In the Classroom
π Backbonding in Carbonyl Complexes and Carbon–Oxygen Stretching Frequencies: A Molecular Modeling Exercise Craig D. Montgomery Department of Chemistry, Trinity Western University, Langley, BC V2Y 1Y1 Canada;
[email protected] Increasingly, molecular modeling is being employed to demonstrate various chemical principles within the undergraduate chemistry curriculum (1). Previously we described modeling exercises that can enhance experiments and illustrate principles pertaining to inorganic chemistry such as the transition state in a nitrito to nitro isomerization, the relative energies of the various conformations of ferrocene and its derivatives (2) and the mechanism of pentacoordinate pseudorotation (3). Herein we describe an exercise that illustrates the effect of various factors on π backbonding to carbonyl ligands. Performing such an exercise allows the student a hands-on opportunity by which they are better able to visualize and understand the demonstrated effects, similar to actually performing the lab experiment. Furthermore the student may view the molecular orbitals corresponding to the M–CO π interaction as well as the competing interaction between the metal and co-ligands. Examples of carbonyl complexes are used frequently in introductory organometallic chemistry courses and texts to illustrate the 18-electron rule and also to introduce the concept of π backbonding (4). Since the carbonyl ligand is able to receive π electron density back from the metal into an antibonding orbital (Figure 1), the accompanying decrease in the νCO value can serve as a qualitative measure of the degree of such backbonding. The effects of the metal center, the metal charge, and the other co-ligands on the metal may all be examined by consideration of various series of complexes. Also while νCO values are commonly used as indicators of the extent of dπ–pπ* backbonding in carbonyl complexes, a better indicator may be that of C–O force constants, bond orders, or bond lengths (5). Therefore we also list calculated C–O bond lengths (the greater this value, the greater the π backbonding). The series M(CO)n (M = Cr, n = 6; M = Fe, n = 5; M = Ni, n = 4) will be used to demonstrate the effect of varying the metal center, the series [M(CO)4]n− (M = Fe, n = 2; M = Co, n = 1; M = Ni, n = 0) will be used to demonstrate the effect of the charge on the metal and the series W(CO)5L (L = PCl3, PMe3, SMe2) to demonstrate the effect of co-ligands.
M
filled dπ orbital
C
O
empty pπ* orbital
Figure 1. The dπ–pπ* backbonding in a carbonyl complex.
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Procedure The molecular modeling software that was used in this study was HyperChem1 although other desktop modeling software could likewise be used. PM3 semi-empirical methods were chosen because, among the HyperChem semi-empirical methods that can be used for transition-metal compounds (PM3, ZINDO/1, ZINDO/S, and extended Huckel), the PM3 method provided the best results in this case when compared to literature values. Extended Huckel calculations are also useful for calculating the molecular orbitals, however not all software packages (such as Spartan) include extended Huckel calculations and PM3 was found to yield very similar results. The following procedure was used: 1. In each case, the molecule is drawn and converted into a three-dimensional structure using the build command. 2. The geometry of the structure is optimized by semiempirical methods (PM3).2 3. The vibrational spectrum is calculated for the optimized geometry by semi-empirical methods (PM3).2 4. Semi-empirical single-point calculations (PM3 or extended Huckel) of the optimized structures are also done for W(CO)5L (L = PCl3, SMe2), yielding molecular orbitals including those corresponding to the M–L π–interactions.
Results and Discussion The exercise has been undertaken by students in a fourth-year organometallic chemistry course where the topic of π backbonding is considered. The students are required to model the three series of compounds in order to calculate the νCO values and then explain the observed trends in the calculated values. They had no difficulty completing the exercise; typically the actual time spent on the computer doing the modeling is about an hour. The results obtained were numerically identical to those reported in this article. Thus far, the exercise has been employed after the lectures on π backbonding had been delivered. The students were required to model the various compounds, calculate the νCO values, and explain the observed trends. The students found the exercise to be useful in illustrating various factors affecting π backbonding. In particular the plotting of the orbitals responsible for the M–CO π interaction allows the student to visualize the effect. Alternately one may employ the exercise in the course before the corresponding lecture material and have the students attempt to explain the various observed trends, such as why there is a decrease in the νCO value with increasing
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In the Classroom A
Table 1. Calculated and Experimental CO Stretching Frequencies for M(CO)n C omple x
νCO/cm᎑1
Phas e
Re f
2000.4
ga s
08
2023.0
ga s
09
2057.6
ga s
10
C al c
A v g C al c
A v g L it
C r(C O)6
2080. (T1u)
2080
F e (C O )5
2074. (E’); 2138. (A2”)
2095
N i(C O)4
2099. (T2)
2099
Table 2. Calculated and Experimental CO Stretching Frequencies for [M(CO)4]n−− Complex
Lit
Calc CO Bond Length/Å
νCO/cm᎑1 Calc
Phase or Solvent
Ref 10
Ni(CO)4
2098
2057.6
1.161
gas
[Co(CO)4]−
1980
1891.0
1.183
DMF
12
[Fe(CO)4]2−
1847
1788.0
1.216
H2O
13
B
NOTE: The IR-active T2 stretching mode is considered.
