In the Laboratory
Separation and Identification of a Mixture of Group 6 Transition-Metal Carbonyl Compounds Using GC–MS in the General Chemistry Curriculum
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Lawrence K. Fong Department of Chemistry, City College of San Francisco, San Francisco, CA 94112;
[email protected] The current literature for undergraduate experiments utilizing gas chromatography coupled with mass spectroscopy (GC–MS) has, for the most part, been geared for the typical second year and beyond chemistry classes (e.g., organic chemistry, quantitative analysis). Very few experiments have been designed for the first-year general chemistry curriculum (1). In our second semester of general chemistry, we utilize GC–MS in our studies concerning transition-metal complexes. A characteristic feature of transition metals is their ability of form compounds in which carbon monoxide acts as a ligand. These types of complexes are commonly referred to as transition-metal carbonyl complexes. Many of these compounds serve as important starting materials for the syntheses of other transition-metal complexes, as well as having importance in structure analyses and industrial processes. Whereas the complexes studied consist of only mononuclear species (e.g., Ni(CO)4), polynuclear transition-metal carbonyl compounds also exist (e.g., Co2(CO)10; ref 2, 3). Our students are asked to study the GC–MS data for the group 6 transition-metal carbonyl compounds, Cr(CO)6, Mo(CO)6, and W(CO)6. These complexes were chosen for several reasons: • Unlike most undergraduate experiments utilizing GC–MS, these are not the classical organic-type molecules; this avoids having to cover basic organic chemistry prior to studying mass spectroscopy. • The complexes are soluble and volatile. • The complexes are readily separated using gas chromatography. • The fragmentation patterns for these complexes are particularly easy to understand (unlike organic molecules of similar molar mass). These complexes fragment by sequential loss of CO to ultimately form the transition-metal cation, M+ (4). • Transition metals exhibit numerous isotopes; this allows our students to calculate the atomic mass based on the natural abundance of the isotopes (5). In addition, the atomic weight of the transition-metal complexes can be approximated without considering the minor carbon and oxygen isotopes. • Because of the “clean” fragmentation patterns, fragments with a 2+ charge can also be observed in the mass spectrum.
The students are asked to: 1. Determine the identity of the transition-metal complexes by molar mass determination. 2. Determine the quantity of Cr(CO)6 and W(CO)6 in an unknown sample utilizing Mo(CO)6 as an internal standard.
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3. Analyze the mass spectrum of W(CO)6. 4. Determine the atomic mass of W using the mass spectral data.
Learning Outcomes In completing this experiment, students will learn: (i) the use of gas chromatography as a means of separating a mixture of compounds, (ii) the use of mass spectroscopy to identify and determine the abundance of isotopes, (iii) the determination of the mass of a molecule or atom, and (iv) the use of an internal standard in quantitative mass spectral analysis. Experimental Logistics Our lab class typically accommodates 20–24 students and runs for a period of three hours. We currently have a total of three labs, but only two are held at the same time. Nevertheless, there arises the question of how all the samples can be run during the laboratory period. Typically, the instructor illustrates to the class how the data are entered into the PC sequence log table. Following this, he/she will initiate the first set of runs (our autosampler holds a total of eight samples). Each run requires a total time of nine minutes. As such, each complete set of four runs for each group of students (2–3 students per group) takes a total of 36 minutes. Because of the large number of students, the chemistry laboratory assistants continue to run samples after the laboratory time is over. The data are collected on floppy disks, and students will use the following laboratory period to analyze their data. Once our students have submitted their samples, they are given mass spectral data of the pure metal carbonyl compounds to analyze during the remainder of the laboratory period. Hazards Hexane is extremely flammable and must be kept away from heat, sparks, and open flames. Avoid breathing hexane vapors. All stock solutions should be prepared in a fume hood by the laboratory instructor. The transition-metal carbonyl compounds are toxic by inhalation. Skin contact with metal carbonyls should be avoided. These materials should be handled with gloves in an adequate hood. Despite the toxic nature of transition-metal carbonyl compounds, the small quantities used in the sample analyses minimize safety concerns. Care should be taken to avoid undue exposure of the metal carbonyls to light as these materials are photochemically sensitive.
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In the Laboratory
Figure 1. Mass Spectrum of W(CO)6.
