WANG Bingwu, XU Guangxian & CHEN Zhida

Science in China Ser. B Chemistry 2004 Vol.47 No.2 91 97 91 Study on the covalence of Cu and chemical bonding in an inorganic fullerene-like molecule, [CuCl] 20 [Cp*FeP 5 ] 12 [Cu-(CH 3 CN) 2 Cl ] 5, by a density functional approach WANG Bingwu, XU Guangxian & CHEN Zhida State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Correspondence should be addressed to Xu Guangxian (email: gxu@pku.edu.cn) Received December 3, 2003 Abstract The electronic structure and chemical bonding in a recently synthesized inorganic fullerene-like molecule, {[CuCl] 20 [Cp*FeP 5 ] 12 [Cu(CH 3 CN) 2 Cl ] 5 }, has been studied by a density functional approach. Geometrical optimization of the three basic structural units of the molecule is performed with Amsterdam Density Functional Program. The results are in agreement with the experiment. Localized MO s obtained by Boys-Foster method give a clear picture of the chemical bonding in this molecule. The reason why CuCl can react with Cp*FeP 5 in solvent CH 3 CN to form the fullerene-like molecule is explained in terms of the soft-hard Lewis acid base theory and a new concept of covalence. Keywords: inorganic fullerene-like molecule, chemical bonding, covalence, copper. DOI: 10.1360/03yb0177 The titled inorganic fullerene-like molecule (hereafter abbreviated as IFM) was recently synthesized by Bai et al. [1], which attracts a lot of interests from inorganic and organometallic chemists, and questions are raised for this smart molecule: ( ) Why CuCl can react with Cp*FeP 5 in solvent CH 3 CN to form IFM? ( ) What is the nature of chemical bonding? ( ) What is the covalence of Cu in this molecule? In this paper we intend to answer these questions in terms of the soft-hard Lewis acid base theory and a new concept of covalence, verified by quantum chemical calculations using the density functional theory. 1 Elucidation of the reaction between CuCl and Cp*FeP 5 Cp*FeP 5 is a ferrocene-like molecule, where Cp* is a penta-methyl-cyclopentadienyl radical and P 5 is iso-electronic with Cp (both CH and P have 5 valence electrons), so P 5 can replace a Cp group in ferrocene, FeCp 2. The difference between FeCp 2 and Cp*FeP 5 is that the latter is a strong soft Lewis base, because each P atom contains a lone electron-pair, which may react with soft Lewis acids, such as CuCl. Each P atom of the P 5 pentagon is therefore linked with a Lewis acid CuCl to form a Cu P dative bond, as shown in the central part of fig. 1. Each CuCl molecule requires 3 dative bonds Copyright by Science in China Press 2004

92 Science in China Ser. B Chemistry Fig. 1. Chemical bonding in a half shell of IFM, [Cp*FeP 5] 6[CuCl(PH 3)] 10, corresponding to the ball-stick model in fig. 2 of Bai s original paper [1]. The five µ 2 -Cl anions and the five µ 2 -Cu(CH 3CN) 2 cations, which serve as bridges connecting the other half shell to a whole sphere is also shown in the figure. to saturate the covalence of Cu (the reason will be explained later). Therefore, Cu is linked with three P atoms, 2 Cu atoms and 4 P atoms to form a hexagon. Thus the half spherical shell of IFM contains 6 pentagons and 10 hexagons, as shown in fig. 1. Fig. 2(a) shows the basic structural unit A, [Cp*FeP 5 ] 2 [CuCl(PH) 3 ] 2, of this half shell, in which the third Cu P bond is replaced by Cu PH 3, in order that the unit A may be a discrete