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Anion Effects on the Adsorption of Acetylene by Nickel Halides Helen Y. Huang, Ralph T. Yang,* and Ning Chen Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 Received August 21, 1998. In Final Form: May 27, 1999 The adsorption of acetylene on different nickel halides has been studied by ab initio molecular orbital calculations. The strength of adsorption on different nickel halides is calculated by ab initio molecular orbital calculation and it follows the order NiF2 > NiCl2 > NiBr2 > NiI2. The calculated heats of adsorption are 25.97, 20.42, 18.24, and 16.42 kcal/mol, respectively. The calculations are performed on C2H2, NiX2, and bonded C2H2-NiX2, at the HF/3-21G level for geometry optimization and B3LYP/3-21+G** level for a detailed analysis of the electronic distribution using natural bond orbital (NBO) theory. The bonding between acetylene and NiX2 involves three parts: (1) σ-donation (overlap of the 2px orbital of C with the 4s orbital of Ni), (2) electron redistribution (from the 4s orbital to the 3dxz orbital of Ni, and (3) d-π* back-donation (from the 3dyz orbital of Ni to the 2py*, or π*, orbital of C). The back-donation dominates the bonding. The three steps combined yield the minimum total energies.
Introduction Separation of acetylene is important in two types of industrial processes: acetylene production and the removal of acetylene and its derivatives as a preseparation step for other separation processes. These are discussed as follows. Acetylene is produced from hydrocarbons by various thermal cracking processes followed by its separation from other hydrocarbon cracking products at near-ambient temperatures. The separation is presently accomplished by costly solvent extraction processes employing liquid solvents such as DMF (dimethylformamide). Separation by gas-solid adsorption would be highly desirable, particularly by the efficient pressure swing adsorption processes.1 However, no appropriate sorbents are available since the known sorbents either do not have the high selectivities for acetylene (over other hydrocarbons) or adsorb acetylene irreversibly. Olefin-paraffin separations represent a class of most important and also most costly separations in the chemical and petrochemical industry. Cryogenic distillation has been used for over 60 years for these separations.2 They remain the most energy-intensive distillations because of the close relative volatilities. Weak chemical complexation provides new possibilities for separations since such bonds are selective (hence high capacity) and reversible.3 Highly efficient π-complexation adsorbents have recently been developed in our laboratory for selective olefin adsorption.4-8 These sorbents contain highly dispersed Ag+ or * To whom correspondence should be addressed. E-mail: yang@ umich.edu. (1) Yang, R. T. In Gas Separation by Adsorption Processes; Butterworth: Boston, 1987; reprinted in paperback, Imperial College Press and World Scientific Press: River Edge, NJ, 1997. (2) Keller, G. E.; Marcinkowsky, A. E.; Verma, S. K.; Williamson, K. D. In Separation and Purification Technology; Li, N. N., Cale, J. M., Eds.; Marcel Dekker: New York, 1992. (3) King, C. J. In Handbook of Separation Process Technology; Rousseau, R. W., Ed.; Wiley: New York, 1987. (4) Yang, R. T.; Kikkinides, E. S. AIChE J. 1995, 41, 509. (5) Cheng, L. S.; Yang, R. T. Adsorption 1995, 1, 61. (6) Wu, Z.; Han, S. S.; Cho, S. H.; Kim, J. N.; Chue, K. T.; Yang, R. T. Ind. Eng. Chem. Res. 1997, 36, 2749. (7) Padin, J.; Yang, R. T. Chem. Eng. Sci. 1999, in press. (8) Rege, S. U.; Padin, J.; Yang, R. T. AIChE J. 1998, 44, 799.