Analysis
Identification of the Metal Carbonyl Compounds To determine the identities of the metal carbonyl compounds, the students may either run a GC–MS of the pure material, or, as is done in our labs, be allowed access to computer files that contain prerecorded data. To facilitate the determination, students are given a list of the possible metal carbonyl compounds. By matching up the atomic masses with the parent peaks, they are able to identify the compounds. The possible identities are: V(CO)6, Cr(CO)6, Mo(CO)6, W(CO)6, Fe(CO)5, Ru(CO)5, Os(CO)5, and Ni(CO)4. Under the conditions described in the Supplemental Materials,W the following retention times were observed for the transition-metal carbonyl compounds: Cr(CO)6–2.9 minutes; Mo(CO)6–4.8 minutes; and W(CO)6–8.7 minutes. The observed parent peaks in the mass spectra were as follows: Cr(CO)6+ m兾z = 220 amu; Mo(CO)6+ m兾z = 264 amu; and W(CO)6+ m兾z = 352 amu. Fragmentation Pattern for W(CO)6 The majority of metal carbonyl compounds undergo similar fragmentations in the mass spectrometer. In particular, the sequential loss of a carbon monoxide ligand from the parent ion, 184W(CO)6+ (m兾z = 352), is observed (Figure 1). With the successive loss of CO, fragments are observed at m兾z = 324, 296, 268, 240, 212, and 184. These peaks are due, respectively, to the following: 184W(CO)5+, 184W(CO)4+, 184 W(CO)3+, 184W(CO)2+, 184W(CO)+, and 184W+. About
Table I. Experimental Isotope Abundance and Atomic Mass for Tungsten Isotope
Relative Abundance
Fractional Abundance
Atomic Mass/ amu
180
W
—
—
—
182
W
4522
0.2691
181.9
183
W
2385
0.1419
182.9
184
W
5130
0.3053
183.9
186
W
4768
0.2837
185.9
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each peak, there appears a unique pattern of peaks. These are due to the natural abundance of the isotopes of W and such information will be used to determine the atomic mass of W. Other interesting peaks are observed in the mass spectrum. In particular, some doubly charged ions are observed (6). For instance, peaks are seen at m兾z = 176, 162, 148, 134, 120, 106, and 92. These correspond to 184W(CO)62+, 184 W(CO)52+, 184W(CO)42+, 184W(CO)32+, 184W(CO)22+, 184 W(CO)2+, and 184W2+.
Determination of Atomic Mass of W The atomic mass of W can be calculated from the fractional abundances of the isotopes, fi (7). The atomic mass is the mass of the weighted average of the isotopes and can be expressed as follows, Atomic mass = ∑ fi Mi where Mi is the atomic mass of the isotope. Given the sensitivity of our instrument, not all of the naturally occurring isotopes of W are detectable. This being the case, of the five naturally occurring isotopes of W, our students are told to look for the following four: 182W, 183W, 184W, and 186W (180W is not detectable). From the tabulated printout of the relative abundance and atomic mass (Table 1), the atomic mass of W is determined to be 183.79 amu. This compares to the literature value of 183.85 amu (0.033 % error; ref 8).1
Determination of the Concentration of Cr(CO)6 and W(CO)6 in the Unknown Mixture To determine the concentration of Cr(CO) 6 and W(CO)6 in an unknown mixture (9), calibration curves are prepared for each metal carbonyl with Mo(CO)6 as an internal standard. Our students generate these curves by preparing three solutions (in hexane) that contain known quantities of Cr(CO)6, Mo(CO)6, and W(CO)6. In addition, specific ions for each metal carbonyl are targeted to determine the quantity of the metal carbonyl in solution. In order to minimize confusion, the parent ion peaks for each metal carbonyl compound were chosen. To detect for Cr(CO)6, the peak at m兾z = 220 amu is targeted. For Mo(CO)6, the peak at m兾z = 264 amu is targeted and for W(CO)6, the peak at
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In the Laboratory
m兾z = 352 amu is targeted. Data from the three solutions are used to make a graph of [Cr(CO) 6 ]兾[Mo(CO) 6 ] versus Cr(CO)6 abundance兾Mo(CO)6 abundance, as well as a graph of [W(CO) 6 ]兾[Mo(CO) 6 ] versus W(CO) 6 abundance兾Mo(CO)6 (note that [ ] refers to the molar concentration of the respective complexes). These graphs can be used to determine the quantity of Cr(CO)6 and W(CO)6 in an unknown mixture. To do so, our students add a known quantity of Mo(CO)6 to their unknown solution (internal standard). By using the equation of the line, they determine the needed concentrations (see Supplemental MaterialsW for details). The calibration curves students typically produce are excellent (R2 = .97 or better) and experimental results are within 5% of the accepted value. Student Reception
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Note 1. One reviewer of this paper suggested the use of the exact masses, not just the integer values determined from the experimental mass spectral data. This will allow students to determine the mass defect. This could certainly be accomplished by supplying students with the necessary information; however, given the sensitivity of the HP G18000 GCD system, 180W, with a natural abundance of 0.14, is not detected by the instrument. The author gratefully acknowledges this suggestion.
Literature Cited
This lab is done towards the end of the term when our students are familiar with data analysis and with the use of Excel to analyze data. One of the main difficulties students encounter is the concept that GC–MS is in reality two techniques that are “merged” into one. To overcome this, we introduce chromatography in an earlier experiment, and relate back to it when this experiment is being conducted. Another difficulty students encounter is dealing with the large quantity of data that are generated. To overcome this, we set aside the following laboratory period for students to analyze their data in our computer studio. This allows them to ask the instructor for guidance. The majority of our students who do this experiment find it interesting and extremely applicable in a real world setting. For example, in introducing the topic of GC–MS, it is mentioned that this technique is routinely used in drug analysis. Conclusion This experiment demonstrates the application of gas chromatography–mass spectrometry in the separation and quantification of a mixture of transition-metal carbonyl compounds. In addition, by observing the isotopic distribution of tungsten, our students are able to calculate an atomic mass for tungsten that agrees well with the literature value. Acknowledgments Funding provided by the National Science Foundation DUE-ILI program (ILI-IP grant 97-51604) for the purchase of the GC–MS and by the DUE-ILI program (DUE-9851317) for the purchase of computers used for data analysis are gratefully acknowledged. The NSF Math and Teacher Education Program (MASTEP) provided funding (NSF grant 95-953786) for the development and implementation of this experiment.
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