Structure and Bonding in MPb5– (M = Cu, Ag, and Au): A Combined

Article pubs.acs.org/JPCA

Structure and Bonding in MPb5− (M = Cu, Ag, and Au): A Combined Investigation by Theoretical Calculations and Photoelectron Imaging Spectroscopy Lijuan Zhao,† Hua Xie,‡ Zhiling Liu,‡ Jie Wang,† Xiaopeng Xing,*,† and Zichao Tang*,‡ †

College of Materials Sciences and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

‡

ABSTRACT: Bimetallic clusters of MPb5− (M = Cu, Ag, and Au) have been studied using density functional theory and photoelectron imaging spectroscopy. These anionic clusters and their neutrals were determined to be a Pb5 trigonal bipyramid with the coinage metal atom on its triangular facet. This structure of each MPb5− or MPb5 was found to be more than 0.5 eV lower than other structural candidates and that of each MPb5− has a HOMO−LUMO gap of larger than 1.2 eV. The chemical bonding between M and Pb5 in MPb5− was dominantly attributed to the interaction between the outer s orbital of M and the lowest unoccupied molecular orbital (LUMO) of Pb5. The inner d orbitals of M and the occupied orbitals of Pb5 unit only make a little contribution. The different bonding behaviors of Cu, Ag, and Au, which are noticeable in many other species, have little effect on the Pb5 counterpart in MPb5−, indicating Pb5 unit acts partially like a large artificial atom. Additionally, photoelectron spectra of MPb5− (M = Cu, Ag, and Au) provide good experimental data to evaluate different theoretical approaches dealing with relativistic effects in clusters containing heavy atoms. solution of liquid ammonia.37 The crystals containing this double charged anion were obtained with sodium cations sequestered by crown ethers.30 Pb52− unit has similar structure and skeletal orbitals as those of the trigonal bipyramid B5H52− or C2B3H5, and their closo structures and enhanced stabilities are rationalized using Wade’s rule.37 Previous studies using mass spectroscopy showed the enhanced stabilities of MPb5− (M = Cu, Ag, and Au).4 Recently, silver-doped lead cluster anions AgPbn− (n = 5−12) were studied using photoelectron imaging spectroscopy and theoretical calculations.38 Even the predicted orbital levels of AgPb5− were not perfectly consistent with the experimental spectrum, the structure were tentatively determined to a Pb5 trigonal bipyramid combining with the Ag atom. Though all Cu, Ag, and Au have an oxidation state of +1, these elements are more electronegative than the alkali metal elements. The Pauling value of Au is even larger than that of Pb and comparable to that of C.39 Therefore, the chemical conditions in MPb5− (M = Cu, Ag, and Au) should be different from those under which Pb52− exist. Studies on these clusters will give more meaningful information about the characters of Pb5 units. In this article, we report investigations on the structures and bonding of MPb5− (M = Cu, Ag, and Au) using density

1. INTRODUCTION In the past decades, people were interested in atomic clusters of Si, Ge, Sn, and Pb, hoping to find similar stable species as carbon fullerenes. Experimental studies in gas phase using mass spectroscopy,1−5 photoelectron spectroscopy,6−14 and ionmobility spectrometry15,16 revealed many fundamental characteristics of these clusters and indeed found many stable structures with enhanced stability.1,12−14 Many theoretical calculations were also carried out to interpret these experimental observations.17−28 Though the general characters of Pb clusters are metallic, which means spherical and flexible structures,10,15 there are some sizes that are rigid enough to resist doping by heteroatoms, solvating in solutions, or packing in bulky crystals. These clusters include Zintl anions of Pb with closo-structures, which usually carry multiple negative charges and coexist with positive alkali metal cations,12−14,29,30 and some neutral units existing in bulky solids.17,25 Now, there is a growing interest in using the closo-structures of group IV elements in the bottom-up syntheses of nanostructured materials and these clusters bridged or modified by coinage metal atoms have been prepared.31−36 Studies on bimetallic clusters of Pb/M (M = Cu, Ag, and Au) lead to profound understanding of the stability and bonding of the closo-clusters of Pb and help to find suitable building blocks for nanostructured materials. Pb52− is a typical Zintl anion, which was originally found in the alloys of lead and alkali metals and later transferred to the © 2013 American Chemical Society

Received: December 28, 2012 Revised: February 15, 2013 Published: February 25, 2013 2325

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Figure 1. Six lowest lying structures for every MPb5− and MPb5 (M = Cu, Ag, and Au). The symmetry and the electronic state of each structure are indicated. The numbers in parentheses are the theoretical energies (eV) relative to the corresponding lowest lying anionic structure. All results from calculation at B3LYP/aug-cc-PVTZ-pp level.