Cu+ ions on the surface for olefin bonding. However, acetylene chemisorbs strongly (and dissociates) on Ag+ and Cu+ to form acetylides (e.g., HCtCAg and C2Ag2), and the acetylides are highly unstable and are susceptible to detonation.2 Therefore, acetylene must be preseparated prior to olefin-paraffin separations. Also, for safety reasons, the trace amount (ppm level) of acetylene contained in air must be preseparated prior to all air separation processes, including cryogenic processes. The most selective and also reversible sorbents for acetylene adsorption (over other small hydrocarbon molecules, e.g., C2H4, C2H6, C3H6, C3H8) have been recently developed in our laboratory.9,10 These sorbents contain Fe2+, Co2+, or Ni2+, cations dispersed in monolayer form on high-surface-area substrates (γ-Al2O3, SiO2, and MCM41). For bond strength with acetylene, the cations follow the trend Fe2+ > Co2+ > Ni2+, and the silica-based substrates were superior over alumina. Design and understanding of the new sorbents require a fundamental understanding of the sorbent-sorbate bonding. Molecular orbital theory is ideally suited for this purpose. We have employed a semiempirical molecular orbital (MO) technique to study adsorption of small hydrocarbon molecules on sorbents containing Ag+ and Cu+ salts.11 Compared with the semiempirical MO techniques, the ab initio methods provide meaningful solutions to the electronic Schro¨dinger equations for all molecular orbitals in a molecule. A large number of ab initio calculations have been made in studies of catalysis, surface reactions, and adsorption on silica and zeolites.12-26 (9) Yang, R. T.; Foldes, R. Ind. Eng. Chem. Res. 1996, 35, 1006. (10) Padin, J.; Yang, R. T. Ind. Eng. Chem. Res. 1997, 36, 4224. (11) Chen, J. P.; Yang, R. T. Langmuir 1995, 11, 3450. (12) Sauer, J. Chem. Rev. 1989, 89, 199. (13) Beran, S. Quantum-Chemical Studies of Zeolites. In Theoretical Aspects of Heterogeneous Catalysis; Moffat, J. B., Ed.; van Nostrand Reinhold: New York, 1990. (14) Kobayashi, H.; Yamaguchi, M.; Tanaka, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1 1985, 81, 1513. (15) Matthews, P. S. C. In Quantum Chemistry of Atoms and Molecules; Cambridge University Press: Cambridge, UK, 1986. (16) Kassab, E.; Seiti, K.; Allavena, M. J. Phys. Chem. 1988, 92, 6705. (17) Kassab, E.; Fouquet, J.; Allavena, M.; Evleth, E. M. J. Phys. Chem. 1993, 97, 9034.
10.1021/la981079c CCC: $18.00 © 1999 American Chemical Society Published on Web 08/14/1999
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Although the majority of the ab initio calculations on adsorption involved “chemisorption” or strong “physisorption”, ab initio molecular orbital methods do not make such a distinction. However, they are well suited for our purpose of calculating the heat of adsorption of acetylene on nickel halides as we will disccuss in later sections; the nature of bonding between the adsorbent and adsorbate is π complexation, which is a weak chemical interaction. Adsorption systems that have been studied by using ab initio molecular orbital theories include O2/C2H4/vanadia14 and the adsorption of H2O, NH3, CO, and various hydrocarbons on silica and zeolites (as referenced above). Among the commercially available ab initio molecular orbital calculation packages, Gaussian 9427 is the most popular one. The Gaussian 94 package has been used recently in our laboratory to study adsorption of O2, N2, and C2H4 on AgX (X ) halide) and Ag-zeolite.28,29 In this work, we studied the anion effects on selective acetylene adsorption by using the ab initio molecular orbital calculation methods. The MO results correctly predicted the trend of the anion effects. Moreover, on the basis of the MO results, a MO theory involving electron redistribution in Ni2+ and 3dyz-2py* (π*) back-donation was proposed for the adsorption of acetylene on Ni2+. Calculation Details Selection of Basis Sets. Gaussian 94 program package27 was used for all the calculations. The most extensively used method involves the minimal basis set STO3G,30,31 created by replacing the Slater-type atomic orbital (STO) with three Gaussian functions. The split-valence basis sets are higher level basis sets with more basis functions used for each valence atomic orbital. The 3-21G basis set is the simplest one among the split-valence basis sets.32,33 The notation for the 3-21G basis set denotes an s-type inner-shelf function with three primitive Gaussian functions, an inner set of valence s- and p-type functions with two primitive Gaussian functions, and another outer sp set function with one primitive Gaussian function. Split-valence basis sets allow orbitals to change size, but not shape. Polarized basis sets remove this limitation by adding orbitals with angular momentum beyond what is required for the ground state to the description of each atom. The only two available polarization functions for the 3-21G basis set are d- and p-type functions, and they can be added as 