentation suggested by the students themselves can lead to the formulation of a semi-research project. Also the dinuclear cyclopentadienyl carbonyl which has formed the subject of this paper has been the starting point of at least two active research fields. One, as indicated above, concerns the oxygen-bonded adducts of polynuclear carbonyls (30). The other concerns the structural and kinetic investigation (31) of a series of related isoelectronic compounds [(h"-CsHs)MLz]z, (where M = W,
Mn, and Fe, and L = CO and NO) and also compounds of (32). Both these the type (h5-CsH5)2Fe2(C0)3.(PO(C6H5)3) fields provide an abundance of material for semi-research projects which could follow the foundation laid by the integrated exercise. Another suitable area, not included in the above experimental scheme for the integrated exercise, concerns the calculation of rate constants for the exchange in the fluxional system and hence the deduction of the thermodynamic parameters of the exchange process. This can be done by comparison of the variable-temperature nmr spectra with computer-generated spectra (16,33,34).
,.".",. ,,a"",
161 Anpliei. R. J., "Synthesis & Technique in Inorganic Chemistry." W. B. Saundcm, Philadelphia. ,968, p. 131. (71 King, R. B., "Orgmornetallie Syntheaea,"Vol. I, Academic Press. New York, 1965. n l d.. -. l~.
(8) (91 (101 ill1 (12) (19) (14)
Conclusion
These experiments on [(h5-CsH&Fe(C0)2]2 are an attempt to apply the criteria given above to the problem of designing an integrated practical exercise for the undergraduate inorganic course. Since the exercise must be presented to the students in a way which challenges them to work critically, and in conjunction with given literature references confronts them with a real scientific situation, then it is clear that the form of the laboratory sheets used must also contribute im~ortantlvto the success of the exercise. (Since requirements will vary in different departments, details of method sheets are not given here but sample sheets for the above exercise will be supplied on request.) The author h o ~ e sthat this DaDer . . will encouraee others to submit further integrated exercises in other fields of inorganic chemistry, so that a range of such experiments would eventually become available.
-
Literature Cited
131 "Rcpurt uf the Inorganic Chemistry Suh~Commifloaof the Curriculum Committee." J . CHEM. EDUC.. 49.526 (1972). (41 Gorman. M., J. CHEM. EDUC.. 50,772 (1973). (51 A b d E. W.. end Stone. F. G. A , Quart. Rau. Chem. Snc. 23. 325 11969); 24. 498
1151 (16) (17)
1181 (19) 1201
Ref. (71.p. 15L. Wiggins. P. W..Edur. Chem.. lO,51(1973I. Kleinborg, J., "Inorganic Syntheses: Vul. 7. McCraw-Hill. New York,1913. D. 110. Pans, 6..snd Sutcliiie, H., "Practical Inorganic Chemistry." 2nd Ed.. Chapman and Hall, London, 1974.~.206. Kae8z.H. D.. C h m B r i t . 9.344 ( ~ 9 1 3 ) . T d m a n , C. A.,R.LC. X r u . 1.337 (1972). Manning, A. R., J. Chem.Sor., A. 1319 (1968). Ruliift, J. C., Cott0n.F. *..and Mark8.T. J., J . A m w Chem. Sac. 92.2155 119701. Bullitt, J . G.,CnfUm,F.A..and M8rks.T. J..lnorg. Chem.. 11.671 (19721. Cotton. F. A. and Wilkinron. G.. "Advanced Inorzanic Chmisuu.". 3rd Ed.. Wilev~, 1nieraci&. New York. 1912. 36. Nuack. K. J..lnon. Nuci.Chom. 25,1383 11968). Mills.0.S.. Act. Crysiollng~..11.620i19581. Davidron. G. D., "lntmductory Croup Theory for ChemiaU." Elsevier, Londun. 1971. Coftnn, F. A,. and Wilkinson, G.. "Advanced Inorganic Chcmisvy." 3rd Ed., WileyInleracionce, New York, 1972, p. 695. Bovey, F. A , "NMR Speefrureapy." Academic Press, New York, 1968. Chap. 7. Cottun, F. A.,AccounlsChem. R e s , 1,257 (19681. Vrieze, K., end van Lwuwen. P. W. N. M.. Progr. Inorg Chsm.. Vol 14, Wiley-InUrreienca. 1989, p. 1. Merkr,T. J..elal.. J . Or!lanomefc~l.Chrm.. 33,C35 (1971). Pearwn, R.G., J. A m w Chem Soc.. 85.3533 11963). Shriver.D. F.. Chem. Arit.. 8,419 119721. Nelson. N. J..ef =I.. J . A m s r Chem S o r , 9I,SL73(L9691. Cuttun, F. A.. Sfsmrnreich. H., and Wilkinson, G., J. lnorr. N u c l Chem.. 9, 3
b.
.
.
,19591~ . .
1:lOI Kr1sthuff.J. S.. and Shrive?. D. F., lnorg. Chem., 13,499i19741. 1211 Kirehner. R. M.. Marks. T. J.. Krirthoff. J. S.. and lbers, J. A.. lnurg Chem.. 13.
.
,A""
.,.
,,a7*, ,.".
(12) Ref. i?ll. 13. 1407 11974). 1:Ill Whiloiidos. G. M..andFleming. J. S.. J A m r i Chem Soc.. 89.2655 11967). 1141 Cottan. F. A.Fa1i.r. J . %and Musc0.A.. J Ampr Chem Sur.. 90.1438 119681
11) Rme,T. L..md Seyse.R. J.. J. CHEM. EDUC.51.127 119741. (21 Barnett. K. W.. J.CHEM. EDUC.. 51.422 (1974).
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