Photoelectron Imaging and Theoretical Studies of Group 11 Cyanides

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Photoelectron Imaging and Theoretical Studies of Group 11 Cyanides MCN (M ) Cu, Ag, Au) Xia Wu, Zhengbo Qin, Hua Xie, Ran Cong, Xiaohu Wu, Zichao Tang,* and Hongjun Fan* State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ReceiVed: February 12, 2010; ReVised Manuscript ReceiVed: October 27, 2010

Photodetachment of group 11 cyanide anions MCN- (M ) Cu, Ag, Au) has been investigated using photoelectron velocity-map imaging. The electron affinities (EAs) of CuCN (1.468(26)) and AgCN (1.602(22)) are larger, while that of AuCN (2.066(8)) is smaller than those of the free atoms. This intriguing observation was confirmed by theoretical studies and was assigned to the transition between ionic and covalent bond properties. The harmonic frequencies of the extended vibrational progressions in the M-C stretching mode are 460(50), 385(27), and 502(10) cm-1, respectively, which suggests a stronger bond for Au-CN than for Ag-CN. Electronic structure analysis and model calculations suggest that all M-C bonds in group 11 cyanides are best described as single bonds. A model has been proposed to explain how the relativistic effects influence the Au-C bond strength in AuCN. 1. Introduction The AuCN molecule is a part of the gold cyanide system. Most experimental studies have focused on AuCN in the condensed phase.1-4 Previous works suggest that solid AuCN exists as infinite linear chains of alternating gold and cyanide moieties, -Au-CN-Au-CN-Au. Gold atoms are arranged in layers to minimize the Au-Au distance for an “aurophilic interaction”.4 However, few experimental studies have been performed on monomeric group 11 cyanides in the gas phase. Only the photoelectron spectroscopy of AgCN and CuCN,5 the millimeter/submilimeter-wave spectroscopy of CuCN,6 and the microwave spectroscopy of AgCN and AuCN7 have been reported so far. There are several theoretical studies on the group 11 cyanides.8-14 It is now well documented that for these complexes the Au-CN bond is shorter, stronger, and more covalent than the Ag-CN bond. In an earlier work, Nelin et al.8 assigned a single bond to Cu-CN in CuCN and stated that there is essentially no back-donation. Later on, Frenking et al.11 suggested that covalent interactions are the driving force for the formation of the group 11 cyanides M-CN (M ) Cu, Ag, Au), but the finally formed bonds bear more electrostatic character. It is interesting that the nature of the Au-CN bond is in debate. ¨ and co-workers stated that the Au-CN In contrast, PyykkO bond length is only slightly longer than the sum of the triplebond covalent radii, and suggested a multiple-bond character in the Au-CN bond of AuCN.13 In the present work, we performed photoelectron imaging studies on photodetachment of MCN- (M ) Cu, Ag, Au) at two photon energies: 532 nm (2.331 eV) and 355 nm (3.496 eV). Accurate electron affinities (EAs) and M-CN stretching vibrational frequencies were reported. Density functional theory (DFT) and ab initio caculations were carried out on the MCN0/species. Electronic structure analysis and model calculations suggest that all M-CN bonds in group 11 cyanides are best * To whom correspondence should be addressed. Tel: +86-41184379365. Fax: +86-411-84675584. E-mail: (Z.T.) [email protected]; (H.F.) [email protected].