3-21G**. Furthermore, adding a diffuse (18) Moffat, J. B., Ed. In Theoretical Aspects of Heterogeneous Catalysis; Van Nostrand Reinhold: New York, 1990. (19) Teunissen, E. H.; van Duijneveldt, F. B.; van Santen, R. A. J. Phys. Chem. 1992, 96, 366. (20) Teunissen, E. H.; van Santen, R. A.; Jansen, A. P. J. J. Phys. Chem. 1993, 97, 203. (21) Brand, H. V.; Curtiss, L. A.; Iton, L. E. J. Phys. Chem. 1993, 97, 12773. (22) Haase, F.; Sauer, J. J. Phys. Chem. 1994, 98, 3083. (23) Haase, F.; Sauer, J. J. Am. Chem. Soc. 1995, 117, 3780. (24) Suzuki, T.; Tamon, H.; Okazaki, M. Chem. Lett. 1994, 2151. (25) Hill, J. R.; Sauer, J. J. Phys. Chem. 1995, 99, 9536. (26) Shah, R.; Payne, M. C.; Lee, M. H.; Gale, J. D. Science 1996, 271, 1395. (27) Frisch, M. J.; et al. (34 names). In Gaussian 94, Revision A.1; Gaussian, Inc.: Pittsburgh, PA, 1995. (28) Chen, N.; Yang, R. T. Ind. Eng. Chem. Res. 1996, 35, 4020. (29) Huang, H. Y., Padin, J.; Yang, R. T. J. Phys. Chem. B 1999, 103, 3206. (30) Hehre, W. J.; Stewart, R. F.; Pople, J. A. J. Chem. Phys. 1969, 51 (6), 2657. (31) Collins, J. B.; Von Schleyer, P. R. J. Chem. Phys. 1976, 64 (12), 5142. (32) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102, 939. (33) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre, W. J. J. Am. Chem. Soc. 1982, 104, 2797.
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function, the large-size version of s- and p-type functions, can be important for systems in which electrons are relatively far from the nucleus, e.g., large atoms such as Br and I. The addition of a diffuse function for 3-21G basis set is employed by adding a “+” sign in front of G, i.e., 3-21+G. Arguments abound on the proper balance between the level of model chemistry (i.e., method + basis set) and the computational cost. A comprehensive study has been made for the best combinations of methods and basis sets, by comparing many model chemistries with the highest calculation levels of G1 and G2 theory for a variety of “real world” cases.34 Thus the following general strategy has been adopted in the field: use a relatively low model chemistry for geometry calculations, and use a higher level for energy and orbital (e.g., NBO) calculations. The nickel atom and the halide atoms, especially Br and I, have a large number of electrons. Also, it has been known that the computational cost of the all-electron ab initio calculations increases as N4, where N is the number of electrons. Hence, they will generate a large number of basis functions even with small basis sets such as STO3G and 3-21G. Moreover, the available basis sets for iodine are restricted to STO-3G, 3-21G, and G-311G. Therefore, in this work, HF/3-21G is used for geometry optimization, and then energies and NBO calculations are performed on the B3LYP/3-21+G** level. Later discussion on model chemistry verification supports the choice of B3LYP/321+G**//HF/3-21G. Geometry Optimization, Electron Correlation, and Density Functional Theory. Geometry optimization is the first step in all calculations. Calculations for all other parameters such as charges, populations, and energies are all based on the geometrically optimized system. In geometry optimization, the geometry is adjusted until a stationary point on the potential surface is found, which means the structure reaches energy minimum. The computational models used are NiX2, C2H2, and bonded NiX2-C2H2. The adsorption models were chosen to be as simple as possible with only a single NiX2 interacting with one acetylene molecule. The main reason for this is that in order to analyze the model using π-complexation theory the same meaningful conclusions from a study with clusters would always be achieved as far as the analysis of the two-way donor-acceptor model is concerned since the metal bears the same electronic configuration.35 On the other hand, the purpose of this work is to study the anion (F -, Cl -, Br -, I -) effects on the adsorption of C2H2 on NiX2, so what we are concerned with most in this study is the trend of the properties of the adsorption systems, while the absolute values are not as important. Moreover, since the same theory and basis set were used for all the calculations, they are generally of nearly equal accuracy for all the elements, so they can reveal reliable trends as well as possible experimental inaccuracies. Electron correlation effects, which are neglected in the Hartree-Fock approximation, is very important for multiple-electron systems, since the energy contribution arising from the electrons interacting with one another increases with the number of electrons that are involved. All post-SCF methods, such as Moller-Plesset, include electron correlation to the basic Hartree-Fock model. On the other hand, the methods based on density functional (34) Foresman, J. B.; Frisch, A. In Exploring Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, PA, 1996. (35) Miralles-Sabater, J.; Merchan, M.; Nebot-Gil, I.; Viruela-Marin, P. M. J. Phys. Chem. 1988, 92, 4853.