PVTZ-pp. Vibration analyses were implemented to make sure that the final structures were real minimum points. All these calculations were performed using Gaussian 09 program.40 The most possible structures for each MPb5− were further analyzed using ADF software41 with different options dealing with relativistic effects. We performed single-point calculation of these structures using density-functional theory (DFT) with the function of PW91 and basis sets of TZ2P. In these DFT calculations, the relativistic effects were considered using the spin−orbit option implemented in ADF.42,43 In this option, the calculations employs double group symmetry and electrons are associated with the general Kramer’s symmetry. The orbital levels from this calculation were used to interpret the electronic properties and bonding in MPb5−. To make comparisons, we also performed single-point calculations at PW91/TZ2P level using the scalar option in ADF, in which only the so-called scalar relativistic corrections are included.44−46

functional theory with inclusion of relativistic effects and photoelectron imaging spectroscopy. From the theoretical results and experimental spectra, we conclude that MPb5− or MPb5 (M = Cu, Ag, and Au) always contain a Pb5 bipyramid unit. Even the bonding characters of Cu, Ag, and Au are quite different, the geometries and orbital levels of Pb5 in MPb5− (M = Cu, Ag, and Au) are very similar to those of naked Pb5, indicating it can be viewed as a stable unit under complicated chemical conditions.

2. COMPUTATIONAL METHODS Our calculations considered the singlet and triplet multiplicities of MPb5− and the doublet and quadruplet multiplicities of MPb5. The optimization for each cluster on each spin state started from more than fifteen different structures covering nearly all geometrical possibilities. These structures were initially optimized using density-functional theory (DFT) with the function of B3LYP and the basis sets of Lanl2dz. Six lower lying structures for each species were further optimized using the same B3LYP method and larger basis sets of aug-cc2326

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Figure 2. Photoelectron imaging results of MPb5− (M = Cu, Ag, and Au) (the left column) and the theoretical density of states (DOS) curves for the structural candidates I (the central column) and III (the right column). In the photoelectron imaging results, each inset shows the symmetrized raw image (upper) and the reconstructed image (bottom) after inverse Abel transformation. The double arrow indicates the direction of the laser polarization. The photoelectron bands in the experimental spectra and the theoretical DOS curves are labeled using X, A, B, etc. The anisotropy parameters of the bands in experimental spectra are indicated. The theoretical DOS curves on the central and right columns were generated according to calculations using PW91/TZ2P method with the spin−orbit option and are shifted to align the HOMO to the theoretical vertical detachment energies.

3. EXPERIMENT The experiment was performed on an instrument that has been described elsewhere.47,48 Briefly, cluster anions were generated by laser vaporization of a target compressed from mixed powders of lead and coinage metals. The nascent anionic cluster were cooled and expanded into the source chamber with helium carrier gas, entering perpendicularly to the acceleration region of a TOF mass spectrometer. After separation by this TOF, the anionic clusters of interest were introduced into the photodetachment region and crossed with a laser beam. The third harmonic output of Nd:YAG laser (355 nm/3.496 eV) was used for all these bimetallic clusters. Photoelectrons were analyzed by a collinear velocity-map photoelectron imaging analyzer, which was built according to a modified design of Eppink and Parker.49 These electrons were mapped onto an image detector consisting of a microchannel plate (MCP) assembly and a phosphor screen. The positions of photoelectrons were recorded by a charge-coupled device (CCD) camera and one image data was obtained by accumulating 50000−100000 laser shots. The original 3D distribution was reconstructed using the Basis Set Expansion (BASEX) inverse Abel transform method,50 and the photoelectron spectrum and the anisotropy parameters of transition bands were acquired by calculations based on one central slice of the 3D distribution. The spectrometer was calibrated using the spectra of Ag− and Au− at 355 nm. Energy resolution of this system is better than 5%, which is 50 meV at electron kinetic energy (eKE) of 1 eV.