described as single bonds. A simple model was also proposed to explain how the relativistic effects influence the Au-CN bond strength in AuCN. 2. Methodology 2.1. Photoelectron Imaging. The experiments were carried out using a collinear photoelectron velocity-map imaging spectrometer equipped with a laser vaporization source. The apparatus is described in detail elsewhere.15 Briefly, the MCN- anions were generated by laser vaporization of the corresponding metal targets (purity 99.99%). A small amount of acetonitrile molecules (CH3CN, analytically pure) without additional purification were seeded in helium (99.99%), by bubbling the He noble carrier gas through the acetonitrile liquid at room temperature (about 300 K). The backing pressure of the gas mixture was adjusted between 1 to 3 atm to get the wanted anions. The negative clusters were extracted perpendicularly from the beam by a -1.2 kV high voltage pulse and were subjected to a McLaren-Wiley time-of-flight mass spectrometer (TOFMS)16 with a mass resolution better than 400. The imaging spectrometer is located at the end of the linear TOF mass spectrometer. The anions MCN- were crossed with a laser beam (532 or 355 nm from a Nd:YAG laser), generated photoelectrons and the neutral MCN. The photoelectrons were extracted by the modified velocity-map imaging (VMI) electrodes, based on the original design of Eppink and Parker.17 The photoelectrons are extracted collinearly with respect to the parent ion beam, similar to that used by Neumark.18 After passing through a 36.7 cm TOF tube, the photoelectrons were mapped onto a detector consisting of a 40 mm diameter microchannel plate (MCP) assembly and phosphor screen. The two-dimensional (2D) images on the phosphor screen were recorded by a charge-coupled device camera with a 10 Hz repetition rate. The energy resolution was about 30 meV at electron kinetic energy (eKE) of 1 eV. The photoelectron image of Au- was used for the spectrometer calibration. All the raw images were reconstructed using the Basis Set Expansion (BASEX) inverse Abel transform method, then the photoelectron

10.1021/jp1013708  2010 American Chemical Society Published on Web 11/19/2010

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Figure 1. Photoelectron images and spectra for MCN- (M ) Cu, Ag, Au) obtained at 532 nm (2.331 eV). The left side shows the raw photoelectron image (top on the left) and the reconstructed (bottom on the left) one after inverse Abel transformation. The double arrow shows the direction of the laser polarization.

spectra and angular distributions were extracted.19 Each image is averaged with between 50 000 and 100 000 laser shots at 10 Hz repetition rate. 2.2. Electronic Structure Calculations. All geometry optimizations and energy calculations were carried out at the open shell couple-cluster UCCSD(T) level using MOLPRO 2006.1.20 The pseudopotential basis sets aug-cc-pVTZ21 were used for the C and N atoms and aug-cc-pVTZ-pp basis sets22,23 for the other atoms. Frequency calculations have been done at the same level, and the unscaled harmonic stretching frequencies are reported. The calculated EAs and vertical detachment energies (VDEs) have been corrected by zero-point energies (ZPE). Hybrid density functional theory (DFT) methods B3LYP24,25 were used to analyze the bond situations, and natural bond

orbital (NBO) analysis26 has been studied using cc-pVTZ(-f)++ basis sets. Generalized gradient approximate (GGA)-DFT method BLYP25,27 as implemented in the ADF package28,29 has been chosen for scalar relativistic correction and energy partition calculations, utilizing triple-ζ Slater basis set (ZORA/TZP) and the frozen-core approximation for 1s-2p of Cu, 1s-3d of Ag, and 1s-4f of Au. 3. Results 3.1. Photoelectron Imaging. The 532 nm photoelectron images and the photoelectron spectra of MCN- (M ) Cu, Ag, Au) are shown in Figure 1. Each spectrum shows only one transition from the ground electronic state of the anion to that of the neutral MCN, and peaks in the direction parallel

Studies of Group 11 Cyanides MCN (M ) Cu, Ag, Au)

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TABLE 1: Calculated and Experimental Adiabatic (EA) and Vertical (VDE) Detachment Energies, M-CN Stretching Vibrational Frequencies, and Anisotropy Parameters at 532 nm for Detachment Processes of MCN- (M ) Cu, Ag, Au) VDE (eV) Cu CuCN Ag AgCN Au AuCN a

M-CN freq (cm-1)

EA (eV)

exptl

calcd

1.468(26) 1.466a

1.46

1.650(20) 1.636a

1.59

2.189(4)

2.08

exptl

calcd

1.236(33) 1.468(26) 1.466a 1.304(28) 1.602(22) 1.588a 2.309(1) 2.066(8)

1.18 1.42 1.22 1.53 2.17 1.97

exptl

calcd

β

460(50) 480a

474

0.5

385(27) 390a

389

0.84

502(10)