Adsorption of C2H2 on Nickel Halides
theory, by including some of the electron correlation effects, have gained increasing acceptance, mainly because they require less computation and are actually more accurate than traditional electron correlation methods. Density functional theory methods partition the electronic energy into several components which are computed separately: kinetic energy, electron-nucleus interaction, Coulombic repulsion, and an exchange-correlation term that accounts for the remainder of the electron-electron interactions. In this work, B3LYP density functional theory method combined with 3-21+G** basis set is used for high-end calculations, such as total SCF energies and NBO analyses. Density functional theory (DFT) was applied to analyze the metal-olefin bond features in terms of natural bond orbital (NBO) scheme. In recent years, DFT has been recognized as a promising approach in the field of ab initio calculations. It is found that the methods originated in DFT are advantageous, since they provide an accurate description of the metal-ligand interactions without losing the simple chemical interpretation arising from a single-determinant scheme.36 In particular, it has been found that a great improvement over the standard DFT can be achieved by a hybrid method consisting of HF and DFT, leading to the so-called self-consistent hybrid (SCH) approaches.37 The SCH approaches are able to provide reliable geometric, thermodynamic, and spectroscopic parameters for a wide class of metal-ligand interactions, ranging from covalent bonds to weak noncovalent interactions..38-40 The advantage of these methods is that they incorporate electron correlation at an affordable computational cost, so they are efficient tools for studying molecular properties of large molecules such as compounds of transition metals. Natural Bond Orbital (NBO). Atomic charge, orbital energy, and population are important pieces of information for determining electronic configuration, net charge association, and, hence, the nature of the bond. However, their quantification is difficult. Among the numerous schemes proposed for atomic population analysis, only that of Mulliken41 population analysis has been widely used. Unfortunately, Mulliken populations fail to give a useful and reliable characterization of charge distribution in many cases.42-44 Natural population analysis (NPA) seems to be a promising alternative to conventional Mulliken population analysis. It gives a better description of the electron distribution in compounds of high ionic character, such as those containing metal atoms.45 The NPA includes a series of calculations such as the determination of natural atomic orbitals (NAOs), natural hybrid orbitals (NHOs), natural bond orbitals (NBOs), and natural localized molecular orbitals (NLMOs). It performs population analysis and energetic analysis that pertain to localized wave-function properties. It is very sensitive for calculating localized weak interactions, such as charge transfer, hydrogen bonds, and weak chemisorption. There(36) Ziegler, T. Chem. Rev. 1991, 91, 651. (37) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (38) Ricca, A.; Bauschlicher, C. W. J. Phys. Chem. 1994, 98, 12899. (39) Barone, V.; Adamo, C. J. Phys. Chem. 1996, 100, 335. (40) Halthausen, M. C.; Heinemann, C.; Cornhl, H. H.; Koch, W.; Schwarz, H. J. Chem. Phys. 1995, 102, 4931. (41) Mulliken, R. S.; Ermler, W. C. Diatomic Molecules: Results of Ab Initio Calculations, Academic Press: New York, 1977; p 33. (42) De Profit, F.; Marin, J. M. L.; Geerling, P. Chem. Phys. Lett. 1996, 250, 393. (43) Luthi, H. P.; Ammeter, J. H.; Almlof, J.; Faegri, K. J. Chem. Phys, 1982, 77, 2002. (44) Collins, B.; Streitwieser, A. J. Comput. Chem. 1980, 1, 81. (45) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83 (2), 735.