4. RESULTS 4.1. Theoretical Structures of MPb5− and MPb5 (M = Cu, Ag, and Au). In Figure 1, we present the six lowest lying structures for each MPb5− and MPb5 (M = Cu, Ag, and Au) after optimization at the B3LYP/aug-cc-PVTZ-pp level. The six structures of CuPb5− include two singlet structures (I and III) and four triplet ones (II, IV, V, and VI). Those of neutral CuPb5 include two doublet structures (I′ and V′) and four quadruplet ones (II′, III′, IV′, and VI′). I and I′ are the lowest lying isomers for CuPb5− and CuPb5, respectively. Both of them contain one Pb5 trigonal bipyramid with the Cu atom on one triangular facet and are more than 0.5 eV lower than other structural candidates. The singlet structure III has a corresponding neutral structure of V′ with doublet state. Though the neutral structures of II′, III′, and IV′ are similar to those of anonic II, IV, and V, respectively, all these neutrals are quadruplet and their doublet states are not real minimum points. We did not find a neutral structure similar to the anionic VI. Similarly, the theoretical structures of MPb5− (M = Ag and Au) include singlet (I, III, and VI) and triplet (II, IV, and V) ones, and those of MPb5 (M = Ag and Au) can be doublet (I′, III′, and V′ of AgPb5, I′ and V′ of AuPb5) or quadruplet (II′, IV′, and VI′ of AgPb5, II′, III′, IV′, and VI′ of AuPb5). Structures I and I′ are always at their lowest spin states and are the lowest structures for MPb5− and MPb5 (M = Ag and Au), respectively. They similarly contain one Pb5 trigonal bipyramid 2327

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Table 1. Symmetries, Electronic States, Relative Energies, ADEs and VDEs of Six Lowest Lying Structures of MPb5− (M = Cu, Ag, and Au) Determined at the B3LYP/aug-cc-PVTZ-pp Levela ADE (eV) ΔE (eV) calc

anions CuPb5−

AgPb5−

AuPb5−

a

I Cs 1A′ II C4v 3A1 III C3v 1A1 IV Cs 3A″ V Cs 3A″ VI C2v 3B2 I Cs 1A′ II C4v 3A1 III C3v 1A1 IV C2v 3B2 V Cs 3A″ VI C1 1A I Cs 1A′ II C4v 3A1 III C3v 1A1 IV C2v 3B1 V Cs 3A″ VI C1 1A

0.00 0.56 0.95 0.97 1.03 1.14 0.00 0.60 0.77 1.01 1.03 1.15 0.00 0.59 0.74 0.93 1.00 1.02

corresponding neutral structure I′ Cs 2A′ I′ Cs 2A′ V′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ III′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ V′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′ I′ Cs 2A′

CuPb5

AgPb5

AuPb5

VDE (eV)

calc

expt

calc

expt

2.49 1.93 2.58 1.52 1.46 1.35 2.48 1.88 2.60 1.47 1.45 1.33 2.49 1.90 2.83 1.56 1.49 1.47

2.42(4)

2.60 2.37 2.69 2.04 2.03 1.87 2.58 2.27 2.72 1.92 1.96 2.27 2.57 2.25 2.95 2.07 2.05 2.42

2.52(4)

2.40(4)

2.47(4)

2.49(4)

2.54(4)

The experimental ADEs and VDEs are also listed to make comparisons.

orbital (HOMO) of these anionic structures will cause dramatic structural changes, and the generated MPb5 have different lower lying structures at the lowest doublet state. For each MPb5, we only found two lower lying neutral structures at the doublet state, one containing a Pb5 bipyramid with M on its triangular facet (I′ for CuPb5, AgPb5, and AuPb5) and the other containing a similar Pb5 bipyramid with M on its apex (V′ for CuPb5 and AuPb5, III′ for AgPb5). Because the M of MPb5− (M = Cu, Ag, and Au) binds with more than one Pb atoms in II, IV, V, or IV, it is more likely that these anionic structures change to I′ of MPb5, in which M is on a facet. Therefore, the theoretical ADE of II, IV, V, or IV of each MPb5− was computed as the energy difference between the anionic structure and the neutral I′ of MPb5. Experimentally, the VDE and ADE of one cluster were determined by measuring the maximum and the threshold of X band in the spectrum, respectively. The uncertainty of each experimental energy value was estimated by the product of the kinetic energy of corresponding photoelectrons and the resolution of our apparatus in that range. Table 1 displays the theoretical and experimental VDEs and ADEs of MPb5− (M = Cu, Ag, and Au). We found that the differences between theoretical VDE and ADE of II, IV, V, or VI are larger than 0.35 eV, indicating a broad X band with a long tails on the low energy side. This is in contrast to the experimental spectra shown in Figure 2, in which X bands always have sharp rising edges and fwhm of about 0.1 eV. Additionally, most of the theoretical VDEs for II, IV, V, and VI of MPb5− (M = Cu, Ag, and Au) are not consistent with the experimental values. For the other two candidates of each MPb5−, the energy of structure I is always about 1.0 eV lower than that of III and the theoretical VDE of I is closer to the experimental value. The theoretical ADEs corresponding to photodetachments from I of MPb5− to I′ of MPb5 are also consistent with the experimental values. Because I and I′ are the lowest lying structures of MPb5− and MPb5, respectively, the obtained ADE of MPb5− can also be viewed as the electron affinities (EAs) of MPb5.