molecule. The solvent mixtures CH 2 Cl 2 /CH 3 CN play an important role in the formation of IFM, because CH 3 CN is not only a solvent but also a soft Lewis base, which reacts with CuCl to form the complex ion-pair, Cu(CH 3 CN) 2 Cl, which serves as bridges to connect the two half shells to a spherical molecule. The half shell (fig. 1) has 5 outmost Cu atoms, each of which is linked by a µ 2 -Cl anion to each of the corresponding 5 outmost Cu atoms of another half sphere. This connecting structural unit B is shown in fig. 2(b). Furthermore, the half sphere has also 5 outmost P atoms, each of which is linked by a Cu complex cation, Cu(CH 3 CN) 2, to each of the corresponding 5 outmost P atoms of another half sphere. This connecting struc- tural unit C is shown in fig. 2(c). 2 Geometry optimization of the basic structural units of IFM In order to verify the above description of the bonding of IFM, we first performed geometry optimizations of the three structural units A, B and C with the Amsterdam Density Functional (ADF) program package version 2003.2 [2]. The local density approximation (LDA) with local exchange and correlation potentials of Vosko, Wilk and Nusair (VWN) were used [3]. Becke s non-local exchange correction [4] and Perdew s non-local correlation correction [5] were added in each SCF consistent cycle. We adopted the TZ2P basis sets in ADF, which consists of a triple-ζ basis sets for all atoms and adding two polarization functions for atoms from H to Ar. The Cu(1s-3p), Fe(1s-3p), Cl(1s-2p), P(1s-2p), C(1s), N(1s) inner cores are kept frozen. The numerical integration procedure applied for the calculations was the polyhedron method developed by Velde and his coworkers [6,7]. The BFGS (Broyden-Fletcher-Goldfarb-Shanno) [8 11] procedure was used in the geometry optimizations. Geometry optimizations were performed on all

Study on the covalence of Cu and chemical bonding in [CuCl] 20[Cp*FeP 5] 12[Cu-(CH 3CN) 2 Cl ] 5 molecule by a density functional approach 93 Fig. 2. (a) The basic structural unit A, [Cp*FeP 5] 2[CuCl(PH 3)] 2, half shell of IFM; (b) the basic structural unit B, µ 2 -Cl anion bridge connecting the two half shells of IFM; (c) the basic structural unit C, µ 2 -Cu(CH 3CN) 2 cation bridge connecting the two half shells of IFM. the freedom of three structural units, and the initial structures were taken from the crystallographic X-ray data of IFM. The average deviation of the calculated Cu P and Cu-Cl bond lengths from those from the X-ray data of the whole IFM is 0.007 nm (table 1). 3 Ab initio calculations of SCF MO s localized by Boys-Foster method Boys-Foster [12,13] method for localization of molecular orbitals was used to analyze the wave functions produced by the self-consistent field procedure. For each pair of orbitals, a 2 2 unitary transformation is performed. The Jacobi type transformation is ψ (i) = cos(t) ψ (i) sin(t) ψ (j), Table 1 Optimized geometry of the basic structural units A, B and C Structural units Unit A Unit B Unit C Bond length/nm, bond angle/( ) Becke- Perdew X-ray data in IFM molecule Cu-Cl (end) 0.2250 0.2206 Cu P 0.2388 0.2292 P Cu P 100.9 101.6 Cu Cl (brid.) 0.2523 0.2330 Cu-Cl (end) 0.2282 0.2227 Cu Cl-Cu 169.3 165.0 Cu N 0.2067 0.1997 Cu P 0.2349 0.2270 P Cu P 117.4 110.5 N Cu N 109.1 120.0

