J . Phys. Chem. 1988, 92, 645-650
645
Ligand Exchange in (~5-Cyclopentadienyl)nickelLigand Systems. A Theoretical Study Linda Throckmorton and Dennis S. Marynick* Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065 (Received: November 3, 1986)
-
This study uses PRDDO, an approximate molecular orbital method, to evaluate the energetics of organometallic exchange L' X,M-L' + L. These calculationsare compared to a large body of experimental information reactions of the form X,M-L on (~5-cyclopentadienyl)nickel(II)system. The PRDDO method can reproduce relative transition metal-ligand exchange energies within groups of related molecules: alcohols, aldehydes, and ethers. Applying a least-squares relationship to correct PRDDO hE"s produces linear correlationsbetween calculated hE"s and experimental AH'S for the alcohols, ethers, and aldehydes. The PRDDO corrected AE's yield the correct relative order for the alkylamines when compared to their proton affinities. However, when compared to experimental data on the (~5-cyclopentadienyl)nickel(II)system, Me,N and Me2NH are inverted. Substituent effects are examined by substituting either fluorines or methyl groups for hydrogens in a number of molecules. Fluorines substituted for hydrogens on the Cp ring enhance Ni-Obonding by withdrawing electron density from the ring and weakening the Ni-ring bond. Fluorines substituted for hydrogens on the ligands have the opposite effect, weakening the Ni-O bond by withdrawing electron density from the a-carbon and localizing the lone pairs on the oxygen. Fluoro-substituted ethanol and diethyl ether ligands have some bi- or tridentate character. Substituting methyl groups for hydrogens on the Cp ring enhances the Ni-ring bond by donating electron density into the ring which in turn donates excess electron density to the nickel. This results in a slightly weaker Ni-0 bond.
+
Introduction Gas-phase measurements of transition metal-ligand bonding energies, the enthalpy of reaction 1, provide thermochemical data which can be used to evaluate the energetics of organometallic reaction mechanisms and catalytic processes. Such measurements AH = D(M'-L) (1) ML1' + L2 = ML2' L1 supply necessary information from which to develop bonding models. While there have been recent experimental determinations of metal-ligand binding energies, D(M'-L), employing ion cyclotron resonance spectroscopy (ICR) techniques for transitionmetal and alkali metal ions,'-9 there have been few studies using organometallic^.'^'^ Attempts to utilize organometallics as precursors for measurements of ligand binding energies have been less successful, due to the formation of polynuclear complexes and extensive ligand substitution Theoretical ligandexchange energy calculations, however, can always be constrained to produce the desired exchange of ligands, making it possible to study systems which cannot be easily observed experimentally. Such studies can increase our understanding of metal-ligand bonding and the effects of substituents within a series of related ligands. Our interest in this area centers around the application of the PRDDO (partial retention of diatomic differential overlap) approximate molecular orbital method to study transition metal-ligand bonding within organometallic complexes. PRDD0'6s17is an approximate molecular orbital method, based on Hartree-Fock theory, which closely reproduces ab initio calculations with the same basis set, while requiring much less
+
(1) Staley, R. H.; Beauchamp, J. L. J . Am. Chem. SOC.1975, 97, 5920. (2) Wwdin, R. L.; Beauchamp, J. L. J. Am. Chem. SOC.1978,100,501. (3) Burnier, R. C.; Byrd, G. D.; Freiser, B. S. J . Am. Chem. SOC.1981, 103, 4360. (4) Uppal, J. S.; Staley, R. H. J . Am. Chem. SOC.1982, 104, 1229. (5) Uppal, J. S.; Staley, R. H. J. Am. Chem. SOC.1982,104, 1235, 1238. (6) Jones, R. W.; Staley, R. H. J . Am. Chem. SOC.1982, 104, 2296. (7) Byrd, G. D.; Freiser, B. S. J. Am. Chem. SOC.1982, 104, 5944. (8) Babinec, S. J.; Allison, J. J. Am. Chem. SOC.1984, 106, 7718. (9) Cassady, C. J.; Freiser, B. S. J. A m . Chem. SOC.1985, 107, 1566. (10) Corderman, R. R.; Beauchamp, J. L. J . Am. Chem. SOC.1976, 98, 3998. (11) Foster, M. S.; Beauchamp, J. L. 1.Am. Chem. SOC.1975,97,4808. (12) Weddle, G.H.; Allison, J.; Ridge, D. P. J. Am. Chem. SOC.1977, 99, 105. (13) Corderman, R. R.; Beauchamp, J. L. Inorg. Chem. 1978, 17, 68. (14) Beauchamp, J. L.; Stevens, A. E.; Corderman, R. R. Pure Appl. Chem. 1979, 51, 967. (15) (a) Macholdt, H.-T.; Elias, H. Inorg. Chem. 1984,23,4315. (b) For very recent work in this general area, see: Kang, H.; Beauchamp, J. I. J. Am. Chem. SOC.1986, 108, 5663 and references therein. (16) Halgren, T. A,; Lipscomb, W. N. J. Chem. Phys. 1973, 58, 1569. (17) Marynick, D. S.; Lipscomb, W. N. Proc. Natl. Acad. Sei. USA 1982, 79, 1341.
0022-3654/88/2092-0645$01.50/0
computing time, therefore allowing the calculation of wave functions for large systems. Although the calculation of a bond dissociation energy is difficult even for small molecules, primarily because of the necessity of including electron correlation, Hartree-Fock theory does properly handle systems which dissociate to closed-shell fragments. For general ligand-exchange reactions, X,M-L
+ L'
---*
X,M-L'
+L
(2)
the perplexities inherent in breaking bonds are avoided if L and L' are similar ligands (e.g., substituted alcohols, aldehydes, etc.). Therefore, even though the basis sets employed are small and the wave functions are of the single-determinant Hartree-Fock type, we will show that the PRDDO method is capable of predicting relative binding energies for groups of closely related ligands. In previous work'* on the structure, conformation, and ligand binding in iron-olefin complexes of the general formula Fe(CO)4(C,X4), where X = H, F, C1, and CN, the calculated AE's for the reaction Fe(C0)4(C2H4) + c2x4
-
Fe(CO)dC&)
+ C2H4
were found to be consistent with available experimental data on related systems. The relative binding energies of cis and trans 1,2-disubstituted olefins were also calculated for reactions of the type (E)-CHX=CHX Fe(CO),-(2)-CHX=CHX Fe(C0)4-(2)-CHX=CHX (Z)-CHX=CHX
+
-.