with Ag or Au on one triangular facet and are more than 0.5 eV lower than their isomers. III′ of AgPb5 and V′ of AuPb5 are doublets and are the corresponding neutral structures for III of AgPb5− and AuPb5−, respectively. We did not find the neutral doublet structures corresponding to II, IV, V, and VI of AgPb5− and AuPb5−. 4.2. Photoelectron Imaging Spectroscopy of MPb5− (M = Cu, Ag, and Au). The left column in Figure 2 displays the photoelectron imaging results of MPb5− (M = Cu, Ag, and Au). The inset in each panel shows the raw image (top) and the transformed one (bottom), and the double arrow beside the raw image indicates the direction of the laser polarization. The experimental spectra for MPb5− (M = Cu, Ag, and Au) are very similar, each of which contain four transition bands labeled as X, A, B, and C. The C bands are partially cut off due to the limitation of photon energy. The intensities of X, A, and B are quite similar in one spectrum and their β values are close to zero, corresponding to isotropic transitions. For all three spectra, the X bands are around 2.5 eV and the intervals from X to A and those from A to C are around 0.5 eV. One difference among the three spectra is that the interval between B and C of AuPb5− is smaller than that of CuPb5− or AgPb5−.

5. DISCUSSION 5.1. Determining the Structures of MPb5− and MPb5 (M = Cu, Ag, and Au). Theoretically, the vertical detachment energy (VDE) of an anionic structure was computed as the energy difference between the anion and the neutral calculated at the equilibrium geometry of the anion, whereas the adiabatic detachment energy (ADE) was defined as the energy difference between the anion and its corresponding neutral at their respective equilibrium geometries. All theoretical VDEs and ADEs were computed on the basis of the structures and energies obtained at the B3LYP/aug-cc-PVTZ-pp level. Note that many anionic structures, such as II, IV, V, and IV of MPb5− (M = Cu, Ag, and Au), do not have corresponding neutral structures at their lowest doublet states. This implies that removing a single electron from the highest occupied molecular 2328

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Figure 3. Theoretical orbital levels of structures I of MPb5− (M = Cu, Ag, and Au) and the bipyramid structure of Pb5 according to calculations using PW91/TZ2P method with the spin−orbit option. Those of MPb5− (M = Cu, Ag, and Au) are shifted to align their HOMO with their theoretical vertical detachment energies. The dark bars stand for the occupied orbitals and the gray bars stand for the empty ones. The main components of the listed orbitals are also indicated.