negative charge as one moves from Ni(CO)4 through to [Fe(CO)4]2−. Following completion of the exercise, the lecture material detailing this topic will be delivered. Finally, the student can return to the exercise for a more complete comprehension of the topic. In all cases, the models converged successfully to an optimized structure and the νCO values were obtained. The optimized structures of Ni(CO)4 and W(CO)5(PCl3) are shown in Figure 2 . The calculated νCO values were generally higher that the experimental values. This is commonly the case for semi-empirical calculations such as PM3 owing to the fact that the potential energy is approximated using a harmonic or quadratic function; in reality the potential energy function is flatter. Other factors that cause calculated vibrational frequencies to be high may include not properly accounting for the coupling of vibrational modes as well as the neglect of correlated electron motion in SCF calculations. Nevertheless the various effects were illustrated well by the trends in the calculated values in this study. For a further discussion on the accuracy of vibrational calculations using semi-empirical methods, see ref 6. Korzeniewski et al. showed that PM3 performed the best out of a group of three semi-empirical methods (PM3, AM1, and MNDO) and produced calculated stretching frequencies values that were about 10% too high (6). As expected, we found in our study that higher-level calculations such as ab initio or DFT did provide more accurate values. For example, the νCO value for Ni(CO)4 (first optimized by PM3) was calculated as 2040 cm᎑1 with ab initio calculations (with a 3-21G basis set) as compared to a literature value of 2057.6 cm᎑1 (and the PM3 calculated value of 2098 cm᎑1). Likewise other theoretical studies of metal carbonyls have obtained excellent results using ab initio and DFT calculations (7). However clearly such calculations are too time consuming to be practical for a student exercise and the semi-empirical calculations here do illustrate the trends well.
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Figure 2. The optimized structure for (A) Ni(CO)4 and (B) W(CO)5(PCl3).
The values of the carbonyl stretching frequencies are somewhat solvent dependent and therefore we have noted the phase or solvent in each case. In each case the band(s) chosen for consideration are IR-active modes and are specified.
M(CO)n Series The νCO values for the M(CO)n series of complexes are reported in Table 1 along with literature values. In the M(CO)n series, the different molecular symmetries give rise to differing numbers of vibrational modes and bands. For the comparison, we have chosen the method employed by Crabtree (11), that of utilizing the weighted average of the νCO values of the IR-active modes. As one moves across the first row of transition metals, as from Cr to Ni in this case, one might expect an increase in the π donation to each CO since the metal is becoming more electron-rich and there are fewer CO ligands to share the donated π electron density. However the effect calculated here (and observed in the literature) is the opposite. Instead the dominant factor is the corresponding increase in effective nuclear charge and electronegativity values across the first row of transition metals. As a result, the π-donor ability of the metal decreases and thus one observes an increase in the νCO values. [M(CO)4]n− Series The νCO values for the next series, [M(CO)4]n−, are reported in Table 2. There are two νCO values for such compounds of Td symmetry, an A1 symmetric stretch (appearing in the Raman spectrum) and a T2 asymmetric stretch
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In the Classroom Table 3. Calculated and Experimental CO Stretching Frequencies for W(CO)5L Ligand
Calc
Lit
Calc CO Bond Length/Å
νCO /cm᎑1
Solvent
A
Ref
PCl3
2064
1985.0
1.169
cyclohexane
14
PMe3
1992
1946.5
1.181
cyclohexane
15
SMe2
1961
1935.0
1.189
hexane
16
NOTE: Several CO stretching modes are observed; the value of the A1 stretch of the carbonyl trans to L is reported.
