Organic Ligand Sandwich Clusters

Aug 2, 2010 - Tingting Zhang , Zhi Tian , Liyan Zhu , Xiuyun Zhang , Qian Chen , Jinlan Wang. Computational and Theoretical Chemistry 2013 1013, 46-51...
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Ab Initio Study on Mixed Inorganic/Organic Ligand Sandwich Clusters: BzTMC60, TM ) Sc-Co Liyan Zhu, Tingting Zhang, Mengxi Yi, and Jinlan Wang* Department of Physics, Southeast UniVersity, Nanjing, 211189, People’s Republic of China ReceiVed: July 2, 2010; ReVised Manuscript ReceiVed: July 16, 2010

We have systematically investigated mixed inorganic/organic ligand sandwich clusters comprised of 3d transition metal (TM) atoms with C60 and benzene (Bz) molecules, BzTMC60, by using all electron density functional theory. We found the bonding type between TM and C60 in the ground state evolves from η6 (TM ) Sc-Cr) to η5 (TM ) Mn) and then to η2 (TM ) Fe, Co) with increasing number of d electrons of TM. The BzTMC60 clusters (TM ) Sc-Co) are of high stability through ionic-covalent interactions. The BzCrC60 cluster has the lowest binding energy due to its largest spin-flip energy and the weakest ionic bonding interaction between the CrBz unit and C60. With the exception of BzTiC60 being triplet, all the BzTMC60 clusters energetically prefer the lowest available spin states, e.g., the ground spin state is either a singlet (with an even number of electrons) or a doublet (with an odd number of electrons). Moreover, the magnetic properties of BzTMC60 show clear dependence to the bond type between TM and C60, and the η5-ligand configurations tend to be in high spin states. I. Introduction Organometallic sandwich clusters comprised of transition metal (TM) atoms and organic molecules have been extensively investigated due to their exceptional structural, electronic, magnetic, and optical properties, as well as their promising applications in nanoelectronics/spintronics since the first discovery of ferrocene, Fe(Cp)2 (Cp ) cyclopentadienyl).1,2 Multidecker sandwiched TM-benzene (Bz ) C6H6)3 and lanthanide (Ln)-cyclooctatetraene (COT ) C8H8)4,5 clusters have been successfully synthesized and exhibit novel magnetic properties.6-18 For example, it has been found through Stern-Gerlach experiments19 that the magnetic moments of VnBzn+1 clusters are linearly dependent on the cluster size, which is an average value over different spin states of low-lying isomers.6 Additional theoretical studies10,20 have revealed that the V- and Mn-Bz infinite molecular wires are half metallic ferromagnets, as well as their isoelectronic counterparts V- and Mn-borazine.21,22 The Ln-COT sandwich clusters also show size-dependent magnetic behavior similar to V-Bz clusters. The magnetic moments, however, are much higher than those of the 3d TM-Bz sandwich clusters,23 which are mainly contributed by the highly localized 4f electrons of Ln atoms.17 Moreover, relativistic density functional theory calculations suggest that the interior Eu-COT bonding is ionic, but the Eu-COT interaction at the two ends is hybrid covalent-ionic in Eun(COT)n+1 multidecker clusters.17 Additionally, Eun(COT)n+1 clusters coupled to gold electrodes show a nearly perfect spin filter effect, which is vital for applications in future spintronics.18 Besides the planar ligands Cp, Bz, COT, and borazine, spherical fullerenes can also coordinate to TM atoms forming multidecker sandwich clusters. Nakajima et al.24-28 have successfully synthesized TMn(C60)m+ cationic clusters in which TM and C60 are suggested to form chain or ring structures for early TMs or rice-ball structures for late TMs. The spherical C60 cage, consisting of 20 hexagonal rings and 12 pentagonal rings, exhibits complicated reactivity and can form different types with * To whom correspondence should be addressed, [email protected].