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Figure 1. Two adsorbate-adsorbent models resulting from geometry optimization. Table 1. Total SCF Energies and Energies of Adsorption (∆E) by the Optimized Model (Figure 9); 1 hartree ) 627.5 Kcal/mol type
HF/3-21G
energy (hartrees)
∆E (kcal/mol)
cross cross cross cross
C2H2 NiF2 NiCl2 NiBr2 NiI2 NiF2-C2H2 NiCl2-C2H2 NiBr2-C2H2 NiI2-C2H2
-76.3960 -1697.328863 -2414.1634 -6619.6611 -15275.2256 -1773.766213 -2490.5919 -6696.0861 -15351.6477
-25.97 -20.42 -18.24 -16.42
fore, the NBO program46 included in Gaussian 9427 was used for studying the electron density and charge distribution of the adsorption systems. Energy of Adsorption Calculations. The determination of bond energies of the metal-olefin bonds is important for predicting stable structures and adsorption mechanisms. In this work, the bond energies have been calculated using the optimized geometries and according to the following expression:
Eads ) Eadsorbate + Eadsorbent - Eadsorbent-adsorbate (1) where Eadsorbate and Eadsorbent are the total energies of the adsorbate (C2H2) and the bare adsorbent (NiX2), respectively, and Eadsorbent-adsorbate is the total energy of the adsorbate/adsorbent system. A higher Eads corresponds to a stronger adsorption. Results and Discussion Geometry-Optimized Adsorbate-Adsorbent Models and Energies of Adsorption. NiF2 has the rutile crystal structure (of the SnO2 type), whereas NiCl2, NiBr2, and NiI2 all have the hexagonal (rhombohedral) CdCl2 structure.47 The common feature of these four salts is that the nickel atom is directly connected to four halide atoms in either the X or Y direction, and these halide atoms are connected to other nickel atoms to form the crystal structure. This feature leads to two types of NiX2-C2H2 adsorption model, plane type and cross type, as shown in Figure 1. Geometry optimizations for the NiX2-C2H2 molecular systems resulted in either of these two types. The total SCF energies and the energies of adsorption (calculated using eq 1) are summarized in Table 1. The calculated energies of adsorption show that the crosstype adsorption model leads to higher energies of adsorption than the plane type, indicating that the cross-type configuration is more stable than the plane type. Therefore, our further discussion will be focused on the cross-type configuration. (46) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1, 1995. (47) Wells, A. F. In Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1984; p 413.