correspond to the orbitals localized on Pb5 unit. This is in contrast to many other alloy clusters, in which replacements of Cu, Ag, and Au dramatically change their electronic properties.48,51−53 For another example, bands X, A, and B in each spectrum have similar intensities and β values. That is because these bands correspond to same type of skeletal orbitals on Pb5, which are combinations of pure 6p of Pb atoms. In Pb52−, each Pb vertex contributes two 6p electrons to fill the skeletal orbitals. Together with the two extra electrons, the total electrons on these orbitals are twelve. According to Wade’s rule, the 2n + 2 (n = 5) skeletal electrons endow Pb52− a closo structure and enhanced stability.37 Mulliken population analysis on MPb5− indicate that the neat charges on Pb5 unit are −0.86, −0.38, and +0.01 for M = Cu, Ag, and Au, respectively. However, the Pb5 units in MPb5− (M = Cu, Ag, and Au) have bipyramid structures similar to that of Pb52− and all three MPb5− have enhanced stability and large HOMO− LUMO gaps. This indicates that the bipyramid Pb5 unit can exist as a stable unit under different charge states besides forming Pb52− protected by cationic alkali metal counterparts. 5.3. Considering the Relativistic Effects in MPb5− (M = Cu, Ag, and Au). The relativistic effects are expected to have strong influence on the properties of the clusters containing heavy metal atoms.54−57 We studied the orbital levels of MPb5− (M = Cu, Ag, and Au) after considering relativistic effects using different approaches and found that the same situation happens for these three species. As an example, Figure 4 shows the frontier orbitals of AgPb5− from three different theoretical approaches. The left column shows the results of the calculation at B3LYP/aug-cc-PVTZ-pp level. The central column and the right one indicate those from the calculations at PW91/TZ2P level with the scalar option and the spin−orbit option, respectively. We find the orbital levels shown on the left column are close to theoretical results at BHLYP/def2-TZVP level,38 and the occupied orbital levels from PW91/TZ2P level

We further generated the theoretical density of states (DOS) curves of I and III according to DFT calculations with inclusion of the spin−orbit option. The HOMO of each anionic structure was aligned to its theoretical VDE, and then every occupied orbital was replaced by a Gaussian peak with fwhm of 0.1 eV. This width roughly equals to those of the bands in our experimental spectra. The DOS curves of I and III are displayed in the central and right columns in Figure 2, respectively. Though the predicted intervals among the bands of structures I are smaller than the experimental observations, resulting in extra D bands appeared within the predicted spectra, these DOS curves are qualitatively consistent with the experiments than those of structures III. In a word, the theoretical and experimental results show that structure I is the most possible structure for each MPb5−, and it generates neutral structure I′ in photodetachment. 5.2. Bonding in MPb5− (M = Cu, Ag, and Au). Figure 3 displays the orbital levels in structures I of MPb5− (M = Cu, Ag, and Au) and the bipyramid structure of Pb5 based on calculations at PW91/TZ2P level with the spin−orbit option. We found that the occupied frontier orbitals (from HOMO to HOMO−4) in MPb5− are skeletal orbitals on Pb5 unit, which are combinations of Pb 6p. The HOMO−5 in MPb5−, dominantly coming from the combination of the outer s of M and the LUMO of Pb5, is the only orbital contributing to bonding between M and Pb5 unit. The d orbitals of M and long pair orbitals of Pb5 in MPb5− are very similar to the corresponding ones on single M atom and naked Pb5 unit. We noticed that MPb5− (M = Cu, Ag, and Au) have HOMO− LUMO gaps larger than 1.2 eV, indicating these structures are chemically inert. The orbital levels and components listed in Figure 3 can interpret the characters of experimental spectra in Figure 2. For example, three spectra of MPb5− (M = Cu, Ag, and Au) are very similar. This is due to the fact that all observed bands 2329

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naked Pb5. This indicates the Pb5 unit can exist under complicated chemical conditions and could play a role in bottom-up syntheses of nanomaterials. Additionally, we found that DFT calculations with the spin−orbit option can qualitatively predict the orbital levels of cluster containing Pb atoms, even the energy intervals among frontier ones tend to be underestimated.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-0411-84379365. Fax: +86-0411-84675584. E-mail: X.X., [email protected]; Z.T., [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21073186, 21103186, 21103226), the Ministry of Science and Technology of China, the Chinese Academy of Sciences, and the Present Fund of GUCAS. The calculated results in this paper are obtained on the Deepcomp7000 of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences.

Figure 4. Orbital levels of structure I of AgPb5− according to calculations using B3LYP/aug-cc-PVTZ-pp with none relativistic option (left), PW91/TZ2P with the scalar option (central), and PW91/TZ2P with the spin−orbit option (right). In all cases, the orbital levels are shifted to align the HOMO with the theoretical vertical detachment energy of AgPb5−. The dark bars stand for the occupied orbital and the gray bars stand for the empty ones. The LUMO, HOMO, and bonding orbital between Ag and Pb5 are indicated.