469

0.78

Experimental results from ref 5.

to the laser polarization axis. The photoelectron anisotropy parameters (β) of the transition of MCN (M ) Cu, Ag, Au) at 532 nm are summarized in Table 1. All the observed vibrational progressions were assigned to the M-CN stretching mode. We tentatively attributed the 0-0 transition to the electron affinity (EA) and assigned the maximum intensity to the vertical detachment energy (VDE). All these data are also collected in Table 1. In Figure 1a, the images for photodetachment of CuCNobtained at 532 nm show that the vibrational spacing is 460(50) cm-1, and EA and VDE are 1.468(26) eV. Figure 1b shows the imaging results of AgCN at 532 nm. The EAs, VDEs, and vibrational frequencies of CuCN and AgCN are in good agreement with the correspondence values measured by Wang et al.5 However, the previously observed weak feature of AgCN near 1.12 eV,5 due to either minor isomers or electronically excited states of the anions, did not appear in our spectrum. Figure 1c shows the imaging results for photodetachment of AuCN- at 532 nm. The 532 nm image reveals well-resolved Au-CN stretching vibrational progression within the 1Σ+ band.9 To our best knowledge, this is the first experimental determination of the EA and the stretching frequency of AuCN in the gas phase. The photodetachment of AuCN- at 355 nm was also measured, and no new features were observed at high binding energy (3.496 eV). It means that the first excited state of AuCN lies in above 3.496 eV. Above all, the most intriguing observations in our experimental results are the change of the EA and vibrational frequencies of MCN with the Cu, Ag, and Au, summarized in Table 1. The increased EA of CuCN (1.468 eV), AgCN (1.602 eV), and AuCN (2.066 eV) might be predicted based on the electron affinities of metal atoms, Cu (1.236 eV), Ag (1.304 eV), and Au (2.309 eV). However, when comparing the EA of the MCN (M ) Cu, Ag) with that of M atom, the EA of MCN (M ) Cu, Ag) is larger than the EA of M, while for M ) Au, it is smaller. Such a distinction suggests that the interaction between Au atom and cyanide group is special. Moreover, the trend of the M-CN stretching vibrational frequencies for these three metals is Ag < Cu < Au. With respect of these, theoretical calculations were employed to obtain an insight of the special interaction in AuCN. 3.2. Theoretical Results. The geometric structures and vibrational frequencies of MCN have been well studied in the literature. Our calculation results are also summarized in Table 1. All values are very similar to the literature calculations at the CCSD(T)/cc-pVQZ level,13 and are in good agreement with the experimental results. The calculated homolytic bond dissociation energies using the fragments M and CN (99.6 kcal/mol for CuCN, others see Scheme 2) show that previous DFT results11 are pretty reliable as well. The

heterolytic bond dissociation energies using the fragments M+ and CN- are calculated to be 187.8 kcal/mol for CuCN, 168.8 kcal/mol for AgCN, and 209.5 kcal/mol for AuCN, respectively. According to Badger’s and Gordy’s rules, the shorter the bond length and the stronger the force constant or vibrational stretching frequency, the larger the bond dissociation energy. However, there are exceptions to this rule, for instance, if there occurs an avoided crossing between two diabatic curves, as in the present case (see Figure 2). Around the equilibrium distance, the curve is strongly ionic according to the bonding analysis by Frenking,11 and the small internuclear distance and large vibrational frequency correspond to the large heterolytic diabatic dissociation energy, following the rules. However the “true” adiabatic homolytic dissociation energy does not follow the rules. For the anions, the M-CN- bond lengths are computed to be 1.892, 2.144, and 2.036 Å for CuCN-, AgCN-, and AuCN-, respectively, which are longer than those in the neutral molecules by 0.065-0.111 Å. The computed M-CNstretching frequencies are 389(CuCN-), 290(AgCN-), and 359 cm-1 (AuCN-), respectively. The calculated EAs and VDEs are tabulated in Table 1. The average error of the calculated EA and VDE is 0.07 eV, and the maximum error is 0.11 eV. All calculated results are in accord with the experimental results. 4. Discussion 4.1. The Transition between Ionic and Covalent Bonds. The EA of the ionic MCN molecule is expected to be larger than that of the free metal atom, since it is easier to put an electron into a cationic metal center than into a neutral one. If we put an electron into the covalent MCN molecule, the electron will go to the M-CN antibonding orbital, which is pretty high

Figure 2. Sketch of the crossing of two molecular diabatic, ionic and covalent, potential energy curves.