+
These calculated ligand-exchange energies are consistent in all cases with the experimental trends. These preliminary studies suggest that the probability of obtaining relative binding energies and observing substituent effects for closely related ligands is favorable; however, an extensive theoretical study of a system for which there is a large amount of experimental data is needed. One such system, involving related ligands, for which there is a great deal of experimental information is ($-cyclopentadienyl)nickel( 11).
+
-
+
(CpNi-L)' L' (CpNi-L')' L (3) These ICR experiments used CpNiNO (Cp = v5-C5H5,cyclopentadienyl anion) to analyze 30 ligands bound to CpNi' in which the parent ion CpNiNO' was employed directly as the initial metal-ligand species.l0 Beauchamp and Corderman's worklo on these gas-phase AH'S is one of the most extensive and successful experiments on ligand-organometallic binding energies with which (18) Axe, F. U.; Marynick, D. S. J . Am. Chem. SOC.1984, 106, 6230. (19) Cox, A. P.; Thomas, L. F.; Sheridan, J. Nature 1958, 181, 1157. (20) Cox, A. P.; Brittain, A. H. Trans. Faraday SOC. 1970, 66, 557.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
to compare theoretical relative binding energies. The CpNiL' complexes are tractable; Le., there are no polynuclear complexes formed. The molecules are large but can be handled by the PRDDO method, although the largest systems studied here would be difficult to handle by ab initio techniques, because of their near-total lack of symmetry. The experimental results refer to gas-phase values without the complication of solvent effects. A comparison of the experimental AH'S to the calculated AE's verifies that while PRDDO calculations do not yield exact binding energies, they do correctly predict trends in the binding energies within three groups of related ligands: alcohols, aldehydes, and ethers. These calculations will push the application of small basis set molecular orbital theory to the limits; however, we have reason to believe that the PRDDO method should be capable of producing useful chemical information. First, the nickel complex has a d8 configuration with a +2 charge and all of the ligands should act as simple a-donors. Second, all of the primary ligands are neutral and monodentate, thus eliminating problems with three-center bonds, bridging groups, bidentate ligands, etc. Thud, the CpNiL+ moiety is simple electronically. In addition, the central atom does not change from one complex to another so that the only apparent effects for closely related ligands should be the substituent effects. By calculating exchange energies instead of bond dissociation energies, we minimize problems such as correlation energy corrections and eliminate problems presented by changes in spin multiplicities, while still obtaining meaningful chemical information. Finally, we note that our methodology forces us to compare calculated AE's to experimental M s . Thus, effects such as vibrational zero-point energy and thermal population of excited vibrational states are not explicitly taken into consideration in our calculations; however, as will be shown below, these effects need not be considered to obtain excellent correlations between calculated and experimental results. We present here a study of the bonding in these systems by our approximate method. As a means of exploring the metalligand bond, we have varied the ligands attached to the metal while maintaining a heteroatom-metal bond between oxygen or nitrogen and nickel(I1). By examining a series of related complexes, in which the substituents on the oxygen or nitrogen have been changed, we can make qualitative estimates of the effects of various substituents on the metal-ligand bond strengths.
Calculations Since Ni2+ is d8, there are two reasonable spin multiplicities possible for CpNiL': a closed-shell singlet and an open-shell triplet. Placing the Cp ring in the xy plane and the Ni-L bond on the z axis, the closed-shell state has an empty d,z orbital, while the open-shell state corresponds to a configuration d2,*d1,,, d2xz-yz, d1 , d2,. Actually, there are many possible open-shell configuYT rations; however, d-orbital participation in these systems is so small (see below) that the actual configuration chosen is irrelevant. Calculations were done on the closed-shell and open-shell states. The unrestricted Hartree-Fock (UHF) method was employed to approximate the open-shell state. Comparison of the calculations for the closed-shell versus the open-shell case for each transition-metal complex disclosed no significant differences in the overlap populations, the relative ligand-exchange energies, the calculated charges on the nickel in the complex, or the calculated charges on the heteroatom in the ligands. This clearly demonstrates the unimportance of d orbitals in describing the Ni-L bonding. As fluorines were substituted for hydrogens on the ligands to expand the study of substituent effects, calculations for the closed-shell case would not converge to any reasonable state and it became clear that the open-shell triplet state was more appropriate for these systems. Therefore, the discussion of the exchange energies and bonding is based on open-shell unrestricted Hartree-Fock calculations.21 Ideally, a one-to-one correspondence between calculated and experimental ligand-exchange energies would exist; however, the (21) Roothaan, C. C. J. Reo. Mod. Phys. 1960, 32, 179.