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Table 2. Geometry-Optimized Structural Data: Bond Length and Bond Angle HF/3-21G Ni-X Ni-C C-C C-H
C2H2
NiF2
NiCl2
1.689
NiBr2
2.238
2.370
NiI2
NiF2-C2H2
Bond Length (Å) 2.596
1.187 1.051
X-Ni-X C-Ni-C
131.7
179.9
180.0
NiCl2-C2H2
1.709 2.267 1.197 1.056
Bond Angle (deg) 180.0 113.1 30.6
NiBr2-C2H2
2.258 2.345 1.197 1.057 130.3 29.6
2.394 2.349 1.196 1.057 130.5 29.5
NiI2-C2H2 2.640 2.361 1.196 1.057 133.8 29.3
Table 3. Net Charges of Atoms in the Adsorption Systems Calculated by Natural Bond Orbital (NBO) Method (X ) Halide) B3LYP/3-21+G** Ni X C H
C2H2
-0.2452 0.2452
NiF2
NiCl2
NiBr2
NiI2
NiF2-C2H2
NiCl2-C2H2
NiBr2-C2H2
NiI2-C2H2
1.431 70 -0.715 85
1.322 68 -0.661 34
1.069 52 -0.534 76
1.039 73 -0.519 86
1.586 08 -0.721 59 -0.363 16 0.291 71
1.367 53 -0.645 96 -0.329 79 0.291 98
1.184 74 -0.539 36 -0.343 14 0.290 13
1.144 08 -0.522 21 -0.339 96 0.290 12
The optimized structural data are summarized in Table 2. The calculated CtC bond length for free acetylene is 1.187 Å, which is slightly shorter than the standard value of 1.21 Å.48 However, for all adsorption systems, the CtC bond length is increased to around 1.2 Å, indicating that the CtC bond is weakened upon adsorption. Next, from NiF2 to NiI2, the Ni-C bond length increases from about 2.267 to 2.361, which indicates that the interaction of NiX2 with C2H2 is the strongest for NiF2 and weakest for NiI2. This result is consistent with the order of calculated energies of adsorption. Also, with the increase in the ionic radius of halides from F, Cl, Br, to I, the bond angles vary accordingly. The increases in the X-Ni-X bond angle are in the same sequence with the change in the halide radius, indicating a repulsion effect by the halide ions. Finally, the calculated C-H bond lengths for all models are within 3% of the standard values.48 This result supports the reliability of our further calculations. The changes in structure parameters of C2H2 and the formation of Ni-C bonds can be explained through π-complexation theory as discussed earlier. In the interaction of C2H2 with a transition metal, donation and backdonation processes take place. The first consequence of the donation and back-donation processes is the weakening of the C-C bond strength (donation of π-bonding electron from C2H2 to the s orbital of metal and back-donation of d electron from metal to the antibonding π orbitals both have the effect of weakening the C-C bonding). A stronger deformation of the molecule indicates a stronger adsorption of olefin on MX. Net Charges. The net charges on individual atoms calculated with the NBO method are shown in Table 3. It is not surprising that the charges on the Ni atom are always positive, since electrons are attracted by the halide atoms, and the trend follows the order of electronegativity of the halides. Correspondingly the charges on the two halide atoms are negative and the negative charge increases with electronegativity. However, a careful analysis of the charges on the nickel and halide atoms shows that there is an imbalance between the positive charge of Ni and the negative charges on the two halide atoms for all four adsorption models. The net positive charge of NiX2 is actually balanced by the negative charges of the two carbon atoms of C2H2. Therefore, there is electron transfer from Ni to acetylene. This will be discussed shortly along with the electron occupancy analysis by NBO.
The trend of anion effects on the adsorption of olefin can be interpreted in terms of the different electronegativities of the anions, i.e., following the order F > Cl > Br > I. An element with a stronger electronegativity would “attract” more electrons from the metal bonded to it, while a metal with more positive charges would be a better acceptor to form π bond with the olefin.11 The NBO charge analysis results shown in Table 3 manifests this trend. From the table, it is obvious that the positive charge on Ni drops from NiF2 to NiI2, which corresponds to the stronger interaction of NiF2 with C2H2 than NiI2 with C2H2. Orbital Energy, Occupancy, and How the Adsorption Bond Is Formed. The general theory for π complexation developed by Dewar49 has been used as the basic theory to interpret our results on selective adsorption of olefins by Ag+ and Cu+ 4,11,28,29 and adsorption of N2 on Ag-zeolite.28 The bonding involves σ donation (i.e., donation from the 2p orbitals of olefin to the 5s orbital of Ag+) and d-π* back-donation (i.e., donation from the 4dyz orbitals of Ag+ to the 2p* antibonding orbitals of olefin.) The d-π* back-donation accounts for only minor contribution to the bond.11,28 The bonding of acetylene to Ni2+ is considerably more complicated. Blizzard and Santry50 discussed the bonding based on Dewar’s theory and hybridization of valence orbitals of Ni2+ with their semiempirical CNDO/2 molecular orbital results. Understanding of the actual bonding between C2H2 and Ni2+ requires ab initio MO calculations, as performed in this study. In the adsorption of C2H4 on Ag+, we discussed electron redistribution from 4dz2 orbital to the 4dyz orbital within Ag+.28 For electron redistribution to occur between two orbitals, it is essential that the orbitals are matched in spatial symmetry as well as close overlap of orbital energies. The Gaussian 94-NBO calculation results on orbital energies and orbital occupancies are given in Tables 4 and 5, respectively. The bonding of C2H2 to NiX2 is discussed as follows. First, the 4s orbital of the Ni atom (i.e., LUMO) for all four Ni-X2 and the 2px/2py orbitals of C (i.e., HOMO) in free acetylene are within very close energy levels. These average gaps are about 0.1 hartree (2.5 eV), which makes it possible for electron transfer from 2px/2py of C to 4s of Ni. This is the σ donation, as shown in Figure 2A. This is consistent with the decrease in the 2px occupancy of the C atoms upon C2H2 adsorption (Table 5). However, the 4s orbital occupancy of Ni does not increase; on the contrary,
(48) Weast, R. C. In Handbook of Chemistry and Physics, 6th ed.; CRC Press: Cleveland, OH, 1987.