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REFERENCES

(1) Neukermans, S.; Janssens, E.; Chen, Z. F.; Silverans, R. E.; Schleyer, P. V.; Lievens, P. Extremely Stable Metal-Encapsulated AlPb10+ and AlPb12+ Clusters: Mass-Spectrometric Discovery and Density Functional Theory Study. Phys. Rev. Lett. 2004, 92, 163401− 1−4. (2) Neukermans, S.; Wang, X.; Veldeman, N.; Janssens, E.; Silverans, R. E.; Lievens, P. Mass Spectrometric Stability Study of Binary Msn Clusters (S = Si, Ge, Sn, Pb, and M = Cr, Mn, Cu, Zn). Int. J. Mass Spectrom. 2006, 252, 145−150. (3) Jackson, P.; Fisher, K. J.; Gadd, G. E.; Dance, I. G.; Smith, D. R.; Willett, G. D. The Gas Phase Reactivity and Theoretical Structures of Germanium Ions Generated by Direct Laser Vaporisation. Int. J. Mass Spectrom. Ion Processes 1997, 164, 45−69. (4) Xing, X. P.; Tian, Z. X.; Liu, H. T.; Tang, Z. C. Magic Bimetallic Cluster Anions of M/Pb (M = Au, Ag and Cu) Observed and Analyzed by Laser Ablation and Time-of-Flight Mass Spectrometry. Rapid Commun. Mass Spectrom. 2003, 17, 1411−1415. (5) Xing, X. P.; Tian, Z. X.; Liu, H. T.; Tang, Z. C. A Comparative Study of Cation and Anion Cluster Reaction Products: The Reaction Mechanisms of Lead Clusters with Benzene in Gas Phase. J. Phys. Chem. A 2003, 107, 8484−8491. (6) Cui, L. F.; Wang, L. M.; Wang, L. S. Evolution of the Electronic Properties of SnN− Clusters (N=4−45) and the Semiconductor-toMetal Transition. J. Chem. Phys. 2007, 126, 064501−1−8. (7) Gantefor, G.; Gausa, M.; Meiwesbroer, K. H.; Lutz, H. O. Photoemission from Tin and Lead Cluster Anions. Z. Phys. D: At. Mol. Clusters 1989, 12, 405−409. (8) Bai, J.; Cui, L. F.; Wang, J. L.; Yoo, S. H.; Li, X.; Jellinek, J.; Koehler, C.; Frauenheim, T.; Wang, L. S.; Zeng, X. C. Structural Evolution of Anionic Silicon Clusters Sin (20 ≤ n ≤ 45). J. Phys. Chem. A 2006, 110, 908−912. (9) Gupta, U.; Reber, A. C.; Melko, J. J.; Khanna, S. N.; Castleman, W., Jr. Stability and Electronic Properties of Isoelectronic Heteroatomic Analogs of Sn52−. Chem. Phys. Lett. 2011, 505, 92−95. (10) Negishi, Y.; Kawamata, H.; Nakajima, A.; Kaya, K. Photoelectron Spectroscopy of Tin and Lead Cluster Anions: Application of Halogen-Doping Method. J. Electron Spectrosc. Relat. Phenom. 2000, 106, 117−125.

with the scalar option are also similar. However, the DFT calculation with the spin−orbit option gave quite different orbital levels on the right column of Figure 4 and the corresponding DOS curve in Figure 2 is qualitatively consistent with the experiment spectrum. When we more cautiously compared the results from DFT including the spin−orbit option with the experimental spectra, we found the predicted intervals among the frontier orbitals are about 60% of the experimental ones. We rechecked the calculation results and experimental spectra of AgPbn− (n = 2−4) in ref 58 and PbnC6H5− (n = 2−4) in ref 11 and observe the same trend. The theoretical intervals among the frontier orbital for these species are around 50−70% of the experimental observations. Overall, DFT calculations with the spin−orbit option employing double group symmetry can qualitatively predict the frontier orbital levels in small cluster containing Pb and the theoretical energy intervals among frontier orbitals from this method tend to be systematically smaller than the experimental observations.

6. CONCLUSIONS Structures of MPb5− and MPb5 (M = Cu, Ag, and Au) were determined according to theoretical calculations and photoelectron imaging results. All these clusters contain a trigonal bipyramid Pb5 with the coinage metal atom on one facet. The VDEs and ADEs of MPb5− (or EAs of MPb5) were reported. The bonding between M and Pb5 unit in MPb5− was dominantly attributed to the interaction between the valence s orbital of M and the LUMO of Pb5. Three MPb5− are geometrically stable and chemically inert, and the Pb5 units in MPb5− have similar structures and orbital levels to those of the 2330

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