Throckmorton and Marynick
, - PRDDO AE's fkcaUmole) a
PRDDO AE's (kcaL'mole1 h
Figure 1. Comparison of uncorrected PRDDO relative binding energies to experimental AITs for the (CpNi+-L) system relative to AD(CpNiC-CHSNC)= O: (a) alcohols; (b) aldehydes; (c) ethers; and (d)
amines. The lines have been omitted for clarity. practical necessity of employing approximate molecular orbital theory with a small basis set makes such correlations unlikely. In fact, limited correlations between the PRDDO and experimental energetics do exist, and are illustrated in Figure 1. If comparisons are limited to individual groups of alcohols, aldehydes, or ethers (Figure 1, a, b, and c, respectively), good linear correlations are found. The nearly identical slope of these three lines suggests that an overall correlation of the form
(4) should exist, where BL is an additivity correction which is different for ethers, aldehydes, and alcohols, and where a and are linear least-squares parameters. Setting 6&h,& arbitrarily to zero yields the least-squares parameters a = 0.853, P = -2.162 kcal/mol, 6aldehydm = 6.0, and Bethers = 3.2 with a correlation coefficient of 0.984. This excellent correlation makes it clear that the PRDDO method is reliably reproducing the experimental substituent effects; however, there are several drawbacks to the above correction scheme. First, two oxygen donors (water and acrylaldehyde) cannot be correlated in this fashion and had to be omitted from the above least-squares procedure. Second, the amines (Figure Id) show an inverse correlation between calculated and experimental values. Finally, this procedure requires four adjustable parameters (a,p, dal&hyde, and 6&m) to correlate only 12 exchange energies. We therefore searched for an alternative correction procedure. Most of the calculated ligand-exchange energies which do not correlate well with experiment involve a ligand which was included in the original parametrization of PRDD0.16 While errors in the PRDDO total energies are generally highly systematic (the energies are nearly always lower than the corresponding ab initio values), this is not true for molecules in the original parametrization set. These molecules tend to have calculated energies which agree very well with the ab initio values. This has the effect of introducing a nonsystematic error for these ligands only. To compensate for this effect, we have developed another correction procedure which is capable of correlating all of the ligand-exchange energies with a maximum of only three parameters, and is thus far superior to the procedure described above. We start by correcting for the systematic errors in the PRDDO energies. The bulk of these errors are of a one-center nature; Le.,
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 647
(q5-Cyclopentadienyl)nickel Ligand Systems they are due to the number and type of atoms in the molecule and not to the details of the molecular geometry.22 In a previous paper, we discussed these errors and the development of a set of atomic correction factors based on 150 open-chain molecules which reduce the errors in the calculated total energy of PRDDO wave functions by a factor of 8 relative to ab initio reference calculation~.~~
Eoc = EPRDDO
+ Cnkek k
(5)
Here, E, represents the PRDDO energy corrected for one-center errors, nk denotes the number of atoms of type k, and ek is an atomic correction factor determined by least-squares fits of corrected PRDDO total energies to reference SCF values. Atomic corrections were developed for hydrogen, boron, carbon, nitrogen, oxygen, fluorine, phosphorus, sulfur, and chlorine only. The correction factors were restricted to non-transition-metal atoms because of complications associated with the variable d-orbital occupancies of these systems. Conceptually, we first determine the uncorrected PRDDO AE‘s. Then, based on the atomic correction factors for the number and type of atoms present, the energies of all species in eq 3 are corrected according to eq 5. The “correction” does not change the calculated AE’s, since the number and type of atoms are the same on both sides of the equation; however, it does bring the energy of each species closer to the exact ab initio energy and eliminates the systematic errors made by the PRDDO method.23 The energies of the free ligands are now further corrected to the exact a b initio value
where k is the difference between the corrected calculated PRDDO energies of the free ligands and their respective ab initio energies. This procedure eliminates the nonsystematic errors (relative to a b initio values) for the free ligands, some of which were in the original PRDDO parametrization. In fact, because we ultimately use ab initio values for the free ligands, the PRDDO energies of the free ligands are not actually needed. The corrected exchange energy AEc is calculated from the a b initio energies of the free ligands and the E , values of the metal complexes. Since it is not possible to apply eq 6 to the metal complexes, overcompensation for the errors occurs. To deal with this problem, we developed two parameters, a and @,based on a least-squares fit of the data, which rectifies the overcompensation
+
AEcLs = a(AEC) p
(7)
This procedure not only corrects the problems with ligands in the original PRDDO parameter set, but also allows all ethers, aldehydes, alcohols, and water to be correlated with the same values of a and p. With a = 0.281 and = -8.848 kcal/mol, we obtain a correlation coefficient of 0.966 for all 14 oxygen donor ligands, - AEcLsl is only 0.6 and the average absolute deviation IAHeicxpt kcal/mol (Table I). In addition, when this procedure is applied to the amines, the corrected exchange energies correlate very well with experiment (correlation coefficient is 0.98 1) if one additional additive factor p’ is added to eq 7 (p’ = 5.910 kcal/mol, average absolute error = 0.7 kcal/mol, with a = 0.288 and p = -8.832 kcal/mol). Because of the excellent correlation with experiment, this procedure will be used to calculate all ligand-exchange energies in this paper. Although our procedure may at first seem arbitrary, it is based on a sound statistical analysis of systematic PRDDO erromZ3It is also worth noting that our final AEcu had only two adjustable parameters, a and p (three if the amines are included), since the atomic correction factors (eq 5 ) and the ab initio energies of the free ligands (eq 6) are predetermined. These two (or three) parameters allow the quantitative correlation of 14 (or 18) ligand-exchange energies. (22) Halgren, T. A,; Kleier, D. A,; Brown, J. H.; Brown, L. D.; Lipscomb, W. N. J . Am. Chem. SOC.1916, 100, 6595. (23) Throckmorton, L.; Marynick, D. S . J . Comput. Chem. 1985,6,652.
