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Binding of Small Gas Molecules by Metal-Bipyridyl Monocationic Complexes (Metal = Cu, Ag, Au) and Possible Bond Activations Therein Gourhari Jana, Sudip Pan, and Pratim Kumar Chattaraj J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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The Journal of Physical Chemistry

Binding of Small Gas Molecules by Metal-Bipyridyl Monocationic Complexes (Metal = Cu, Ag, Au) and Possible Bond Activations Therein Gourhari Jana,1 Sudip Pan,*,2 and Pratim K. Chattaraj*,1 1

Department of Chemistry and Centre for Theoretical Studies,

Indian Institute of Technology Kharagpur, Kharagpur, 721302, India 2

Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados Unidad

Mérida. km 6 Antigua carretera a Progreso. Apdo. Postal 73, Cordemex, 97310, Mérida, Yuc., México * Corresponding authors: [email protected] (SP), [email protected] (PKC)

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Abstract The viability of a series of small gas molecules (H2, N2, CO, CO2, H2O, H2S, C2H2, CH4, CH3Cl, C2H4, and C2H6) bound [M-(bipy)]+ (bipy = bipyridyl; M = Cu, Ag, Au) complexes is investigated at the PBE0/cc-pVTZ/cc-pVTZ-PP level with a special emphasis on the possible bond activation within the bound ligands. While the bond dissociation energy, enthalpy change, and free energy change are computed to show the stability of the complexes with respect to the dissociation into [M-(bipy)]+ and free gas molecule (L), natural bond orbital, electron density, and energy decomposition analyses in conjunction with natural orbitals for chemical valence are carried out to characterize the nature of L-M bonds. For a given L, the L binding ability is the highest for Au followed by Cu and Ag complexes, except for quite loosely bound CO2. For all ligand cases, the dissociation processes from the respective bound complexes are endergonic in nature at room temperature except for the H2, CH4, C2H6 bound Ag complexes and CO2 bound Ag and Au complexes. The interaction between L and M centers is supported by orbital and ionic

interactions with latter being more dominant over the former. The nature of interaction in L-M bonds is also studied by the electron density analysis. The delocalization index and local energy density values support the covalent character in L-M bonds in most of the cases. These M centers can act as a mild bond activation agent for L, Au being the best candidate in this series for this purpose. Particularly, the H-H bond in H2, C=C bond in C2H4, C≡C bond in C2H2, and C-H bonds in CH4 and C2H6 (the last two are for Au) are elongated along with a significant red-shift in the corresponding stretching frequency, compared to those in free molecules. These can be explained by the significant π-back-donation populating the lowest unoccupied antibonding molecular orbital of L in these complexes.

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Introduction Transition-metal complexes containing one or more organic ligands are of paramount interest,1-5 specifically because of their possible utilities in bond activation, catalysis, dyesensitized solar cells, etc. Focusing on the group 11 elements, the so-called coinage metal (Cu, Ag, and Au) bound complexes are interesting species in many areas of metal-organic chemistry, owing to their application as luminescent materials and building blocks to construct supramolecular networks. In addition, their distinctive reactivities have enabled dramatic recent advances in chemical organic synthesis,6-9 gas absorption, bond activation, various chemical and biochemical processes,10-16 chromatography,17,18 industrial catalytic processes, and other photophysical and photochemical applications.19-22 The interaction between M+ (M = Cu, Ag, Au) and an olefin is well studied as it has important application in chemistry for chromatographic separation.23 Copper–ethylene adducts are of interest as models for the ethylene (the smallest plant hormone) receptor site in plants.24,25 Three fascinating organometallic ‘‘spoke–wheel’’ complexes having coinage metal ions, bonded to three norbornene molecules in trigonal planar fashion were isolated and characterized using different methods including X-ray crystallography.26 Consequently, isolable coinage metal– alkene adducts and their structures, bonding and properties are of significant interest. However, in contrast to Cu and Ag, only a limited number of structurally characterized Au(I)–alkene adducts is known.27-38 The bidentate chelating donor based on nitrogen ligand, bipyridyl (bipy) appears to be the ligand of choice for stabilizing the species with M-C2H4 moiety.39-44 Such complexes have important usage in industry. For examples, while Ag-catalyzed oxidation of ethylene to ethylene oxide is a major industrial process,45-49 Au-based complexes serve as excellent catalysts for the selective epoxidation of propene and other alkenes.6,50,51 Recently, [Cu-(bipy)]+ complexes bound with additional ligands like different alkenes52 and alkynes53 have received considerable attention keeping focus on the photophysics and photochemistry thereof. More recently, Weber and co-workers54 studied the interaction between L (L = N2, Cl, H2O) and [Cu-(bipy)]+ complexes and the effect of interaction on the electronic spectrum of the complex by employing both experimental and computational methods. Similarly, various bipy derivatives of Ag and Au are also known in the literature.31,32,55,56 Therefore, it 3