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Figure 3. Frontier orbitals of CuCN and AuCN. Orbital energies in eV are given above each orbital (the MOs are drawing with isodensity contour of 0.05 au).

in energy. Hence the EA of covalent MCN is anticipated to be smaller than the EA of M. Our model calculations clearly demonstrated these trends, where EA of ionic KCN (0.904 eV) is found to be larger than the EA of K (0.368 eV). Considering our experimental results, the observed EAs of CuCN and AgCN are larger than those of the Cu and Ag atoms, respectively. Thus, we refer these species to the ionic bonding features. The experimental EA of AuCN is smaller than that of the Au atom, therefore the AuCN exhibit more covalent bond feature. The visible abnormal EA’s trend suggests an interesting transition from ionic bond properties to covalent bond properties from CuCN to AgCN to AuCN. Figure 3 also shows that Au-CN bears stronger covalence interaction and weaker ionic interaction than Cu-CN. This trend is further supported by charge analysis where the computed natural charges of M in MCN are 0.67 for Cu, 0.67 for Ag, and 0.43 for Au. Concerning the total bond strength, the heterolytic Au-CN bond dissociation energy (referring to M+ and CN-)11 is stronger than the Ag-CN bond by 40.7 kcal/mol, and the Cu-CN bond by 21.7 kcal/mol. The strongest bond dissociation energy of Au-CN corresponds to the largest frequency among the MCN. 4.2. The Role of the Relativistic Effects. The relatively strong Au-CN bond is widely believed to arise from the relativistic effects that lead to the contraction of the 6s orbital, the destabilization of 5d shell, and the enhancing of the 5d6s hybridization.14 This statement is supported by our calculation. NBO26 analysis suggests that the hybridization of the metal in MCN is s(95.8%)d(3.7%) for the Ag-CN bond, and s(84.8%)d(15.1%) for the Au-CN bond. The natural electron configuration of M in MCN is 4s0.453d9.88 for Cu, 5s0.424d9.91 for Ag, and 6s0.855d9.72 for Au. The highest d orbital in AuCN is dz2. As shown in Scheme 1, the hybridization of the dz2 and 6s orbitals results in two new orbitals. The one with low orbital energy is doubly occupied

SCHEME 1: Influence of Hybridization to Ligand-Metal d Electrons Repulsion

and dominated by dz2. Comparing with the conventional dz2, this hybridized orbital bears smaller lobes on the z-axis (where the ligand is), which gives smaller repulsion to the ligand. Hence, we propose that the relativistic effects enhance the Au-CN bond in AuCN by reducing the repulsion between metal d electrons and the ligand. This reduced repulsive force discussed above relies on the fact that the highest d orbital is occupied. So it is only important for d10 metals. To verify our interpretation, we have computed RhCN and IrCN molecules where the metal atoms have d8 configuration. The results are shown in Scheme 2. Indeed, we found the Ir-CN bond is slightly longer and weaker than the Rh-CN bond. Therefore these model calculations strongly support our hypothesis. 4.3. Bonding Analysis of Group 11 Cyanides MCN. An earlier study by Frenking et al.11 suggested that the π bond is not very important in MCN (M ) Cu, Ag, Au). Later on, Zaleski-Ejgierd et al.13 suggested that the Au-CN bond in AuCN bears multibond character. The frontier orbitals of CuCN and AuCN are depicted in Figure 3. For both molecules, the LUMO+6 and LUMO+7 are essentially CN π* orbitals with negligible Au/Cu contributions, which suggest that the back-donation to CN- is rather small. The HOMO-6/HOMO-7 orbitals represent the bonding interaction