TABLE I: Relative Ligand-Exchange Energies on q5-CpNiL+for Various Ligandso PRDDO
H20 MeOH EtOH i-PrOH t-BuOH MezO Et20 i-Pr20 HCHO MeCHO EtCHO i-PrCHO C2H3CHO t-BuCHO Me2C0 NH3 MeNH, Me2NH Me3N
-10.9 -11.5 -8.9 -6.6 -6.1 -12.6 -9.6 -5.8 -20.1 -15.2 -14.4 -13.8 -8.6 -13.0 -10.4 +4.8 +1.5 -4.4 -8.5
-14.2 -12.0 -10.5 -9.0 -7.2 -9.6 -6.1 -4.5 -14.3 -11.6 -9.9 -8.5 -8.2 -6.6 -9.1 -3.9 -2.4 -1.9 -1.4
-15.2 -12.0 -10.1 -8.1 -6.8 -10.7 -6.7 -4.0 -13.8 -10.6 -9.5 -8.5 -8.3 -8.0 -6.9 -5.3 -2.4 -0.8 -1.2
1.o 0.0 0.4 0.9 0.4 1.1 0.6 0.5 0.5 1.1 0.4 0.0 0.1 1.4 2.2 1.4 0.0 1.1 0.2
“All data in kcal/mol. bValues are relative to AD(CpNi+-CH3NC) = 0. D(CpNi’-CH3NC) = 57.7 i 5 kcal/mol based on a literature ~ ~the ’ measured valvalue of D(CpNi+-NO) = 46 5 k ~ a l / m o l ’ ~and ue of AD(CpNi’-NO) = -1 1.7 kcal/moI.’O CUnless otherwise noted, all experimental AD(CpNi’-L) are from ref 10. “Experimental value from ref 2. eExperimental value from ref 14. /The absolute difference reported is between corrected PRDDO AE’s and experimental AH’S with an average absolute difference of 0.7 kcal for the amines, 0.7 kcal for the oxygen donors including Me2C0, and 0.6 kcal for the oxygen donors excluding Me2C0.
*
Geometries Table I lists the ligands used in the exchange reactions. The ligands are limited to open-chain molecules, as noted above, containing first-row atoms and hydrogens because it is currently not possible to perform PRDDO calculations involving d orbitals on more than one atom per complex. The experimental geometries were used where available. When the geometry of the methyl group substituents of the ligands was not given, the H-C-H and H-C-X angles were assumed to be 109.47’ and the H-C bond length was assumed to be 1.09 A. The optimized coordinates for (CH3)3Nwere obtained from Eades et al.24 The geometries of CH3NHzand (CH3)2NHwere estimated by replacing a methyl group of trimethylamine with a hydrogen. The tert-butyl alcohol and aldehyde molecules were optimized with respect to the location and rotation of the methyl groups starting from experimental information for tert-butyl cyanide. The minimum energy conformation of each ligand bound to the nickel was located, subject to maintaining C, symmetry within the ligand. This also corresponded to the optimal geometry of the free ligand. The geometry of the $-CpNi+ moiety was obtained from a microwave structure of CpNiN0.1g,20The Ni-ring distance was optimized for each CpNiL’ complex. The average distance of 2.10 f 0.02 A was determined and the Ni-ring distance was then fixed at the value, Ni-C = 2.10 A. The experimental Ni-C bond length is 2.1 1 A.2o The Ni-heteroatom bond lengths were o timized for each complex. The average of the Ni-0 (1.879 ) and the Ni-N (1.968 A) bond distances were ascertained and taken as representative of the Ni-0 and Ni-N distances for the C p N i + moiety. S i n c e a previous of PRDDO optimized
w
(24) Eades, R.A.; Weil, D. A,; Dixon, D. A,; Douglass, Jr., C. H. J . Phys. Chem. 1981, 85, 916. (25) Marynick, D. S.; Axe, F. U.; Kirkpatrick, C. M.; Throckmorton, L. Chem. Phys. Lett. 1983, 99, 406. (26) Eades, R. A.; Weil, D. A.; Dixon, D. A,; Douglass, Jr., C. H. J . Phys. Chem. 1981, 85, 98 1. (27) Ros, P.J . Chem. Phys. 1968, 49, 4902. (28) Bernardi, F.; Csizmandia, I. G.; Schelgel, H. B.; Wolfe, S. Can.J . Chem. 1975, 53, 1144
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The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Throckmorton and Marynick TABLE 11: Relative Binding Energies of Related Ligands" pred re1 exchange group
alcohols
ligand n-PrOH n-BuOH
aldehydes
i-BuOH n-PrCHO n-BuCHO i-BuCHO
energies, kcal -9.1 -7 6 -8.4
-8.7 -7.4 -8.2
Relative to AD(CpNi+-CH,NC) = 0. Corrected PRDDO calculated AE's: AE = 0.288(AEPRDD0 + Ac') - 8.832 kcal. See text for discussion of least-squares procedure.
Figure 2. The lowast energy conformations for 12 of the oxygen-donating ligands on the CpNi+ moiety. Top row (from left to right): MeOH, EtOH, i-PrOH, and t-BuOH. Middle row: Me20, Et20,i-Pr20,and HCHO. Bottom row MeCHO, EtCHO, i-PrCHO, and 2-BuCHO.
bond lengths and angles for various low oxidation state transition-metal complexes demonstrated the capability of the PRDDO method to obtain reasonable bond lengths with an average absolute error of 0.04 A, the Ni-heteroatom distances should be acceptable. The angle of bonding was optimized also and indicated that while the aldehydes bind at an average Ni-0-C angle of 143 f 2' (Figure 2), the ethers and alcohols bind in a coplanar fashion with the Ni. That is, the a-carbons of the ethers or the a-carbon and hydroxyl hydrogen of the alcohols are in the same plane with the oxygen-nickel bond and the center of the Cp ring. The amines bond tetrahedrally with the nickel such that the angle between the center of the C p ring, the nickel, and the nitrogen is 180.0'.