metal ions. For example, C60 acts as an η2 type (bridge site over a C-C bond in C60, two bonds) with a Pt atom in C60Pt(PPh3)2,29 while it can also serve as an η5- (five bond) or η6-ligand with different TM atoms.30 Chemical probe experiments on TMn(C60)m+ (TM ) Sc-Ni) clusters show different bonding types between TM and C60. Namely, the C60 acts as an η6-ligand toward early TMs (TM ) Sc, Ti, and V),24 while it adopts an η2 or η3 type26 with late TMs. Theoretically, Andriotis et al.31,32 have confirmed that C60 acts as an η6-ligand when bonded to a V atom, and as an η2- or η3-ligand toward a Ni atom. A recent ab initio investigation of Vn(C60)m clusters, however, has revealed that the V atom is only about 0.08 eV in energy preferable to be bonded to the pentagonal ring (η5) of C60 over the hexagonal ring (η6) in the smallest cluster VC60. However the hexagonal ring (η6) is the most favorable binding site for larger clusters.28 On the other hand, mixed ligand sandwich clusters have also attracted considerable interest both experimentally and theoretically. A variety of mixed ligand sandwich clusters, including CpTMCh (TM ) Ti, V, Cr, Nb, Ta, and Mo, Ch ) C7H7),33-37 CpTMCOT (TM ) Ti, V, Co),38 Cp-V-Bz,39,40 and CpFeBz,41 have been successfully synthesized. On the theoretical side, CpTMCh (TM ) Ti-Co) and CpTMCOT (TM ) Ti-Co)42,43 have been studied in great detail with ab initio calculations. Mixed four- and six-membered ring and three- and sevenmembered ring sandwich clusters, C4H4TMC6H6 and C3H3TMC7H7 (TM ) Fe, Ru, and Os), have been predicted to be stable enough to exist. Such mixed ligand sandwich clusters have also exhibited intriguing electronic and magnetic properties differing from their homoligand counterparts.44-49 For example, mixed ligand Cp-V-Bz sandwich clusters have higher magnetic moments compared to their homoligand counterpart V-Bz sandwich clusters, and the spin stability is significantly enhanced upon the mixture.48,49 The above studies, however, are focused on the mixture of organic ligands. Is it possible to obtain mixed inorganic/organic ligand sandwich clusters? The answer is yes. Sawamura et al.50 have successfully synthesized a hybrid structure of ferrocene

10.1021/jp106129r  2010 American Chemical Society Published on Web 08/02/2010

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Figure 1. Five initial structures classified by the TM site with respect to C60: the hollow site above the center of a hexagonal (H, a) and a pentagonal (P, b) ring, the bridge site over a short (S, c) and a long (L, d) C-C bond, and the top (T, e) site on a carbon atom of C60. Gray, magenta, and red balls represent carbon, hydrogen, and TM atoms, respectively. The C atoms in C60 directly bonded to TM are labeled as gray balls.

and fullerene, CpFe(C60Me5), in which C60 is identified as an η5-ligand. Besides, mixed inorganic/organic ligand BzFeC60 sandwich clusters comprised of C60, Fe, and Bz have also been successfully synthesized through laser vaporization method.51 So far, a theoretical study of their structural, electronic, and magnetic properties of mixed inorganic/organic ligand sandwich clusters is still lacking. In this study, we report a detailed theoretical investigation on the structural, energetic, electronic, and magnetic properties of mixed inorganic/organic ligand sandwich clusters, BzTMC60 (TM ) Sc-Co) and compare them with their homoligand TMBz2 counterparts. II. Computational Method All calculations were performed using the DMol3 package,52,53 with both spin restricted and unrestricted general gradient approximation. The exchange-correlation potentials were parametrized by the BLYP54,55 functional, combined with an all electron double numerical basis with polarization functions (DNP). The accuracy of this method was verified in our previous studies.7,28 The self-consistent field calculations were carried out with a convergence criterion of 10-6 a.u. on the total energy and electron density. All structures were optimized by using the Broyden-Fletcher-Goldfarb-Shanno algorithm without any symmetry constraint. Vibrational frequency computations were employed to filter out saddle points from true minima of the corresponding potential energy surface of the clusters. The convergence of gradient, displacement and total energy differences was 10-3, 10-3, and 10-5 a.u., respectively. In order to obtain the lowest energy configuration, five different structures were considered according to the site of the TM atom bonded to the fullerene C60 as shown in Figure 1. Namely, the hollow site above the center of a hexagonal (H, η6) and a pentagonal (P, η5) ring, the bridge site over a short (S, η2) and a long (L, η2) C-C bond, and the top (T, η1) site on a carbon atom of C60. The structure of BzTMC60 in configuration X (X ) H, P, L, S, or T) is named as BzTMC60-X for convenience. III. Results and Discussion The geometric and energetic properties of BzTMC60 clusters are summarized in Table 1 and their optimized structures are displayed in Figure 2. Note that configuration T is not included as it was optimized to configuration H or S for all BzTMC60 studied here. Moreover, configuration L, similar to configuration T, is also relaxed to other configurations for the cases of TM ) Sc-Mn. As clearly seen in Table 1, BzTMC60-H with the