94 Science in China Ser. B Chemistry ψ ( j) = sin(t) ψ (i) cos(t) ψ ( j). In the Boys-Foster method, it maximizes the sum of the squares of the MO dipole moment integrals and gets the minimal spatial extent, so the localized orbitals are as compact as possible. The set of localized orbitals thus obtained in this paper is a good description of bonding in the unit A (table 2), unit B (table 3) and unit C (table 4). Table 2 Compositions of Boys-Foster localized MO s of unit A, [Cp*FeP 5] 2[CuCl(PH) 3] 2 (only those related to Cu bonds are shown) No of MO s Energy /ev 43 10.40 44 10.39 45 10.39 46 10.37 47 9.83 48 9.82 55 9.60 56 9.59 Compositions (%) 8.1%Cu2(4s) 6.2%Cu2(4p x) 2.8%Cu2(4p y) 53.5%P7(3s) 17.4%P7(3p x) 9.6%P7(3p y) 8.2%Cu1(4s) 5.9%Cu1(4p x) 2.7%Cu1(4p y) 53.2%P8(3s) 15.9%P8(3p x) 7.9%P8(3p y) 3.2%P8(3p z) 8.1%Cu2(4s) 3.9%Cu2(4p x) 4.4%Cu2(4p y) 52.8%P16(3s) 12.2%P16(3p x) 13.3%P16(3p y) 2.1%P16(3p z) 8.1%Cu1(4s) 4.2%Cu1(4p x) 1.6%Cu1(4p y) 3.4%Cu1(4p z) 52.7%P18(3s) 13.6%P18(3p x) 5.2%P18(3p y) 8.8%P18(3p z) 8.9%Cu2(4s) 2.7%Cu2(4p y) 6.8%Cu2(4p z) 40.6%P12(3s) 17.1%P12(3p y) 23.0%P12(3p z) 8.9%Cu1(4s) 7.4%Cu1(4p y) 2.0%Cu1(4p z) 40.6%P13(3s) 35.9%P13(3p y) 3.8%P13(3p z) 6.3%Cu1(4s) 1.5%Cu1(4s) 9.1%Cu1(4p z) 14.2%Cl3(3s) 3.9%Cl3(3p x) 60.8%Cl3(3p z) 6.3%Cu2(4s) 1.5%Cu2(4s) 2.3%Cu2(4p y) 6.8%Cu2(4p z) 14.2%Cl4(3s) 3.6%Cl4(3p x) 18.3%Cl4(3p y) 43.3%Cl4(3p z) Character Cu P dative bonds Cu P dative bonds Cu-Cl σ bond In fig. 3(a), 80.5% of the electronic cloud of MO#43 is still located on P atom, and only 17.1% is donated to Cu atom. It represents the Cu2 P7 dative bond. Similarly, MO#44 represents Cu1 P8 bond, MO#45 represents Cu2 P16 bond, and Mo#46 represents Cu1 P18. In fig. 3(b), 80.7% of the electronic cloud of MO#55 is still located on P atom, only 18.4% is donated to Cu atom. It represents the Cu2 P12 dative Table 3 Compositions of Boys-Foster Localized MO s of Unit B, [CuCl(PH 3)] 2Cl (only those related to Cu bonds are shown) No of MO s Energy/ ev 13 6.83 14 6.77 15 6.72 16 6.71 17 6.70 18 6.70 19 6.32 20 6.30 Composition (%) 4.1%Cu2(4s) 6.4%Cu2(4p x) 25.5%Cl5(3s) 40.7%Cl5(3p x) 21.0%Cl5(3p y) 3.8%Cu1(4s) 6.0%Cu1(4p x) 25.6%Cl5(3s) 37.9%Cl5(3p x) 24.5%Cl5(3p y) 7.3%Cu1(4s) 2.1%Cu1(4p y) 5.8%Cu1(4p z) 44.8%P8(3s) 3.2%P8(3p y) 36.6%P8(3p z) 7.2%Cu2(4s) 2.3%Cu2(4p y) 5.6%Cu2(4p z) 45.0%P9(3s) 3.6%P9(3p y) 36.0%P9(3p z) 7.2%Cu1(4s) 2.6%Cu1(4p y) 5.2%Cu1(4p z) 45.0%P6(3s) 4.0%P6(3p y) 35.4%P6(3p z) 7.1%Cu2(4s) 2.3%Cu2(4p y) 5.4%Cu2(4p z) 45.1%P7(3s) 3.2%P7(3p y) 36.1%P7(3p z) 6.8%Cu1(4s) 2.6%Cu1(4p x) 6.0%Cu1(4p y) 15.7%Cl3(3s) 20.5%Cl3(3p x) 45.7%Cl3(3p y) 6.7%Cu2(4s) 2.7%Cu2(4p x) 5.9%Cu2(4p y) 15.7%Cl4(3s) 21.1%Cl4(3p x) 45.2%Cl4(3p y) Character Cu Cl dative bond Cu P dative bonds Cu-Cl σ bond Table 4 Compositions of Boys-Foster localized MO s of unit C, µ 2 - Cu(CH 3CN) 2 (only those related to Cu bonds are shown) No of MO s Energy /ev 15 15.30 16 15.29 23 12.68 Composition (%) 4.3%Cu1(4s) 3.1%Cu1(4p y) 5.9%Cu1(4p z) 52.6%N4(4s) 9.0%N4(2p y) 25.4%N4(2p z) 4.3%Cu1(4s) 3.0%Cu1(4p y) 5.9%Cu1(4p z) 52.7%N5(4s) 8.5%N5(2p y) 26.0%N5(2p z) 9.7%Cu1(4s) 7.6%Cu1(4p x) 3.4%Cu1(4p y) 38.0%P2(3s) 30.1%P2(3p x) 10.0%P2(3p y) Character Cu N dative bond Cu P dative bond bond. Similarly, MO#56 represents Cu1 P13 bond. In fig. 3(c), 78.9% of the electronic cloud of MO#47 is donated by Cl atom, only 16.9% is donated by Cu atom. It represents the Cu1-Cl3 σ bond. Similarly, MO#56 represents Cu2-Cl4 σ bond. In fig. 4(a), 61.7% of the electronic cloud of MO#13 is still located on Cl atom, and 36.0% is do-