Discussion Relative Exchange Energies. From the plots in Figure 1 it can be seen that, with the exception of acrylaldehyde and water, the PRDDO method can predict the experimental trends within the three group of related oxygen-bound ligands: alcohols, aldehydes, and ethers. For the amines the ordering is b a ~ k w a r d . Applying ~~,~~ the parametrized correction factors discussed above allows all the oxygen donors to be described by one set of correction factors and corrects the problem with the amines as seen in Table I, where the corrected PRDDO AE's and experimental AH'S are tabulated. For the oxygen donors, the average absolute difference between the corrected PRDDO AE's and experimental AH'S is 0.6 kcal/mol, while for the amines, the corresponding value is 0.7 kcal/mol. Thus, 'remarkably accurate correlations are obtained with this procedure. The most probable cause for the dramatic change in the amine correlation after applying the correction procedure lies in the fact that ammonia and methylamine were among a group of simple molecules used in the original parametrization of PRDD0.16 Therefore, PRDDO calculates the absolute energies of these molecules much more accurately than the other molecules. The energy correction procedure utilized here minimizes the problems inherent in molecules used in the parametrization of PRDDO, because the energies of all free ligands are corrected to the exact ab initio value. Water also belongs in this group which explains why it does not fall on either the uncorrected PRDDO ether or alcohol lines. While the corrected PRDDO AE's for the amines certainly enhanced the linearity of the amine correlation, the ordering is not consistent with the experimental values. Beauchamplo reports that the ordering of AD(L-CpNi+) for the alkylamine series is irregular with Me,NH > Me,N > MeNH, > NH,, similar to the behavior the amines exhibit toward Li+., The corrected (29) Ferguson, W. I.; Hardy, N. C. Chem. Phys. Lett. 1980, 71. 9 5 .
PRDDO AE's, as well as the proton affinities for the alkylamines, give a regular series with Me3N > Me2NH > MeNH, > NH,. The difference between the experimental binding energies of CpNi+-Me,N and CpNi+-Me2NH is 0.4 kcal; that between Li+-Me,N and Li+-Me2NH is 0.1 kcal. Such a small difference (0.4 kcal) is certainly well within the intrinsic error of PRDDO, particularly since we are neglecting changes in vibrational zeropoint energy, thermal population of vibrational states, etc. Since the PRDDO methodology enumerates the trends in binding energies correctly within groups of related molecules, it should be capable of predicting the relative binding energies for other molecules within the groups, such as, n-PrOH, n-BuOH, i-BuOH, n-PrCHO, n-BuCHO, and i-BuCHO. PRDDO calculations predict a series with the binding energy of t-BuCHO > n-BuCHO > i-BuCHO > i-PrCHO > n-PrCHO > EtCHO for the aldehydes and t-BuOH > n-BuOH > i-BuOH > i-PrOH = n-PrOH > EtOH for the alcohols (Table 11). Although there are no experimental values for the CpN? system, proton affinities30 are available for comparison as well as ICR studies of transition metal ion-ligand exchange reaction^.^.^ Comparison of those ligands in common with the proton affinities available, Mn+ is d6, and the (Cu+-2L) systhe (Mn+-L) s y ~ t e mwhere ,~ tem,6 where Cu+ is dlO, indicates agreement in trends for the alcohols and aldehydes with the exception of i-PrOH and n-BuOH which are inverted for PRDDO calculations. Metal-Ligand Bond. The electronic structure of the metalligand bond in the oxygen-bound complexes reflects the different ligands involved. The alcohols and related ethers bond coplanar with the nickel, while the aldehydes form an angle of 143 f 2' (Figure 2). Taking water and formaldehyde as the simplest representatives of these groups, the bonding can be examined in greater detail. Both molecules have two lone pairs, one a C-type and the other with local *-symmetry. In both cases, the o lone pair is of a l symmetry with the oxygen a lone pair of water being higher in energy (-0.4407 au) than its formaldehyde counterpart (-0.5397 au). The P-type orbital of formaldehyde of b, symmetry lies in the plane of the two C-H o-bonds. The mixing between the C-H bonding orbital of b, symmetry and the lone pair destabilizes the bl lone pair. Thus, the oxygen T lone pair of formaldehyde is higher in energy (-0.3724 au) than its water counterpart (-0.3908 au). It is therefore more likely to take part in the bonding to the transition metal in order to lower its energy. Bending of the formaldehyde allows the T lone pair to overlap strongly with the nickel 4s and 4p orbitals with local a-symmetry. The net gain is a lowering of the energy by about 3.5 kcal for formaldehyde, and up to 5.0 kcal for the other aldehydes. Previous s t ~ d i e s ~of~protonated ,~* formaldehyde and acetaldehyde indicate the same configuration. The same kind of oxygen P lone pair overlap applies in the case of water, and a previous study of protonated water29found that H30+has a bond angle of 11 1-1 12' and an out-of-plane angle of 17-18'. Since the oxygen P lone pair of water is not as high in energy as its Formaldehyde counterpart, the need for stabilization is not as great. Therefore, a planar geometry is reasonable (30) Armstrong, D. R.; Perkins, P. G.; Stewart, J. J. P. J . Chem. SOC., Dalton Trans. 1973, 8 3 8 . Degrees of bonding are calculated quantities which are very nearly equal to 1.0 for a single bond, 2.0 for a double bond, etc. (31) Muller, J.; Goll, W. Chem. Ber. 1973, 106, 1129.