lowest spin state (singlet or doublet for even or odd electrons, respectively) is identified as the most stable structure for early TM from Sc to Cr, and the BzTMC60-S and BzTMC60-P have much higher energies. The only exception is BzTiC60-H, whose triplet state is energetically more stable than the singlet by only 0.008 eV. The possible existence of BzCrC60-H is also proposed by Gal’pern et al.56 For BzMnC60, the configurations H, P, and S are close in energy. The configuration P in a doublet state is the most stable, in which both Mn-C60 and Mn-Bz distances are the lowest among the three configurations as well. When the d shell of TM is over half filled, e.g., Fe and Co, the ground-state structures adopt configuration S for BzFeC60 and BzCoC60. As aforementioned, the bonding natures between C60 and 3d TM in TMn(C60)m+ clusters largely depend on the TMs. That is, early TMs prefer bonding to C60 in an η6 type,24 while the late ones adopt an η2 or η3 type,26 which is due to the different occupancy of d electron in TMs and the hybridization strength between d electrons of TM and π eletrons of C60.32 The element-dependent coordination variation, that is, that the ground-state structure of BzTMC60 switches from configuration H (η6) to P (η5) and then to S (η2) with increasing d electrons of TM, can be understood from a simple electron counting rule. As the ligand Bz and hexagonal ring in C60 both can provide six valence electrons, the BzTMC60-H clusters with early TMs (TM ) Sc, Ti, and V) have 15, 16, and 17 electrons, respectively, and the BzCrC60-H exactly reach the so-called 18-electron rule.57 BzMnC60-H would violate the above rule since Mn has one more valence electron than Cr. However, the BzMnC60-P in which Mn bonds to the pentagonal ring of C60 still satisfies the 18-electron rule; thus, the BzMnC60-P is energetically more favorable over the BzMnC60-H. When the d shell of TM is over half-filled (TM ) Fe and Co), both BzTMC60-H and BzTMC60-P disobey the 18-electron rule and fewer bonds are formed between TM and C60. The distance RTM-C60, listed in Table 1, is defined as the distance between TM and the center of a hexagonal (pentagonal) ring directly bonded to TM in BzTMC60-H (BzTMC60-P) or that of C-C bond in BzTMC60-S and BzTMC60-L, while the distance RTM-Bz is the vertical distance between the TM atom and the Bz plane. These distances in the ground state structures of BzTMC60 clusters are displayed in Figure 3a. RTM-Bz is comparable with RTM-C60 in BzScC60 and BzTiC60, but it is much shorter than RTM-C60 in the rest of the BzTMC60 clusters. Moreover, both RTM-C60 and RTM-Bz gradually decrease as TM varies from Sc to Cr with the decreasing atom size

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TABLE 1: Structural and Energetic Properties of BzTMC60a TM

site

M

PGS

∆E

BE

∆

RTM-C60

RTM-Bz

RC-C (C60)b

RC-C (Bz)c

Sc

H S P H S P H S P H S P P S H S P L H S L P H

2 2 4 3 1 5 2 2 4 1 5 3 2 4 2 1 3 3 3 2 2 2 4

Cs C1 Cs Cs C2 C1 C3V Cs C1 C3V C1 C1 Cs C2V Cs C1 Cs C1 C3V C2V Cs Cs Cs

0.00 0.27 0.55 0.00 0.07 0.33 0.00 0.20 0.42 0.00 0.27 0.58 0.00 0.10 0.19 0.00 0.18 0.35 0.72 0.00 0.45 0.76 1.25

3.58 3.31 3.03 3.19 3.12 2.86 3.32 3.11 2.90 1.96 1.69 1.38 3.66 3.57 3.48 4.04 3.86 3.69 3.32 2.50 2.05 1.74 1.25

0.20 0.46 0.15 0.28 1.06 0.13 0.64 0.74 0.15 0.91 0.86 0.14 0.14 0.87 0.12 0.93 0.39 0.44 0.36 1.19 0.64 0.15 0.04