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would be interesting to check whether a series of small gas molecules, bound with [M-(bipy)]+ (M = Cu, Ag, Au) would be viable ,which would be certainly important to the experimentalists in order to check the impact on the corresponding photophysics and photochemistry in the complexes due to the complexation. Further, this study would also be important in enriching the gas-phase chemistry of [M-(bipy)]+. In the present study, in order to predict a number of viable [LM-(bipy)]+ complexes we have studied the structure, and stability of [LM-(bipy)]+ complexes (L = H2, N2, CO, H2O, H2S, C2H2, C2H4, CH4, CH3Cl, C2H6; M = Cu, Ag, Au) using quantum chemical calculations. Additionally, the possibility of bond activation in L because of the complexation is also explored. The bond dissociation energy and thermochemical parameters like enthalpy change (∆H) and Gibbs free energy change (∆G) are computed to quantify the L binding ability of these complexes. Further, the nature of bonding is analyzed through natural bond orbital (NBO), electron density,57 and energy decomposition (EDA) analyses.58-61 Particularly, the last analysis in conjunction with natural orbitals for chemical valence (NOCV) theory provides important insight into the reason for bond activations therein.

Computational details Geometries of all the studied systems are fully optimized employing the hybrid exchange-correlation functional, PBE062 in conjunction with a correlation consistent triple zeta quality basis set along with relativistic effective core potential (RECP) for the heavier atoms. The cc-pVTZ basis set63-65 is used for H, C, N, O, S atoms and cc-pVTZ-PP basis set along with RECP is used for the Cu, Ag, Au atoms. The RECPs used for computations are ECP10MDF for Cu, ECP28MDF for Ag, and ECP60MDF for Au.66 Harmonic vibrational frequency analysis is carried out to exemplify that all the optimized structures are at the minima on the respective potential energy surfaces and also to include the zero point energy (ZPE) correction. Nevertheless, we have also compared the theoretically obtained vibrational frequencies of [N2M(bipy)]+ complex at the presently studied level with those of experimentally reported frequencies (see Table S1 in supporting information). It shows a good agreement with the experimental values which implies that the PBE0/cc-pVTZ/cc-pVTZ-PP level is a good choice for our 4


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computations. From, here onwards cc-pVTZ/cc-pVTZ-PP will be abbreviated as VTZ for brevity. The standard counterpoise method proposed by Boys and Bernardi67 is adopted for the correction of basis set superposition error (BSSE) towards the calculated dissociation energy. The dissociation energy (D) is computed as +

+

D = E([M-(bipy)] ) + E(L) - E([LM-(bipy)] )

(1),

which is then corrected from ZPE and BSSE. Here, E denotes the total energy of a system. Natural population analysis (NPA)68 and Wiberg bond index (WBI)69 calculation are performed to compute the charge (q) on each atomic center and bond order, respectively, using NBO scheme. All these calculations are performed using Gaussian 09.C01 program package.70 To look into the nature of bonding, electron density analysis is performed at the PBE0/ccpVTZ/WTBS71,72//PBE0/VTZ level using Multiwfn software,73 where all electron WTBS basis set is used for the Cu, Ag and Au atoms. EDA-NOCV is carried out at the revPBE-D3(BJ)/TZ2P//PBE0/VTZ level using ADF 2013.01program package74. In EDA, the total interaction energy (∆Eint) between two fragments is decomposed into four energy terms. The first one is the electrostatic interaction energy (∆Eelstat), which is calculated classically by taking the two fragments at their optimized positions but considering the charge distribution to be unperturbed on each fragment by other. The next one is the Pauli repulsion energy (∆EPauli), which appears as the repulsive energy between electrons of the same spin and it is computed by employing Kohn-Sham determinant on the superimposed fragments to obey the Pauli principle by antisymmetrization and renormalization. The orbital interaction energy (∆Eorb) originates from the mixing of orbitals, charge transfer and polarization between two fragments. Lastly, the dispersion interaction energy (∆Edisp) represents the dispersion interaction between the two fragments. Therefore, ∆Eint can be written as, ∆Eint = ∆EPauli + ∆Eelstat + ∆Eorb + ∆Edisp

(2).