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 649
(~5-Cyclopentadienyl)nickelLigand Systems TABLE 111: Comparison of Degrees of Bonding and Overlap Populations for v5-(C5H5)NiL+(A), q5-(C5F5)NiL+(B), and $-(C5(CH&)NiLt (C) for Ni-Cd, and Ni-0, and Comparison of Mulliken Charges
ligand (L)
degree of bonding B C (a) Ni-Cring
A
Et20 i-Pr20 HCHO MeCHO EtCHO i-PrCHO t-BuCHO
0.3318 0.3305 0.3268 0.3256 0.3248 0.3291 0.3222 0.3213 0.3354 0.3311 0.3300 0.3296 0.3288
0.3187 0.3174 0.3141 0.3131 0.3123 0.3163 0.3098 0.3091 0.3200 0.3177 0.3167 0.3165 0.3157
H20 MeOH EtOH i-PrOH t-BuOH Me20 Et20 i-Pr20 HCHO MeCHO EtCHO i-PrCHO r-BuCHO
0.5917 0.5917 0.5914 0.6011 0.6067 0.5882 0.5830 0.6018 0.5628 0.5844 0.5899 0.5952 0.5988
0.5960 0.5962 0.5957 0.6056 0.6112 0.5931 0.5878 0.6065 0.5762 0.5903 0.5958 0.601 1 0.6049
H20 MeOH EtOH i-PrOH t-BuOH Me20
0.3305 0.3288 0.3256 0.3245 0.3238 0.3277 0.3214 0.3205 0.3323 0.3299 0.3288 0.3285 0.3276
overlap population B C
A
0.1255 0.1250 0.1231 0.1221 0.1213 0.1244 0.1208 0.1195 0.1280 0.1258 0.1252 0.1248 0.1242
0.1135 0.1129 0.1108 0.1100 0.1092 0.1123 0.1083 0.1070 0.1148 0.1135 0.1128 0.1124 0.1118
0.1270 0.1260 0.1241 0.1234 0.1226 0.1255 0.1220 0.1206 0.1284 0.1272 0.1265 0.1261 0.1255
0.3063 0.3065 0.3062 0.3123 0.3161 0.3038 0.3015 0.3124 0.2779 0.2986 0.3030 0.3059 0.3089
0.3102 0.3103 0.3098 0.3159 0.3196 0.3079 0.3053 0.3160 0.291 1 0.3038 0.3081 0.31 10 0.3140
0.2956 0.2952 0.2951 0.3013 0.3049 0.2922 0.2906 0.3019 0.2758 0.2878 0.2920 0.2949 0.2977
(b) Ni-0
ligand (L) H20 MeOH EtOH i-PrOH r-BuOH Me20 Et20 i-Pr20 HCHO MeCHO EtCHO i-PrCHO t-BuCHO
0.5774 0.5765 0.5766 0.5861 0.5914 0.5724 0.5686 0.5872 0.5576 0.5701 0.5754 0.5803 0.5837
(c) Comoarison of Mulliken Charges charge on Ni charge on 0 A B C A B C -0.4113 -0.3582 -0.3607 -0.3623 -0.3657 -0.2891 -0.2883 -0.2920 -0.2291 -0.3059 -0.3081 -0.3088 -0.3127
-0.4068 -0.3519 -0.3545 -0.3558 -0.3589 -0.2813 -0.2801 -0.2837 -0.2649 -0.3036 -0.3059 -0.3070 -0.3108
-0.4062 0.9468 -0.3526 0.9499 -0.3551 0.9529 -0.3564 0.9510 -0.3598 0.9508 -0.2817 0.9540 -0.2808 0.9605 -0.2854 0.9515 -0.2638 0.9538 -0.3019 0.9520 -0.3041 0.9527 -0.3049 0.9504 -0.3086 0.9512
0.9618 0.9647 0.9663 0.9641 0.9635 0.9681 0.9719 0.9627 0.9675 0.9671 0.9676 0.9648 0.9654
0.9445 0.9489 0.9510 0.9490 0.9488 0.9533 0.9574 0.9499 0.9493 0.9499 0.951 1 0.9489 0.9499
and optimization of the CpNi(H20)' complex indicates the Ni+-H20 moiety is indeed planar. Therefore, alcohols and ethers bond primarily through ligand-to-metal a-donation into the empty 4s and 4p, orbitals on the metal. In the closed-shell singlet case, a small (-0.03 e) amount of donation occurs into the empty dZ2 orbital. Substituent Effects
Several trends are apparent in substituent effects. First, as previously noted by Corderman and Beauchamplo, the AD(LCpNi') increases with increasing substitution of alkyl groups for H on the basic site: M e 2 0 > MeOH > H 2 0 ; Me2C0 > MeCHO > HCHO; and, in the present study, Me3N > Me2NH > MeNH, > NH3. The relative ligand binding energies are enhanced with increasing alkyl substitution on the carbon a to the basic site (Me3COH > MezCHOH > EtOH > MeOH and i-Pr20 > E t 2 0 > MezO) and with increasing alkyl substitution on the carbon remote to the basic site (Me3CCH0 > Me2CHCH0 > EtCHO > MeCHO). PRDDO predicts that AD(L-CpNi') increases as the length of the carbon chain grows: n-BuCHO > n-PrCHO > EtCHO > MeCHO > H C H O and n-BuOH > n-PrOH > EtOH > MeOH > H 2 0 , an occurrence observed for alcohols in the Cu+ system6 and to some extent for the aldehyde^.^^^ The
TABLE I V Population Analyses for Fluoro-Substituted q5-CpNiL+ Complexes deg of bonding overlap pop. charge on 0 ligand (L) (Ni-0) (Ni-0) in complexes free ligand
CH3OH CFiOH CH3CHO CF3CHO CH3CH2CHO CHSCF2CHO CF3CH2CHO CF3CF2CHO CH 3 0CH3 CF3OCH3 CF3OCFj
0.6101 0.5932 0.6070 0.5891 0.61 13 0.5990 0.606 1 0.5940 0.6044 0.5870 0.5713
0.3089 0.2862 0.3024 0.2910 0.3055 0.2978 0.3029 0.2951 0.3050 0.2863 0.2677
-0.3630 -0.3 7 5 3 -0.3028 -0.2679 -0.3070 -0.2775 -0.3068 -0.2774 -0.2925 -0.2829 -0.2908
-0.3408 -0.3485 -0.2400 -0.1969 -0.2442 -0.2078 -0.2415 -0.2051 -0.2756 -0.3030 -0.3190
trends in proton affinities for these molecules are similiar, Le., increasing proton affinities with increasing carbon chain length. The population analyses are consistent with the trends mentioned above. The Ni-0 overlap population (Table IIIb) decreases as Me3COH > Me2CHOH > EtOH MeOH, and Me3CCH0 > MezCHCHO > EtCHO > MeCHO > HCHO. The charge on the oxygen (Table IIIc) correlates well with the Ni-0 overlap populations. To investigate the effect of the cyclopentadienyl ring on the metal-ligand bond, all hydrogens on the ring were replaced with either fluorines or methyl groups. When fluorines are substituted on the ring, electron density is inductively withdrawn from the ring decreasing the Ni-ring overlap (Table IIIa-c). The positive charge on the nickel increases and the Ni-ring bond weakens. With the increased positive charge on the nickel, the oxygen donates more electron density to the nickel to compensate for this increase and the bond between the two is enhanced. When methyls are substituted they donate electron density into the ring which in turn donates some of the excess to the nickel, strengthening the Ni-ring bond while the Ni-0 bond is weakened. Therefore, the fluorines enhance the metal-ligand bond, while methyl groups weaken the bond. Fluorines were also substituted systematically for hydrogens in MeOH, EtOH, Me20, Et20, MeCHO, and EtCHO to further probe the