(49) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71. (50) Blizzard, A. C.; Santry, D. P. J. Am. Chem. Soc. 1968, 90, 5746.
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Table 4. Orbital Energies (in hartrees) for Individual Molecular Orbitals Calculated by NBO B3LYP/3-21+G** Ni 4s Ni 3dxy Ni 3dyz Ni 3dxz Ni 3dx2-y2 Ni 3dz2 C 2s C 2px C 2py C 2pz Xs X px X py X pz
C2H2
-0.1085 -0.0658 -0.0658 0.0142
NiF2
NiCl2
NiBr2
NiI2
NiF2-C2H2
NiCl2-C2H2
NiBr2-C2H2
NiI2-C2H2
-0.1170 -0.3160 -0.3037 -0.2166 -0.2790 -0.3443
-0.1507 -0.3182 -0.3105 -0.2213 -0.2962 -0.3567
-0.1776 -0.2856 -0.2955 -0.1998 -0.2834 -0.3212
-0.1876 -0.2788 -0.2874 -0.1916 -0.2765 -0.3178
-1.2896 -0.4172 -0.4458 -0.4251
-0.9684 -0.3363 -0.3624 -0.3360
-0.9949 -0.2969 -0.2972 -0.3208
-0.8614 -0.2769 -0.2775 -0.2992
-0.03074 -0.2905 -0.1944 -0.2795 -0.2734 -0.3286 -0.2487 -0.1992 -0.0742 -0.2243 -1.2910 -0.4001 -0.3805 -0.3926
-0.0400 -0.2962 -0.1957 -0.2875 -0.2835 -0.3354 -0.2474 -0.1984 -0.0720 -0.2197 -0.9585 -0.3248 -0.3124 -0.3156
-0.0937 -0.2801 -0.1915 -0.2699 -0.2679 -0.3188 -0.2383 -0.1907 -0.0646 -0.2119 -0.9919 -0.2900 -0.2784 -0.2819
-0.1249 -0.2785 -0.1911 -0.2694 -0.2682 -0.3176 -0.2378 -0.1901 -0.0628 -0.2116 -0.8650 -0.2718 -0.2601 -0.2633
Table 5. Orbital Occupancies for Individual Molecular Orbitals Calculated by NBO B3LYP/3-21+G** Ni 4s Ni 3dxy Ni 3dyz Ni 3dxz Ni 3dx2-y2 Ni 3dz2 total 3d C 2s C 2px C 2py C 2pz total 2p Xs X px X py X pz total p
C2H2
NiF2
NiCl2
NiBr2
NiI2
NiF2-C2H2
NiCl2-C2H2
NiBr2-C2H2
NiI2-C2H2
0.5349 1.9915 1.9884 0.3590 1.6721 1.9790 7.9900
0.6138 1.9960 2.0000 0.2328 1.7997 1.9831 8.0117
0.7727 2.0000 1.9974 0.2614 1.9842 1.7876 8.0307
0.7905 2.0000 1.9972 0.2479 1.9841 1.8221 8.0512
1.9795 1.9968 1.7708 1.9516 5.7192
1.9788 1.9918 1.7951 1.8724 5.6593
1.9824 1.9787 1.8442 1.7240 5.5470
1.9787 1.9836 1.8554 1.6960 5.5350
0.3470 1.9836 0.4265 1.7973 1.8545 1.9629 8.0249 1.0325 0.9902 1.2641 1.0453 3.2996 1.9760 1.7713 1.9907 1.9663 5.7283
0.4827 1.9907 0.3781 1.8580 1.8825 1.9622 8.0716 1.0308 0.9896 1.2608 1.0198 3.2711 1.9763 1.7088 1.9872 1.9543 5.5603
0.5972 1.9912 0.3994 1.8473 1.8836 1.9633 8.0847 1.0284 0.9921 1.2616 1.0266 3.2803 1.9797 1.6569 1.9693 1.9282 5.5544
0.6294 1.9910 0.3991 1.8528 1.8965 1.9583 8.0976 1.0280 0.9928 1.2609 1.0251 3.2787 1.9764 1.6400 1.9747 1.9254 5.5401
1.0030 0.9997 0.9997 1.2381 3.2375
Figure 2. Bonding of C2H2 to NiX2, showing (A) electron transfer in σ donation, (B) electron redistribution, and (C) electron back-donation. Note: (A) and (B) are in the plane of the paper whereas (C) is perpendicular to the plane of the paper.