Study on the covalence of Cu and chemical bonding in [CuCl] 20[Cp*FeP 5] 12[Cu-(CH 3CN) 2 Cl ] 5 molecule by a density functional approach 95 Fig. 3. Localized molecular orbitals of unit A. (a) MO#43 46 for 4 Cu P dative bonds; (b) MO#47 and 48 for 2 Cu P dative bonds; (c) MO#55 and 56 for 2 Cu-Cl σ bonds. Fig. 4. Localized molecular orbitals of unit B. (a) MO#13 and 14 for 2 Cu Cl dative bonds; (b) MO#15 18 for 4 Cu P dative bonds; (c) MO#19 and 20 for 2 Cu-Cl σ bonds. nated to Cu atom. It represents the Cu2 Cl5 dative bond. Similarly, MO#14 represents Cu1 Cl5 bond. In fig. 4(b), 84.6% of the electronic cloud of MO#15 is still located on P atom, and 15.2% is donated to Cu atom. It represents the Cu1 P8 dative bond. Similarly, MO#16 represents Cu2 P9 bond, MO#17 represents Cu1 P6 bond, and MO#18 represents Cu2 P7 bond. In fig. 4(c), 81.9% of the electronic cloud of MO#19 is donated by Cl atom, only 15.4% is donated by Cu atom. It represents the Cu1-Cl3 σ bond. Similarly, MO#20 represents Cu2-Cl4 σ bond. In fig. 5(a), 87.0% of the electronic cloud of MO#15 is still located on N atom, and 13.3% is donated to Cu atom. It represents the Cu1 N4 dative bond. Similarly, MO#16 represents Cu1 N5 bond. In fig. 5(b), 78.1% of the electronic cloud of MO#23 is still located on P atom, and 20.7% is donated to Cu atom. It represents the Cu1 P2 dative bond. Similarly, MO#24 represents Cu1 P3 bond. 4 Mulliken s charge distribution of atoms in IFM The Mulliken charge distribution of the structural units A, B and C is shown in table 5. It can be seen

96 Science in China Ser. B Chemistry Fig. 5. Localized molecular orbitals of unit C. (a) MO#15 and 16 for 2 Cu N dative bonds; (b) MO#23 and 24 for 2 Cu P dative bonds. Table 5 Mulliken charge distribution of some atoms of the structural units A, B and C Unit Atom Charge Unit Atom Charge Unit Atom Charge 1 Cu 0.36 1 Cu 0.13 1 Cu 0.23 Unit A 2 Cu 0.36 2 Cu 0.13 2 P 0.80 3 Cl 0.47 3 Cl 0.55 3 P 0.80 4 Cl 0.47 4 Cl 0.55 4 N 0.17 5 Fe 0.60 Unit B 5 Cl 0.37 Unit C 5 C 0.23 6 Fe 0.58 6 P 0.76 6 C 0.42 7 P 0.35 7 P 0.76 7 N 0.17 8 P 0.35 8 P 0.76 8 C 0.23 9 P 0.06 9 P 0.76 9 C 0.42 that Cu has a negative charge of 0.36 and P has a positive charge of 0.35. This is an evidence of the formation of Cu P dative bond. The Fe atoms have even higher negative charges, due to the donation of π electrons from the cyclopentadienyl ring as well as from the P 5 ring to the vacant valence orbitals 4s and 4p of the Fe atom. The population distribution of electrons among the valence orbitals of Fe is 7.26 on 3d, 0.95 on 4p, and 0.36 on 4s. The population analysis is not listed in table 5, but is available on request. The small deviation between the charges of Fe5 and Fe6 is due to the fact that we did not put symmetry restriction in geometrical optimization, so there are small computational errors in bond lengths. P9 atom is nearly neutral, because there are no Cu P bonds. 