Furthermore, EDA-NOCV decomposes the total deformation density (∆ρ(r)) into its individual differential densities (∆ρi(r)), which can be expressed over the pairs of NOCV. Thus, ∆ρ(r) = ∑ ∆ρi(r)

(3).

These computations help in identifying those fragment orbitals, which are contributing the most towards the chemical bond formation. It also provides the information regarding the direction of 5



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the charge flow in between those fragments. On a similar manner, the total ∆Eorb is also decomposed into ∆Eiorb corresponding to each charge transfer channel as ∆Eorb = ∑ ∆Eiorb

(4).

Results and discussion Structures and energetics The optimized structures of all the studied [LM-(bipy)]+ complexes including the bare ones are given in Figures 1 and 2. While [Cu-(bipy)]+ complex is found to have C2v symmetry, its Ag and Au analogues adopt C2 point group symmetry. In terms of orientation of L, Cu and Ag complexes show structural resemblance to each other where most of the complexes are symmetric or nearly symmetric, whereas for Au case, L units (L = N2, CO2, H2O, H2S) are bound with Au in a tilted manner in such a way that it can readily welcome another L to adopt a square planar coordination sphere around it (see Figure S1 in supporting information for better clarity). However, we have restricted ourselves to only one L bound complex, focusing on the possible orientation and bond activations therein. It is noted that H2, C2H2 and C2H4 prefer to coordinate in an η2 mode via two H centers in former and two C centers in the latter two cases, thereby they attain a side-on orientation with respect to M. On the other hand, N2, CO, CO2, H2O and H2S coordinate through only one center with end-on orientation (η1 mode). Note that the geometries of H2, C2H2 and C2H4 bound Au complexes, where Au centers are tetracoordinated, are very similar to those in Cu and Ag complexes. Remarkably, our computations reveal that N2 in [N2Cu-(bipy)]+ prefers an end-on orientation in contrast to the side-on as reported by Weber and co-workers where the former is more stable by 16.3 kcal/mol than the latter. In CO case, the O-bound analogues are also minima on the potential energy surface, but they are significantly higher in energy (by 28.6 (Cu), 21.8 (Ag), and 44.6 (Au) kcal/mol) than the corresponding C-side bound isomers. An η2-CO isomer is only obtained for Cu which is 26.5 kcal/mol higher in energy than the C-side bound end-on isomer. However, the CO in similar isomer for Ag and Au cases automatically goes back to the 6


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η1 mode upon optimization. The related results for these higher energy isomers are provided in Table S2. On the other hand, while in cases of saturated hydrocarbons, CH4 and C2H6, they interact with M centers via two H atoms, CH3Cl interacts through its Cl center. The L-M bond dissociation energy values corrected from both ZPE and BSSE (D0BSSE) for the [LM-(bipy)]+ complexes range from 5.2 to 40.9 kcal/mol for Cu, 0.9 to 23.8 kcal/mol for Ag, and 4.3 to 49.4 kcal/mol for Au complexes with the lowest value for CO2 and the highest value for C2H4 (see Tables 1-3). For a given L, except for loosely bound CO2 the complexation ability of M follows the usual trend of Au > Cu > Ag where the relativistic effect for Au plays a role for its enhanced binding ability. This is remarkable that even H2 interacts quite strongly with Cu (10.5 kcal/mol) and Au (29.0 kcal/mol). The interaction with saturated hydrocarbons, CH4 and C2H6, is relatively weaker than that with unsaturated hydrocarbons, C2H2 and C2H4. This is because not only the latter ones are electron-rich to donate to M centers, but also there are suitable π* orbitals to accept the electron back-donation from M. The interaction becomes stronger than that of CH4 by substituting one H atoms of CH4 by Cl as CH3Cl interacts with the electron-rich Cl center. On the other hand, CO2 interacts quite poorly via its O-end with M center, resulting in quite low D0BSSE values. Between H2O and H2S, H2S provides a much stronger bond with M, presumably because of its larger basicity than H2O. The thermochemical stability of [LM-(bipy)]+ complexes with respect to the dissociation into L and [M-(bipy)]+ can be further understood from the corresponding ∆H and ∆G values computed at 298 K. The positive ∆H values for the dissociation processes imply the endothermic nature of the dissociation, which also follows the same trend as that of D0BSSE either by changing L or M. Most importantly, except for CO2 bound Ag and Au complexes, and H2, CH4 and C2H6 bound Ag complexes, the dissociation processes for all the complexes are endergonic in nature at room temperature, highlighting their stability with respect to dissociation. It may be noted that the corresponding ∆G values for the dissociations of CO2, CH4 and C2H6 bound Cu complexes are very close to zero which shows that slight lower temperature is needed to make them viable. Inclusion of thermal correction and entropic factor do not alter the stability order either by changing L or M. Notice that from the nature of adsorption point of view, the present L-M