it has decreased by about 0.2 upon adsorption (Table 5). A careful examination of the orbital energies and occupancies reveals the very close energy levels between the 4s and 3dxz orbitals of Ni. Since the energy of 4s is slightly higher than that of 3dxz, and these orbitals are in close vicinity (Figure 2 B), redistribution of electrons from 4s to 3dxz upon adsorption becomes easy. The increase in the 3dxz orbital occupancy of Ni upon C2H2 adsorption (Table 5) is clear evidence of electron redistribution. After the formation of the adsorption system (NiX2-C2H2), the increase of 4s orbital energy of Ni and decrease in the 2px/2py orbital energy result in a bigger energy gap between Ni 4s orbital and C 2px/2py orbitals, indicating a stabilizing
effect by electron transfer between the Ni and C atoms and electron redistribution within the Ni atom. The adsorption bonding is not complete without d-π* back-donation. The next closest valence orbital of Ni to 2px/2py orbitals of C is 3dyz. The energy gap between those orbitals is about 0.2 hartree (or 5 eV). Therefore, the d-π* back-donation takes place from the 3dyz orbital of Ni to 2py* (or π* bond) of C2H2 (Figure 2C). As shown in Table 5, the decrease in the electron occupancy of the Ni 3dyz orbital and increase in the electron occupancy of C 2py orbital after adsorption clearly verifies the facts of electron back-donation. As we can also see, the back-donation dominates the interaction of C2H2 with NiX2 based on the
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changes in occupancy. It is important to note that the d-π* back-donation is not to the 2px* orbital of C2H2, but to 2py*, which is in the plane 90° from that of 2px*. In contrast, ethylene does not have the second π bond as C2H2 does; hence it cannot adsorb on NiX2. The d-π* backdonation to the 2py* bond is shown in Figure 2C, which shows the perfect matching of the orbitals (four lobes) and signs of the 3dyz and 2py* orbitals. This perfect matching compensates for the relatively large distance as compared to π complexation of C2H2 with Ag+.28 Acknowledgment. This work was supported by the NSF under Grants CTS-9520328 and CTS-9819008.
Huang et al.
Nomenclature E ) energy HF ) Hartree-Fock HOMO ) highest occupied molecular orbital LUMO ) lowest occupied molecular orbital MO ) molecular orbital NBO ) natural bond orbital SCF ) self-consistent field STO-3G ) Slater-type atomic orbital with three Gaussian functions B3LYP ) Becke three-parameter hybrid with Lee, Yang, and Parr correlation LA981079C