5 Covalence of Cu CuCl is conventionally called mono-valent copper chloride. Here, by the term mono-valent, it is meant that the oxidation state of copper is 1, and is denoted by Cu(I). Since most of Cu(I) organometallics and clusters are molecules, a question is raised: What is the covalence of Cu? Pauling s definition of covalence is very successful in organic chemistry, but has encountered some difficulty in inorganic chemistry and organometallic chemistry. For example, what is the covalence of atoms in molecules CO, NO, Ni(CO) 4, etc? This difficulty is mainly due to the fact that there is no difference between the normal and the dative bonds. Xu [14,15] proposed a new definition of covalence as follows: The covalence of an atom in a molecule is equal to the number of sharing electrons accepted from the neighboring atoms during the formation of the molecule. It is also equal to the number of vacancies in the valence shell of the atom. Thus for the two kinds of covalence, we have ( ) Normal bond A B V A = V B = 1, Bond order = (V A V B )/2 = 1; ( ) Dative bond A B V A = 2, V B = 0, Bond order = (V A V B )/2 = 1.

Study on the covalence of Cu and chemical bonding in [CuCl] 20[Cp*FeP 5] 12[Cu-(CH 3CN) 2 Cl ] 5 molecule by a density functional approach 97 Since the Cu(I) atom in the IFM molecule requires three dative Cu P bonds and one normal Cu Cl bond to saturate its covalence as demonstrated above, its covalence is 2 3 1 = 7. The valence shell of Cu atom has 9 orbitals (d 5 sp 3 ), which can accommodate 18 electrons, while the Cu atom has 11 valence electrons, therefore, it has 7 vacancies, which is just its covalence. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 20041006). References 1. Bai, J. F., Alexander, V. V., Manfred, S., Synthesis of inorganic fullerene-like molecules, Science, 2003, 300: 781 783.[DOI] 2. Amsterdam Density Functional (ADF), Version 2003.2; Scientific Computing and Modeling, Theoretical Chemistry, Amsterdam: Virje University, 2003. 3. Vosko, S. H., Wilk, L., Nusair, M., Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis, Can. J. Phys., 1980, 58: 1200 1211. 4. Becke, A. D., Density-functional exchange-energy approximation with correct asymptotic behavior, Phys. Rev. A, 1988, 38: 3098 3100. [DOI] 5. Perdew, J. P., Density-functional approximation for the correlation energy of the inhomogeneous electron gas, Phys. Rev. B, 1986, 33: 8822 8824. [DOI] 6. Boerrigter, P. M., Velde, G. T., Baerends, E. J., Three-dimensional numerical integration for electronic structure calculations, Int. J. Quant. Chem., 1988, 33: 87 113. 7. Velde, G. T., Baelends, E. J., Numerical integration for polyatomic systems, J. Comput. Phys., 1992, 99: 84 98. 8. Broyden, C. G., The convergence of a class of double-rank minimization algorithms 2, The new algorithm, J. of the Inst. for Math. and Applications, 1970, 6: 222 231. 9. Fletcher, R., A new approach to variable-metric algorithms, Computer Journal, 1970, 13: 317 322. 10. Goldfarb, D., A family of variable-metric algorithms derived by variational means, Math. Comp., 1970, 24: 23 26. 11. Shanno, D. F., Conditioning of quasi-newton methods for function minimization, Mathematics of Computation, 1970, 24: 647 656. 12. Rüdenberg, C. K., Localized atomic and molecular orbitals, Rev. Mod. Phys., 1963, 35: 457 464. 13. Boys, S. F., Construction of some molecular orbitals to be approximately invariant for changes from one molecule to another, Rev. Mod. Phys., 1960, 32: 296 299. 14. Xu Guangxian, Structural rules of cluster compounds and related molecules (I) The (nxcπ) formalism, Chemical J. Chinese Universities, 1984, 1: 100. 15. Xu Guangxian, Structural rules of cluster compounds and related molecules (II) New definition of covalence, Molecular Science, 1983, 1(10): 1 14.