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binding in all cases can be considered as chemisorption, except for weakly bound CO2-Ag, as new chemical bonds are getting formed in between L and M (vide infra). The energy difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (∆EH-L) can also be used as a stability and reactivity indicator for molecular systems. In general, high ∆EH-L value indicates that the chemical entity is reluctant to either accept or donate electron from/to other interacting chemical species. In other words, a system with high ∆EH-L value is less reactive, and hence more stable. In our cases, except for CO2 and H2O bound Cu complexes, corresponding ∆EH-L values are found to enhance in [LM(bipy)]+, compared to those in bare moieties. To explore the stability of complex in presence of a counter-ion, we have considered SbF6‾ as the corresponding anion, and [N2Au-(bipy)]+ and [H2OAu-(bipy)]+ as a case study (see Figure S2). The ZPE corrected dissociation energy, D0 in the resulting complex for the dissociation process, [LAu-(bipy)]+[SbF6]‾ → [LAu-(bipy)]+ + SbF6‾, is computed as: D0 = E0([LAu-(bipy)]+) + E0(SbF6‾) − E0([LAu-(bipy)]+[SbF6]‾)

(5),

where E0 denotes ZPE corrected energy, and L = N2 and H2O. It is found that SbF6‾ interacts with [N2Au-(bipy)]+ and [H2OAu-(bipy)]+ with D0 values of 60.8 kcal/mol and 80.4 kcal/mol, respectively, without affecting the N-Au, N-N, O-Au and O-H bond lengths. Bond activation In addition to the strong bond formation, the complexation induces the activation of bond within L, which is reflected in the lengthening of bonds as well as in the decrease in stretching frequencies as provided in Table S3. Notice that the bond lengthening and consequent red-shift in stretching frequency can be correlated with the extent of bond activation as the weakening of a bond would induce significant reactivity towards undergoing various transformations. The results show that the H-H bond in H2, C=C bond in C2H4, C≡C bond in C2H2, and C-H bonds in CH4 and C2H6 get activated to some extent, which is the highest in case of Au followed by Cu and Ag, for a given L. This is remarkable that even the C-H bonds in CH4 and C2H6 get

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elongated due to such interaction with Au center. In rest of the cases, the bonds in L remain more or less unaffected or undergo only slight lengthening.

Nature of bonding The charges on the Cu, Ag and Au centers are 0.81, 0.83 and 0.71 |e|, respectively, in [M(bipy)]+ complexes. The lower positive charge on Au center can be understood from its higher electronegativity (2.5 in Pauling scale) than that in Cu and Ag (~ 1.9). In fact, the high electronegativity of the former species may have a relation to its very large relativistic effect, which leads to the contraction of the valence 6s orbital, stabilizing it energetically. Therefore, this orbital shows greater propensity to accept electron. In fact, Au can even behave as an anion.75 In case of [LM-(bipy)]+ complexes, the L acquires a net positive charge, except for C2H2 and C2H4, and the charge on the M center gets lowered implying the charge transfer from L to M center. Such charge transfer ranges within 0.07-0.24 e- for Cu, 0.04-0.21 e- for Ag, and 0.09-0.33 e- for Au with the maximum such value for H2S (see Tables 1-3). The calculated WBI values give the idea about the degree of covalency in L-M bonds. While a small WBI value for a bond indicates a non-covalent (van der Waals or electrostatic) type of interaction, a large WBI value shows the dominant covalent character in that bond. In the present cases, WBI values of L-M bonds are found to be considerable for N2, CO, H2S, C2H2, C2H4 and CH3Cl cases which vary within 0.38-0.80 for Cu, 0.31-0.67 for Ag, and 0.52-1.00 for Au with the highest value for CO and the lowest value for C2H4. In case of H2 bound Au complex, the WBI value of H-Au bond is found to be quite high (0.74), implying the formation of an H-Au covalent bond. It may also be noted that for a particular L-M bond, the corresponding WBI value is the largest in Au complex followed by those in Cu and Ag analogues. The electron density analysis is carried out for these complexes by employing the Bader’s atoms-in-molecules theory57 and the corresponding results are tabulated in Table S4. The negative value of the Laplacian of the electron density (∇2ρ(rc)) at the bond critical point (BCP) implies that there is electron density accumulation in between two bonded atoms. Thus, ∇2ρ(rc) < 0 indicates a covalent or shared bond between two atoms, whereas the reverse situation (i.e., ∇2ρ(rc) > 0) is indicative of a non-covalent interaction. Although the above criterion can explain 9