transition metal-ligand bond. The bond angles were retained and the only changes in geometry were the lengthening of the C-H bonds of 1.09 8,to C-F bonds of 1.36 A. The effects of substituting fluorines for hydrogens on the ligands cannot be discussed as a group as could the perfluorocyclopentadienyl ring effects, because of apparent interaction between the substituted fluorines and the nickel. The simplebt of the fluoro-substituted ligands to discuss are C F 3 0 H , CF3CH0, CH30CF3,CF30CF,, CH3CF2CH0,CF3CHzCH0,and CF3CF2CH0. These ligands retain their free molecule conformations on complexation with the CpNi' moiety and bind strictly through the oxygen as monodentate ligands. Table IV lists the Ni-0 overlap population, the degree of bonding30 for Ni-0, and the charge on the oxygen in the complex and in the free ligand. From Table IV, it can be seen that the fluorines weaken the Ni-0 bond in all cases, with strongest effect occurring when all hydrogens are replaced by fluorines. Fluorines replacing hydrogens on the a-carbon have a greater weakening effect on the Ni-0 bond than those on the @-carbon. This effect is evident in their ligand-exchange energies. The binding energy increases as CF30CF3< C F 3 0 H < CF,CHO < H20 < CF3CFzCHO < CF30CH3 < HCHO < CH3CFzCHO < MeOH < C F 3 C H z C H 0 < MeCHO < MezO < EtOH C EtCHO (Table V). The fluorines withdraw electron density from the carbon and force the oxygen lone pairs to be more localized on the oxygen, thus lowering the amount of donation into the nickel 4s orbital and weakening the Ni-0 bond. For the fluoro-substituted ethanol and diethyl ether ligands, the fluorine effects on the Ni-0 bonding are masked by interaction ' between the fluorines on the @-carbons and the nickel. The conformation of the free ligands is changed on complexation to the CpNi+ moiety to allow the maximum interaction between the nickel and the fluorine on the p-carbon which is in the plane with
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Throckmorton and Marynick
650 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 TABLE V List of Fluoro-Substituted and Unsubstituted Ligands in Order of Strongest to Weakest Binding Ligand" AE,~ alcohol ligands kcal/mol ether ligands AE, kcal/mol
CF3CH2OH CF3CF2OH CHSCF20H CF3OH
-7.4 -12.5 -1 2.9 -17.1
(CF3CH2)20 CF3CH20CH2CH3 CF3CF20CH2CF3 CF3CH20CFZCF3 CF3CF20CH2CH3 CH3CF20CH2CH3 CH3CFzOCFZCH3 CH3OCF3 (CHICF2)20 CF3OCF3
-1.8 -4.5 -6.8 -8.6 -9.3 -10.8 -13.4 -14.8 -15.3 -20.0
"Energy corrected according to eq 7 in text. bThe exchange energies are relative to AD[CpNi+-CH,NC] = 0, where D[CpNi+-CH3NC] = 57.7 kcal/mol for the reaction CpNi+-CH3NC CpNi+ + CH3NC.
-
TABLE VI: Population Analysis for Fluoro-Substituted Ethanol and Diethyl Ether n5-CpNiL+ ComDlexes
ligand (L)
deg of bonding (Ni-F") 0.2087 0.2035 0.2229 0.2216 0.2262 0.2247 0.2261 0.2251 0.2221
charge charge overlap on 0 in on Ni in POP. (Ni-Fa) complexes complexes 0.1098 0.1064 0.1132 0.1124 0.1155 0.1 148 0.1152 0.1159 0.1127
-0.3 58 8 -0.3730 -0.2780 -0.3014 -0.2838 -0.2945 -0.2948 -0.3056 -0.2897
0.9504 0.9448 0.9632 0.9579 0.9602 0.9579 0.9579 0.9554 0.9608
" Fluorines on the @-carbonsonly. the nickel as shown in Figure 3a,c. The average Ni-F distance for these fluorines is 2.10 A while that of Ni-0 is 1.89 A. The average Ni-F overlap populations of 0.114 for fluoro-substituted diethyl ether and 0.108 for fluoro-substituted ethanol are clearly significant (Table VI). These results suggest that when fluorines are substituted on the @-carbons, the fluoro-substituted ligands act like bi- or tridentate ligands. Examination of the ligand-exchange energies of these ligands verifies this suggestion (Table V). Both (CF,CH,),O and CF,CH,0CH2CH3 are stronger binding ligands than Et20. When fluorines are substituted on the a-carbons of ethanol and diethyl ether the preferred complex conformation places the fluorines as close to the nickel as the fixed experimental geometry will allow (Figure 3b). Substitution of fluorines on the a-carbon inductively withdraws electron density from the oxygen, weakening the Ni-0 bond and, thus, decreasing the ligand binding energy. Table V presents the order of the decrease as fluorines are substituted on the a-carbons as well as on the /%carbons. Conclusion
This study demonstrates that the PRDDO molecular orbital method can reproduce relative transition metal-ligand exchange energies within groups of related molecules: alcohols, aldehydes, and ethers. Applying a least-squares parameter to correct the PRDDO AE's produces linear correlations between AE's and experimental AH'S for the alcohols and ethers, which bond through u-donation into the empty 4s and 4p, orbitals of the nickel, and the aldehydes, which bond at an angle to allow the oxygen a lone pair to overlap with the empty Ni acceptor orbitals. The PRDDO corrected AE's yield the correct relative order for the alkylamines when compared to their proton affinities. When compared to experimental data, Me3N and Me,NH are inverted. Fluorines substituted for hydrogens on the Cp ring enhance the Ni-0 bond by withdrawing electron density from the ring and
A B C Figure 3. Three examples of changes in conformation when fluorines are
substituted for hydrogens in the ligands. The complexes in the top row indicate the preferred conformation of the unsubstituted ligands. The complexes in the bottom row give the preferred conformation of the fluoro-substituted ligands. The preferred conformations are lower in energy by (A) 11.43 kcal/mol, (B) 2.65 kcal/mol, and (C) 18.42 kcal/mol.