2.03 2.14 2.24 1.97 2.01 2.18 1.85 2.02 2.05 1.81 1.97 1.97 1.88 1.94 1.89 1.88 1.92 1.97 2.00 1.89 1.90 2.00 2.09

2.05 2.12 2.02 1.95 1.89 1.95 1.76 1.75 1.76 1.70 2.17 1.73 1.60 1.84 1.61 1.60 1.63 1.70 1.70 1.69 1.68 1.70 1.76

1.43-1.48 1.53 1.46-1.47 1.42-1.48 1.55 1.46-1.47 1.43-1.46 1.51 1.46-1.47 1.42-1.45 1.52 1.46-1.47 1.46-1.47 1.52 1.41-1.46 1.51 1.46 1.52 1.41-1.46 1.50 1.54 1.46-1.47 1.42-1.46

1.42-1.43 1.40-1.44 1.42-1.43 1.42 1.39-1.46 1.42 1.43 1.42-1.46 1.41-1.44 1.42 1.41-1.42 1.41-1.44 1.42-1.43 1.41-1.44 1.42-1.43 1.41-1.43 1.42 1.42-1.43 1.42 1.42 1.42 1.42 1.42

Ti V Cr Mn Fe

Co

a Spin multiplicity M, point group symmetry PGS, relative energy with respect to the ground state ∆E (eV), binding energy BE (eV), HOMO-LUMO gap ∆ (eV), and the distances of TM-C60, TM-Bz, and C-C (Å). b Bond length of C-C in C60 for those C atoms directly bonded to TM. c Bond length of C-C in Bz.

(Figure 3a), which is consistent with the tendency of the TM-Bz distance in TMBz2 clusters.58 Moreover, C60 fullerene has low reduction potential59 and can serve as an electron acceptor,60 which is also confirmed in BzTMC60 clusters by the charge population analysis (Figure 3b). The C60 is negatively charged in all BzTMC60 clusters with Mulliken charges around -0.5 e, while both TM and Bz are positively charged. Therefore, the ionic bonds induced by charge transfer are formed between the TMBz unit and C60 besides covalent bonds, but such ionic interaction scarcely exists in homoligand TM-Bz sandwich clusters.10 The electrostatic attraction between TMBz and C60 can further stabilize the whole sandwich clusters. Moreover, that the absolute charges on early TMs decrease from Sc to Cr is due to the increasing electronegativity from left to right along the 3d period. To evaluate the stability of BzTMC60 clusters, we calculated the binding energies (BEs) of the ground state structures of the clusters defined as

BE ) E(C60) + E(TM) + E(Bz) - E(BzTMC60) where E( · ) is the total energy of the isolated C60, TM, Bz, and BzTMC60, respectively. As plotted in Figure 3c, all BEs are positive, indicating that the formation of BzTMC60 is an exothermal process. The BzTMC60 (TM ) Sc, Ti, V, Mn, and Fe) clusters have much higher BEs (>3.0 eV), among which BzFeC60 possesses the largest BE of 4.04 eV. Nevertheless, the BzCrC60 cluster has the lowest BE, suggesting its relatively low stability partially originates from the weaker ionic interaction between CrBz and C60, as the lowest Mulliken charges are found located on the CrBz unit and C60 among all mixed inorganic/ organic ligand sandwich clusters (Figure 3b). Moreover, the valence electronic configuration of an isolated Cr atom is 3d54s1, of which six valence electrons have parallel spin orientation giving maximum spin multiplicity. However, the sandwich cluster BzCrC60-H favors the singlet state. Thus, more energy