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the bonding situation in many cases, it is not adequate in describing the bond involving the heavier atoms.76-92 Even for simple molecules like CO and F2, it fails to describe the bond.58;pp312–314,93 Thereafter, Cremer et al. 93 proposed that a bond may be considered as covalent (or at least partially covalent) if it has the local energy density (H(rc)) less than zero, where H(rc) is the sum of local kinetic energy density (G(rc)) and local potential energy density (V(rc)). This argument is based on the fact that the bond formation is the result of a complex interplay between the variation in G(rc) and V(rc) values and a thorough analysis for a series of covalent molecules results in a negative H(rc) value.93 This descriptor is particularly more useful over ∇2ρ(rc) when the covalent bond strength is not that high and the associated electron density (ρ(rc)) at the BCP is low. In fact, H(rc) can also describe the F-F bond in F2. Although the ρ(r) flows out of the bonding region of F-F, the residual density is enough to stabilize the bond through an increase in the negative value of V(rc). In the present cases, although ∇2ρ(rc) is found to be larger than zero, H(rc) values are negative in all cases, except for CO2-Ag bond (see Tables 1-3 and S4). Thus, according to electron density based descriptors these bonds can be classified as covalent in nature. Note that for those cases in which significantly larger WBI values are obtained, the corresponding H(rc) values are also more negative than the other cases. Further, for a given L the degree of covalency follows the order as Au > Cu > Ag. The contour plots of ∇2ρ(r) for L bound complexes are provided in Figures S3-S5 where the solid green lines show the region with ∇2ρ(r) > 0 and the blue dotted lines represent the region having ∇2ρ(r) < 0. It is obvious from these plots that there is no electron accumulation in between L and M centers. Furthermore, delocalization indices (δ)94-96 in L-M bonds are computed which is a measure of the number of pair-wise electrons that no longer correspond to either of the atomic basins exclusively but remain shared between them. Therefore, this definition is in the same line with the notion of the covalency. In other words, δ can be used as an index of bond order. In general, δ follows the same trend as that of the WBI either by changing L or M, albeit the former is significantly higher than the latter. The corresponding δ values reveal that the OC-M (M = Cu, Ag, Au) and H2S-Au bonds have bond order one or even larger than one, whereas the corresponding L-M bonds attain almost 49-85% for Cu, 41-74% for Ag, and 80% for Au of a 10


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typical covalent bond in H2, N2, C2H2, C2H4, H2S (for Cu and Ag), and CH3Cl bound analogues (see Tables 1-3). In case of Au, even for the rest of L, there exists almost 50% covalent character in the L-Au bond. On the other hand, in Cu and Ag cases the degree of covalency is quite small for the remaining L with the least covalency in CO2 bound analogue. To further analyze the nature of L-M bond, EDA-NOCV is carried out taking L atoms as one fragment and [M-(bipy)]+ as another and the results are presented in Tables 4-6. For these LM bonds, the major contribution towards the total attraction originates from ∆Eelstat which lies within the range of 50-69%, whereas ∆Eorb contributes around 26-48% towards the same. Irrespective of M, the least orbital interaction is attained for CO2, followed by H2O. Note that similar to the WBI and δ values, for a given L the magnitude of ∆Eorb follows the order as Au > Cu > Ag. In all the cases, the contribution from ∆Edisp is quite small, being only 1-9% of total attraction. The ∆Eorb is further decomposed into its σ and π contributions. The corresponding deformation densities (∆ρ(r)) for the pair-wise orbital interactions in these complexes are depicted in Figure 3 for Au, and Figures S6 and S7 for Cu and Ag analogues, respectively, where the electron density depletion (∆ρ(r) < 0) and accumulation (∆ρ(r) > 0) are shown in red and blue color, respectively. In other words, the electron density flows from red to blue region using the present color code. Let us first analyze the cases in which the bond activation is obtained, i.e., H2, C2H4, C2H2, CH4 and C2H6 bound cases. In ∆ρ(σ1) plot, the electron density from H-H σorbital in case of H2 and the π-electron density from C≡C and C=C in C2H2 and C2H4, respectively, get shifted to the vacant s orbital of M (LUMO of [M-(bipy)]+) (see Figure 4). Note that some portion of the electron density from M is further transferred to the adjacent N centers. This deformation density channel comprises 27-37% of the total ∆Eorb value. There also exists another σ-channel, ∆ρ(σ2), with a significantly lower contribution than ∆ρ(σ1) (only 1-7% of ∆Eorb, except for H2-Cu (23.9%) and H2-Ag (25.9%) cases). ∆ρ(σ2) plot represents the polarization within the M-L moiety in which the electron density is back-shifted from orbital of M to L with some internal polarization involved within L. The major contribution (54-70%, except for H2-Cu (42.8%) and H2-Ag (36.8%) cases) towards ∆Eorb comes from the M→L π-back-donation involving the 3d orbitals of M and σ* orbital of H2 and π* orbitals of 11