weakening the Ni-ring bond. Fluorines substituted for hydrogens on the ligands have the opposite effect, weakening the Ni-O bond by withdrawing electron density from the a-carbon and localizing the lone pairs on the oxygen. Fluoro-substituted ethanol and diethyl ether ligands produce bi- or tridentate ligands. Substituting methyl groups for hydrogens on the Cp ring strengthens the Ni-ring bond and weakens the Ni-0 bond by donating electron density into the ring which in turn donates excess electron density to the nickel.
Acknowledgment. This work was supported by the Robert A. Welch Foundation (Grant Y-743) and the Organized Research Fund of The University of Texas at Arlington. Registry No. q5-CpNi(H20)+,111846-73-2; q5-CpNi(MeOH)+, 60507-9 1-7; q5-CpNi(EtOH)+, 60507-87-1; q'-CpNi(i-PrOH)+, 6050811-4; q5-CpNi(t-BuOH)+,60508- 16-9; qS-CpNi(MezO)+,60507-94-0; q5-CpNi(Et20)+,60508-1 8-1; q5-CpNi(i-Pr20)+,11 1846-74-3; q5CpNi(HCHO)+, 11 1846-75-4; q5-CpNi(MeCHO)+, 60507-85-9; 9,CpNi(EtCHO)+, 60508-08-9; q5-CpNi(i-PrCHO)+, 60508-09-0; 9,CpNi(C2H3CHO)+,60508-10-3; q5-CpNi(t-BuCHO)+, 60508-12-5; q5-CpNi(Me2CO)+, 60508-14-7; q5-CpNi(NH3)', 60508-20-5; q5CpNi(MeNH2)+, 60508-22-7; q5-CpNi(Me,NH)+, 60508-24-9; q5CpNi(Me3N)+, 60508-23-8; q5-CpNi(n-PrOH)+, 11 1846-76-5; q 5 CpNi(n-BuOH)', 111846-77-6; q5-CpNi(i-BuOH)+,111846-78-7; qsCpNi(n-PrCHO)', 111846-79-8; q5-CpNi(n-BuCHO)+,11 1846-80-1; qS-CpNi(i-BuCHO)+,111846-81-2; q5-(CsF5)Ni(H20)+, 11 1846-82-3; q5-(C5F5)Ni(MeOH)+,111846-83-4; v5-(C5F5)Ni(EtOH)+,111846-84-5; q5-(C5F5)Ni(i-PrOH)+,111846-85-6; qs-(C5F5)Ni(t-BuOH)+,11184686-7; q5-(C5F5)Ni(Me20)+, 111846-87-8; q5-(C5F5)Ni(Et20)+, 11184688-9; qs-(C5FS)Ni(i-Pr20)+,11 1846-89-0; q5-(C5F5)Ni(HCHO)+, 11 1846-90-3; q5-(C5F5)Ni(MeCHO)+,111846-91-4; qs-(C5F5)Ni(EtCHO)', 111846-92-5; q5-(C5FS)Ni(i-PrCHO)+,11 1846-93-6; 9,(CsF5)Ni(t-BuCHO)+,111846-94-7; q5-(C5(CH3),)Ni(H2O)+, 11184695-8; q5-(C5(CH3)S)Ni(MeOH)+,111846-96-9; q5-(C,(CH3),)Ni(EtOH)', 111846-97-0; q5-(CS(CH3),)Ni(i-PrOH)+,111846-98-1; q5(C5(CH3),)Ni(t-BuOH)+, 111846-99-2; q5-(C5(CH3),)Ni(Me20)+, 111847-00-8; qs-(CS(CH3)5)Ni(Et20)+, 11 1847-01-9; q5-(C5(CH3),)Ni(i-Pr20)+, 111847-02-0; q5-(C5(CH3),)Ni(HCH0)+,111847-03-1; q5-(C5(CH,)S)Ni(MeCHO)+, 111847-04-2; q5-(C5(CH3)5)Ni(EtCHO)+, 11 1847-05-3; q5-(C5(CH3)s)Ni(i-PrCHO)+,111847-06-4; q5-(C,(CH3),)Ni(t-BuCHO)+, 111847-07-5; q5-CpNi(CF30H)+,111847-08-6; q5-CpNi(CF3CHO)+,111847-09-7; q5-CpNi(CH3CF2CHO)+,11184710-0; q5-CpNi(CF3CH2CHO)+,111847-11-1; q5-CpNi(CF3CF2CHO)+, 111847-12-2; q5-CpNi(CF30CH,)+,111847-13-3; q5-CpNi(CF30CF3)+, 11 1847-14-4; q5-CpNi(CF3CH20H)+, 11 1868-75-8; q5-CpNi(CF3CF20H)+, 111847-15-5; q5-CpNi(CH3CF20H)+,111847-16-6; q5-CpNi((CF3CH2),0)+,111847-17-7; q5-CpNi(CF3CH20CH,CH3)+, 11 1847-18-8; q5-CpNi(CF3CF20CH2CF3)+, 111847-19-9; qS-CpNi(CF3CF20CH2CH3)+,111847-20-2; q5-CpNi(CH,CF20CH2CH3)+, 111847-21-3; qs-CpNi(CH3CF20CF,CH3)+,11 1847-22-4.