Studies of Group 11 Cyanides MCN (M ) Cu, Ag, Au)

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SCHEME 2: Comparison of Group 9 and Group 11 MCN Complexesa

was reproduced by theoretical studies and was assigned to the transition from ionic to covalent bonding as we go from CuCN to AuCN. The M-CN harmonic stretching frequencies of the ground state of MCN suggest a stronger Au-CN than Ag-CN bond. We propose that s-d hybridization, which is more pronounced in AuCN because of relativistic effects, is able to effectively reduce the repulsive force between the Au d shell and the ligand electrons, thus stabilizing the metal-ligand bonds in MCN for heavy metal atoms with d10 configuration. The covalence of the Au-CN bond is enhanced by the relativistic effects. The short Au-CN bond is most probably just a size effect and not due to the multibond character. Electronic structure analysis and model calculations show that all M-CN bonds in group 11 MCN complexes are best described as a single bond.

a Bond lengths in Å are shown above the M-C bond, and homolytical bond dissociation energies in kcal/mol are given below each molecule.

between the Au/Cu d and CN π orbitals, while HOMO-1/ HOMO-2 represent the corresponding antibonding interactions. Since all these orbitals are occupied, the π-bond interaction and antibond interaction cancel out each other, and the Cu-C and Au-C bond energies do not have significant π contribution. Therefore, we feel that the covalent interactions of Au-CN and Cu-CN bonds are better described as single bonds, and the Au-C bond is more covalence mostly because its σ bond is stronger. The small back-donation in AuCN is not surprising since CNis an anion, and group 11 metals are not good back-donators. Back-donation usually weakens the CN bond strength and lengthens the CN bond length because of the occupation of the C-N π* orbitals. As the comparison, the computed C-N bond length is 1.171 Å for CuCN, which is almost the same as 1.170 Å for AgCN, 1.169 Å for AuCN, and shorter than 1.179 Å in free CN and 1.172 Å in KCN. Actually the C-N bonds in CuCN is longer than in AuCN, which is possibly because its CN fragment bears more negative charge. As a comparison, for the typical strong back-donation ligand CO, the C-O bond length in RuCO is computed to be 1.163 Å, which is much longer than the 1.127 Å in free CO. The covalent radii of Cu, Ag, and Au were studied experimentally and theoretically.30-33 Recent work by Tripathi et al.30 gave covalent radii of 1.46 Å for Ag(I) and 1.37 Å for Au(I). The revised covalent radii table by Cordero et al.31 gives 1.45(5) for Ag and 1.36(6) Å for Au. These values indicate that the covalent radius of Au(I) is smaller than Ag(I) by 0.09 Å, which is very similar to the 0.097 Å difference between the Au-CN and Ag-CN bond lengths. Thus, we prefer the interpretation that the short Au-CN bond is just the common size effect but not due to some multibond character. Compared to the neutral species, the long bond lengths and low stretching frequencies for the anions MCN- suggest a weak M-CN- bond strength. This trend is anticipated, since no matter the bond is ionic or covalent, the M-CN- bond in the anion should be weaker than that in the neutral molecules. In the case of ionic bonding, the M0 in MCN- is bigger than the M+1 in MCN, and the electrostatic interaction between M0 and CN- is much smaller than that between M+1 and CN-. While in the case of covalent bonding the M-CNbond in MCN- is weaker since one electron occupies the M-CN σ* antibonding orbital. 5. Summary and Conclusions We present the photoelectron velocity-map imaging of photodetachment of group 11 cyanide anions MCN- (M ) Cu, Ag, Au). The photoelectron imaging spectra exhibit vibrationally resolved ground-state transitions. The electron affinities of CuCN and AgCN are larger than those of metal atoms, while for AuCN it is smaller. This intriguing behavior

Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant 20773126), the Ministry of Science and Technology of China, and the Chinese Academy of Sciences. References and Notes (1) Zhdanov, G. S.; Shugam, E. A. Zh. Fiz. Khim. 1945, 19, 519. (2) Hibble, S. J.; Cheyne, S. M.; Hannon, A. C.; Eversfield, S. G. Inorg. Chem. 2002, 41, 1042. (3) Hibble, S. J.; Hannon, A. C.; Cheyne, S. M. Inorg. Chem. 2003, 42, 4724. (4) Bowmaker, G. A.; Kennedy, B. J.; Reid, J. C. Inorg. Chem. 1998, 37, 3968. (5) Boldyrev, A. I.; Li, X.; Wang, L. S. J. Chem. Phys. 2000, 112, 3627. (6) Grotjahn, D. B.; Brewster, M. A.; Ziurys, L. M. J. Am. Chem. Soc. 2002, 124, 5895. (7) Okabayashi, T.; Okabayashi, E. Y.; Koto, F.; Ishida, T.; Tanimoto, M. J. Am. Chem. Soc. 2009, 131, 11712. (8) Nelin, C. J.; Bagus, P. S.; Philpott, M. R. J. Chem. Phys. 1987, 87, 2170. (9) Veldkamp, A.; Frenking, G. Organometallics 1993, 12, 4613. (10) Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc. 1999, 121, 411. (11) Dietz, O.; Rayon, V. M.; Frenking, G. Inorg. Chem. 2003, 42, 4977. (12) Lee, D. K.; Lim, I. S.; Lee, Y. S.; Hagebaum-Reignier, D.; Jeung, G. H. J. Chem. Phys. 2007, 126, 244313. (13) Zaleski-Ejgierd, P.; Patzschke, M.; Pyykko¨, P. J. Chem. Phys. 2008, 128, 224303. (14) Wang, X. B.; Wang, Y. L.; Yang, J.; Xing, X. P.; Li, J.; Wang, L. S. J. Am. Chem. Soc. 2009, 131, 16368. (15) Wu, X.; Qin, Z.; Xie, H.; Tang, Z. Chin. J. Chem. Phys. 2010, 23, 373. (16) Wiley, W. C.; McLaren, I. H. ReV. Sci. Instrum. 1955, 26, 1150. (17) Eppink, A.; Parker, D. H. ReV. Sci. Instrum. 1997, 68, 3477. (18) Osterwalder, A.; Nee, M. J.; Zhou, J.; Neumark, D. M. J. Chem. Phys. 2004, 121, 6317. (19) Dribinski, V.; Ossadtchi, A.; Mandelshtam, V. A.; Reisler, H. ReV. Sci. Instrum. 2002, 73, 2634. (20) Werner, H. J.; Knowles, P. J.; Lindh, R.; Manby, F. R.; Schu¨tz, M.; Celani, P.; Korona, T.; Rauhut, G.; Amos, R. D.; Bernhardsson, A.; Berning, A.; Cooper, D. L.; Deegan, M. J. O.; Dobbyn, A. J.; Eckert, F.; Hampel, C.; Hetzer, G.; Lloyd, A. W.; McNicholas, S. J.; Meyer, W.; Mura, M. E.; Nicklass, A.; Palmieri, P.; Pitzer, R.; Schumann, U.; Stoll, H.; Stone, A. J.; Tarroni, R.; Thorsteinsson, T. MOLPRO, version 2006.1; Cardiff University: United Kingdom, 2006. (21) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007. (22) Kendall, R. A.; Dunning, T. H.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796. (23) Woon, D. E.; Dunning, T. H. J. Chem. Phys. 1993, 98, 1358. (24) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (25) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (26) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. (27) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (28) Baerends, E. J.; Ziegler, T.; Autschbach, J.; Bashford, D.; Bérces, A.; Bickelhaupt, F. M.; Bo, C.; Boerrigter, P. M.; Cavallo, L.; Chong, D. P.; Deng, L.; Dickson, R. M.; Ellis, D. E.; van Faassen, M.; Fan, L.; Fischer, T. H.; Fonseca Guerra, C.; Ghysels, A.; Giammona, A.; van Gisbergen, S. J. A.; Götz, A. W.; Groeneveld, J. A.; Gritsenko, O. V.; Grüning, M.;

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