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C2H4 and C2H2. A detailed comparison of MOs with the ∆ρ plots suggests that while ∆ρ(σ1) corresponds to an electron density transfer from HOMO of L to LUMO of [M-(bipy)]+, ∆ρ(π1) and ∆ρ(π2) represent electron transfer from HOMO and HOMO-4 of [M-(bipy)]+ to two degenerate LUMOs of C2H2, and LUMO and LUMO+1 of C2H4, respectively (see Figure 4). Note that the associated ∆Eorb value corresponding to ∆ρ(π2) is significantly smaller as in ∆ρ(π1) the acceptor d orbitals of M and N atoms reside in the same plane and because of the large electronegativity of N, they pull the electron density towards themselves making such electron transfer more effective. Now, since the effect of the σ-donation is over balanced by the π-backdonation, consequently the related bond in L gets elongated and a red-shift in the corresponding stretching frequency is attained. Because of the considerably larger π-back-donation populating σ* orbital of H2 in Au case than those in Cu and Ag cases, the H-H bond fully dissociates in H2Au complex. Further, despite only 24-30% contribution from π-back-donation towards total ∆Eorb the C-H bonds of CH4 and C2H6 in Au complex get enlarged significantly. Note that in addition to the π-back-donation (occupying σ* orbital of C-H to some extent), the ∆ρ(σ1) also weakens the C-H bonds by removing the electron density from there, making the overall impact considerable. For the rest of the systems, similar four significant deformation density channels are observed. However, the σ-contribution (57-82% of ∆Eorb) is significantly more dominant over the π-back-donation (13-37% of ∆Eorb), except for N2 and CO bound analogues. In the former case, M→N2 π-back-donation (HOMO and HOMO-4 of [M-(bipy)]+→π* LUMO of N2) contributes 54.6, 42.7 and 43.6% of ∆Eorb value in Cu, Ag and Au complexes, respectively. Consequently, the largest red-shift in N≡N stretching frequency is noted for Cu (62 cm-1) followed by that in Au (56 cm-1) and Ag (17 cm-1). On the other hand, in case of CO although significant π-back-donation (57.9% for Cu, 49.9% for Ag and 46.4% for Au for the electron transfer from HOMO and HOMO-4 of [M-(bipy)]+ → π* LUMO of CO) is noted, a small blueshift is found for Cu and Ag, and for Au it remains almost unchanged. This can be explained by the change in polarization in C-O bond.97-101 In free state of CO, the bond remains polarized towards more electronegative O end (0.485 e-on C and -0.485 e-on O; total polarization, ∆q = 0.97 e-). ∆ρ(σ1) shows that the transfer of electron density from C end to M also pulls some 12


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degree of the electron density from the O end. On the other hand, in ∆ρ(π1) the electron density is mainly accumulated on C and a small extent on O. Particularly, ∆ρ(π2) presents a change in polarization within C-O bond where the π-back-donation induces an electron density depletion from the O end. As a result, the C-O bonds in the bound complexes become less polarized (∆q = 0.88 e- for Cu and Au, and 0.87 e-for Ag) than that in the free state, resulting in more covalent character in C-O bond. Therefore, π-back-donation and change in polarization act in opposite directions. As a result, the C-O stretching frequency remains unaltered.