is needed to overcome the spin-flip,58 which leads to the BzCrC60-H having the lowest BE. A similar phenomenon was also observed in the homoligand sandwich cluster CrBz2 by Pandey et al.58 The different metal-ligand interaction can be further understood from the highest occupied molecular orbitals (HOMOs) displayed in Figure 4. For BzTMC60 with early TM (Sc, Ti, and V), the HOMOs are all δ bonding orbitals, mainly arising from the dxy or dx2-y2 orbitals of TM and π orbitals of C atoms (Figure 4a-d). However, the HOMO of BzCrC60-H is a doubly degenerate nonbonding orbital showing typical dz2 characteristic (Figure 4e). As for BzMnC60, the HOMO is antibonding in configuration H, whereas it is a strong π (dxz) bonding orbital in configuration P. Thus, the BzMnC60-P is more stable than the BzMnC60-H. For the cases of BzFeC60 and BzCoC60, the HOMOs are all π antibonding in configurations H (Figure 4, structures h and i) and P (Figure 4j), while they become robust π bonding orbitals in configuration S. Therefore, BzFeC60 and BzCoC60 energetically favor configuration S rather than H or P. As the bonding natures of HOMOs in BzCoC60 and BzFeC60 are similar, only the HOMOs of BzFeC60 are presented in Figure 4. In terms of magnetic properties, the ground-state structures of BzTMC60 generally prefer the lowest available spin states, i.e., the ground spin state is either a singlet (with an even number of electrons) or a doublet (with an odd number of electrons) with the exception of BzTiC60, whose singlet is higher than the triplet state by 0.008 eV in energy. Compared to their homoligand TMBz2 counterparts, the BzTMC60 clusters show similarity in magnetic properties except for the case of Ti and Fe,58 of which the singlet and triplet states are identified as the ground spin states for TiBz2 and FeBz2, respectively. The discrepancy might arise from the different bonding types between Fe and ligands in BzFeC60 and FeBz2 since Fe-C60 adopts η2 bonding style rather than η6 type in FeBz2. For BzTMC60 with early TM (TM ) Sc-Cr), the magnetic properties show another clear tendency, namely, BzTMC60-P energetically favors the high

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Figure 2. Equilibrium geometries of BzTMC60 in each configuration. The values given in parentheses correspond to their relative energies (in eV) with respect to the ground states.

spin state, of which the magnetic moment is higher than the ground state (BzTMC60-H) by 2 µB (Figure 5a). Similar magnetic enhancement in configuration P is also observed in mixed ligand VBzCp cluster.48 When a Bz is substituted by a Cp in the VBz2 cluster, the magnetic moment increases by 1

µB as a consequence of one electron transferring from V to Cp. Whereas BzTMC60-S with early TM possesses the lowest spin state, BzCrC60-S favors the quintet over the doublet slightly. For the BzTMC60 clusters with late TM, however, there is no clear tendency regarding the magnetic properties.

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Figure 3. (a) Distance RTM-C60 and RTM-Bz. (b) Charge population on C60 (QC60), TM (QTM), and Bz (QBz). (c) BEs of BzTMC60. Note that the C60 is negatively charged.

To understand the enhanced magnetic moments in BzTMC60-P composed of early TMs (TM ) Sc, Ti, V, and Cr), we carefully examined their occupied frontier energy levels. All HOMOs of BzTMC60-P (TM ) Sc, Ti, V, and Cr) are π bonding orbitals between TM and C60, mainly arising from dxz or dyz states of early TM atoms and π states of ligands. The HOMOs in BzTMC60-H (structures a-e in Figure 4), however, are mainly composed of the dxy or dx2-y2 (Sc, Ti, and V) or dz2(Cr) orbitals of early TM. It means that one β-spin electron occupying the dxy or dx2-y2 (Sc, Ti, and V) or dz2 (Cr) orbital of early TM in BzTMC60-H transfers to the R-spin channel occupying the dxz or dyz orbital. As a consequence, the total magnetic moment in BzTMC60-P is enhanced by 2 µB in comparison with the BzTMC60-H. Detailed occupied frontier energy levels, taking BzVC60 for an example, are presented in Figure 5b. In BzVC60-H, three valence electrons of V (3d34s2) occupy a nondegenerate orbital (dz2) and a set of doubly degenerate orbitals (dxy and dx2-y2) in the R-spin channel, while the rest

Figure 5. (a) Magnetic moments (in µB) of BzTMC60 in each configuration and (b) occupied frontier energy level and isosurface density of BzVC60 in the configurations H (left panel) and P (right panel). Red upward and blue downward arrows correspond to spin up and down electrons, respectively.

occupy a set of doubly degenerate orbitals in the opposite spin channel. Such an electron configuration results in one unpaired electron in BzVC60-H, and thus the total magnetic moment is 1 µB. While in BzVC60-P, the lower symmetry and large spin splitting lead to the splitting of those doubly degenerate orbitals, the dxy orbital in the β-spin channel, degenerate with the dx2-y2

Figure 4. Highest occupied molecular orbitals (HOMOs) of BzTMC60. Note that the HOMOs of BzVC60, BzCrC60, and BzFeC60 in configuration H are doubly degenerate.