(appearing in both the IR and Raman spectrum), with the A1 value being the larger of the two. We have chosen to consider the T2 band. Again the trend in the calculated values mirrors that of the experimental values. As one moves from Ni to Co to Fe, the number of d electrons remains constant at ten, but the oxidation state of the metal becomes increasingly negative. This increases the ability of the metal as π donor and results in a corresponding decrease in the νCO values (and the C–O bond length increases).
B
W(CO)5L Series The effect of π-bonding co-ligands such as phosphines, that may compete for the π electron density on the metal center is illustrated using the series of complexes, W(CO)5L (where L = PCl3, PMe3, SMe2). The results are shown in Table 3. In the case of each of the complexes in this series having C4v symmetry (if one regards the ligand L as spherical), there are three νCO bands appearing in the IR spectrum owing to a doubly degenerate E mode and two A1 modes (the symmetric C–O stretch involving all five carbonyls and the C–O stretch for the carbonyl trans to L). It is this latter A1 mode that is considered here as it best illustrates the competition for dπ electron density on the metal center. Again the expected trend in the calculated νCO values is observed. As the electron-withdrawing ability of L decreases (PCl3, PMe3, SMe2), the π-bonding ability of L decreases and the degree of π bonding to the CO ligands increases. Therefore the νCO values decrease. Again, this is illustrated by the calculated C–O bond lengths. Based on these calculated results, the student may also construct a mini π-acceptor series.
π-Bonding Molecular Orbitals These effects can also be viewed by considering the molecular orbitals. For example, the HOMO-1 orbitals are shown in Figure 3 for W(CO)5L, L = PCl3 and SMe2. It may be necessary to rotate the molecule after calculating and plotting the HOMO-1 orbital to gain the best view of the relative π interactions. In addition, it may be necessary to adjust the orbital contour value; in Figure 3, I have used a value of 0.02. One is able to see the greater M–L π interaction in the case of PCl3 with its more electronegative substituents and the greater M–CO π interaction in the case where L = SMe2.
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Figure 3. HOMO-1 for (A) W(CO)5(PCl3) and (B) W(CO)5(SMe2) showing the M–L and M–CO π interactions.
Conclusions The exercises described in this article allow the student to investigate the various factors that affect dπ–pπ* backbonding to carbonyl ligands. The student may be asked to determine the trends in νCO values and bond orders for each series and explain the effects of co-ligands, the metal center, and the metal charge, before being given the literature values. The visual and hands-on nature of the modeling exercise deepens the students understanding of dπ–pπ* backbonding and further enforces the concepts of IR spectroscopy. Notes 1. HyperChem Professional, version 7.1; Hypercube, Inc: Gainesville, FL, 2002. 2. Geometry optimizations for PM3 semi-empirical calculations had a termination condition of less than 0.1 kcal兾(Å mol) (decreased to 0.01 before vibrational spectrum calculations) with a Polak–Ribiere algorithm. The setup for PM3 semi-empirical calculations typically involved a convergence limit of 0.01 kcal兾mol, RHF spin pairing, and accelerated convergence.
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In the Classroom
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8. 9. 10. 11. 12. 13. 14. 15. 16.
8474. Li, J.; Schreckenbach, G.; Ziegler, T. J. Am. Chem. Soc. 1995, 117, 486. Ehlers, A. W.; Dapprich, S.; Vyboishchikov, S. F.; Frenking, G. Organomet. 1996, 15, 105. Ehlers, A. W.; Frenking, G. Organomet. 1995, 14, 423. Jones, L. H.; McDowell, R. S.; Goldblatt, M. Inorg. Chem. 1969, 8, 2349. Jones, L. H.; McDowell, R. S.; Goldblatt, M.; Swanson, B. I. J. Chem. Phys. 1972, 57, 2056. Bouquet, G.; Bigorgne, M. Spectrochim. Acta 1971, 27A, 139. Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; John Wiley and Sons: New York, 1994; p 42. Edgell, W. F.; Lyford, J. J. Chem. Phys. 1970, 52, 4329. Stammreich, H.; Kawai, K.; Tavares, Y.; Krumholz, P.; Behmoiras, J.; Bril, S. J. Chem. Phys. 1960, 32, 1482. Dahlgren, R. M.; Zink, J. I. Inorg. Chem. 1977, 16, 3154. Bancroft, M.; Dignard–Bailey, L.; Puddephatt, R. J. Inorg. Chem. 1986, 25, 3675. Dombek, B. D.; Angelici, R. J. Inorg. Chem. 1976, 15, 2403.
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