Conclusions The present in silico investigations reveal that [M-(bipy)]+ (bipy = bipyridyl; M = Cu, Ag, Au) complexes are able to bind a series of small molecules like H2, N2, O2, CO, CO2, H2O, H2S, C2H2, CH4, CH3Cl, C2H4, and C2H6 effectively. The corresponding bond dissociation energy values lie within the range of 5.2-40.9 kcal/mol for Cu, 0.9-23.8 kcal/mol for Ag, and 4.3-49.4 kcal/mol for Au complexes with the lowest value for CO2 and the highest value for C2H4. All the dissociation processes are found to be non-spontaneous, except for CO2 bound Ag and Au complexes, and H2, CH4 and C2H6 bound Ag complexes which show the viability of the studied complexes at room temperature. Further, the complexation with M causes an activation in the H-H bond of H2, C=C bond of C2H4, C≡C bond of C2H2, and C-H bonds of CH4 and C2H6 (the last two are for Au). The extent of such activation is the maximum for Au followed by those in Cu and Ag. Except for CO2-Ag bond, all the L-M (L is the binding site of gas molecules) bonds are covalent in nature as reflected in their negative local energy density. The delocalization index also corroborates with the local energy density results. For these L-M bonds, the major contribution towards the total attraction is originated from the electrostatic interaction (50-69%), whereas orbital interaction is responsible for 26-48% of the total attraction. In all cases, there are four major deformation density channels (two σ and two π in nature) which are mainly responsible for the orbital interaction. The significant L←M π-back-donation is responsible for the obtained bond activation as understood from the EDA-NOCV computation. Therefore, the present results show that many small molecules bound [M-(bipy)]+ complexes 13



Page 14 of 36

could be viable in gas phase. In fact, our test with a counter-ion hints that these complexes are stable in presence of SbF6- which is necessary from the experimental point of view to synthesize them in large scale in solid state. Moreover, the present systems will also be interesting candidates to tune the photophysics and photochemistry of [M-(bipy)]+ complexes.

Supporting information Supporting Information Available: The computed and experimentally reported bond lengths and stretching frequencies for [N2-Cu(bipy)]+ complex, results of higher energy isomers, bond length elongation, the stretching frequency and the decrement of stretching frequency, results of electron density analysis, bond angles of the studied complexes, optimized structure of [N2Au-(bipy)]+ and [H2OAu-

(bipy)]+ in presence of SbF6-, contour diagrams of the Laplacian of the electron density of [LM(bipy)]+, The plot of deformation densities in [LM-(bipy)]+ (M = Cu, Ag), and the Cartesian coordinates of the studied systems. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes The authors declare no competing financial interest.

Acknowledgements PKC would like to thank DST, New Delhi for the J. C. Bose National Fellowship. GJ thanks to IITKGP, Kharagpur for his fellowship.

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Goldman, A. S.; Krogh-Jespersen, K. Why Do Cationic Carbon Monoxide Complexes Have High C-O Stretching Force Constants and Short C-O Bonds? Electrostatic Effects, Not σ-Bonding. J. Am. Chem. Soc. 1996, 118, 12159–12166.

(100) Ghara, M.; Pan, S.; Kumar, A.; Merino, G.; Chattaraj, P. K. Structure, Stability, and Nature of Bonding in Carbon Monoxide bound EX3+ Complexes (E = Group 14 element; X = H, F, Cl, Br, I). J. Comput. Chem. 2016, 37, 2202–2211. (101) Saha, R.; Pan, S.; Frenking, G.; Chattaraj, P. K.; Merino, G. The Strongest CO Binding and the Largest C-O Stretching Frequency. Phys. Chem. Chem. Phys. 2017, 19, 22862293.

Figures

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Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Optimized geometries of all the studied [LM-(bipy)]+ (M = Cu, Ag; L = H2, N2, CO, CO2 , H2O, H2S, C2H2, C2H4, CH4, CH3Cl, C2H6) complexes at the PBE0/VTZ level. The values without parentheses and within braces are the bond lengths in Å for Cu and Ag complexes, respectively.

23



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Figure 2. Optimized geometries of all the studied [LAu-(bipy)]+ (L = H2, N2, CO, CO2 , H2O, H2S, C2H2, C2H4, CH4, CH3Cl, C2H6) complexes at the PBE0/VTZ level. The values without parentheses are the bond lengths in Å.

24


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Figure 3a. The plot of deformation densities (∆ρi(r)) of the pair-wise orbital interactions and the associated ∆Eiorb energies obtained from the EDA-NOCV for [LAu-(bipy)]+ (L = H2, N2, O2 and CO, CO2). ∆Eiorb values are given within square brackets in kcal/mol.