Mixed Inorganic/Organic Ligand Sandwich Clusters orbital in BzVC60-H, is pushed far above the Fermi level. Therefore the valence electron occupying the dxy orbital in the β-spin channel in BzVC60-H transfers to the R-spin channel and occupies the dxz orbital in BzVC60-P, which eventually results in an enhancement in magnetic moment by 2 µB when the structure switches from BzVC60-H to BzVC60-P. IV. Conclusion In summary, all electron density functional theory calculations have been performed to investigate the structural, energetic, electronic, and magnetic properties of mixed inorganic/organic ligand BzTMC60, TM ) Sc-Co sandwich clusters. The ground state equilibrium geometries of BzTMC60 evolve with increasing d electrons of TM, following the order of H (η6) f P (η5) f S (η2). Namely, the clusters containing early TMs (TM ) Sc-Cr, with d electrons less than half filled or half filled 3d and 4s shell) prefer configuration H, while configuration P is the most energetically favorable for BzMnC60. When the d shell of TM is over half filled, the ground state favors BzTMC60-S. The element-dependent structural evolution can be well understood by a simple electron counting rule. The BzTMC60 clusters (TM ) Sc-Co) are highly stable through ionic-covalent interactions between TM and C60. The BzFeC60 cluster is found to be the most stable cluster, while BzCrC60 is the least stable one. The lesser stability of BzCrC60 is due to the weaker ionic interaction between the CrBz unit and C60 and the larger spinflip energy in the formation of the mixed inorganic/organic sandwich clusters. Moreover, the lowest spin states are energetically preferred for the mixed inorganic/organic ligand BzTMC60 clusters, with the exception of TM ) Ti, of which the triplet state is slightly more stable than the singlet one. Besides, BzTMC60-P tends to possess higher magnetic moments than the ground state configuration (BzTMC60-H) by 2 µB for the clusters containing early TMs (TM ) Sc, Ti, V, and Cr) because of lower symmetry and large spin splitting in BzTMC60-P. Acknowledgment. This work is supported by the NSF (Grant No. 20873019), NBRP (Contract Nos. 2010CB923401 and 2009CB623200), the Peiyu Foundation of SEU, and the Scientific Research Foundation of the Graduate School of SEU (YBJJ0932), China. We thank Jordan J. Phillips in Department of Physcis, Central Michigan University, for editing the manuscript. The calculations were done using a computational facility at the Department of Physics, Southeast University. References and Notes (1) Coulson, C. A.; Higgs, P. W.; March, N. H. Nature 1951, 168, 1039. (2) Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. J. Am. Chem. Soc. 1952, 74, 2125. (3) Kurikawa, T.; Takeda, H.; Hirano, M.; Judai, K.; Arita, T.; Nagao, S.; Nakajima, A.; Kaya, K. Organometallics 1999, 18, 1430. (4) Kurikawa, T.; Negishi, Y.; Hayakawa, F.; Nagao, S.; Miyajima, K.; Nakajima, A.; Kaya, K. J. Am. Chem. Soc. 1998, 120, 11766. (5) Hosoya, N.; Takegami, R.; Suzumura, J.-i.; Yada, K.; Koyasu, K.; Miyajima, K.; Mitsui, M.; Knickelbein, M. B.; Yabushita, S.; Nakajima, A. J. Phys. Chem. A 2004, 109, 9. (6) Wang, J.; Acioli, P. H.; Jellinek, J. J. Am. Chem. Soc. 2005, 127, 2812. (7) Zhang, X.; Wang, J. J. Phys. Chem. A 2007, 112, 296. (8) Wang, J.; Zhu, L.; Zhang, X.; Yang, M. J. Phys. Chem. A 2008, 112, 8226. (9) Wang, J.; Jellinek, J. J. Phys. Chem. A 2005, 109, 10180. (10) Xiang, H.; Yang, J.; Hou, J. G.; Zhu, Q. J. Am. Chem. Soc. 2006, 128, 2310. (11) Zhu, L.; Wang, J.; Yang, M. J. Mol. Struct.: THEOCHEM 2008, 869, 37. (12) Liu, W.; Dolg, M.; Fulde, P. J. Chem. Phys. 1997, 107, 3584. (13) Liu, W.; Dolg, M.; Fulde, P. Inorg. Chem. 1998, 37, 1067.

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