25



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Figure 3b. The plot of deformation densities (∆ρi(r)) of the pair-wise orbital interactions and the associated ∆Eiorb energies obtained from the EDA-NOCV for [LAu-(bipy)]+ (L = H2O, H2S, C2H2 and C2H4). ∆Eiorb values are given within square brackets in kcal/mol. 26


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Figure 3c. The plot of deformation densities (∆ρi(r)) of the pair-wise orbital interactions and the associated ∆Eiorb energies obtained from the EDA-NOCV for [LAu-(bipy)]+ (L = CH4, CH3Cl and C2H6). ∆Eiorb values are given within square brackets in kcal/mol.

27



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Figure 4a. Plot of deformation densities (∆ρi(r)) of the pair-wise orbital interactions and shape of the most important interacting orbitals(formation of σ-bonding and π-bonding) between L and [M-(bipy)]+ associated with ∆Eiorb energies obtained from the EDA-NOCV for [LM-(bipy)]+ (L= 28


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N2, CO). Associated orbital values are also given. The color code of the charge flow is red→blue.

Figure 4b. Plot of deformation densities (∆ρi(r)) of the pair-wise orbital interactions and shape of the most important interacting orbitals(formation of σ-bonding and π-bonding) between L and [M-(bipy)]+ associated with ∆Eiorb energies obtained from the EDA-NOCV for [LM-(bipy)]+ (L 29



Page 30 of 36

= C2H4,C2H2). Associated orbital values are also given. The color code of the charge flow is red→blue. Tables

Table 1. Both ZPE- and BSSE-corrected dissociation energy (D0BSSE, kcal/mol) of L-Cu bonds, enthalpy change (∆H, kcal/mol) at 298 K, free energy change (∆G, kcal/mol) at 298 K for the dissociation process: [LCu-(bipy)]+ → L + [Cu-(bipy)]+, HOMO-LUMO energy difference (∆EHL, eV), NPA charges on Cu and L (q, au), at the PBE0/VTZ level. The local energy density (H(rc), au) at the BCP of L-Cu bond and delocalization index (δ(L-Cu), au) are computed at the PBE0/VTZ/WTBS level where L is the coordinating center of the ligand. D0BSSE

Complexes

∆H

∆G

[Cu-(bipy)]+

∆EH-L

WBI(L-Cu)

q(L)

4.21

q(Cu)

H(rc)

δ(L-Cu)

0.81

[H2Cu-(bipy)]+

10.5

12.1

3.4

4.97

0.23

0.08

0.61

-0.039

0.49

[N2Cu-(bipy)]+

21.9

23.3

13.8

4.66

0.45

0.06

0.65

-0.039

0.74

+

31.4

32.5

22.1

4.95

0.80

0.16

0.52

-0.064

1.07

5.2

5.4

0.3

4.07

0.19

0.08

0.70

-0.006

0.36

18.9

21.0

13.3

3.98

0.24

0.12

0.67

-0.013

0.46

24.0

24.8

15.3

4.39

0.55

0.24

0.49

-0.031

0.85

34.2

35.4

24.5

5.14

0.42

-0.04

0.69

-0.038

0.67

33.8

34.9

22.9

4.95

0.38

-0.04

0.71

-0.032

0.60

7.5

7.8

0.1

4.35

0.18

0.11

0.63

-0.013

0.42

17.2

17.3

8.7

4.21

0.45

0.54

0.73

-0.021

0.66

8.9

9.0

0.3

4.31

0.18

0.11

0.63

-0.013

0.41

[OCCu-(bipy)]

[CO2Cu-(bipy)]

+

[H2OCu-(bipy)] [H2SCu-(bipy)]

+

+

[C2H2Cu-(bipy)]

+

[C2H4Cu-(bipy)]

+

[CH4Cu-(bipy)]

+

[CH3ClCu-(bipy)]

+

[C2H6Cu-(bipy)]+

30


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Table 2. Both ZPE- and BSSE-corrected dissociation energy (D0BSSE, kcal/mol) of L-Ag bonds, enthalpy change (∆H, kcal/mol) at 298 K, free energy change (∆G, kcal/mol) at 298 K for the dissociation process: [LAg-(bipy)]+ → L + [Ag-(bipy)]+, HOMO-LUMO energy difference (∆EH-L, eV), NPA charges on Ag and L (q, au), at the PBE0/VTZ level. The local energy density (H(rc)) at the BCP of L-Ag bond and delocalization index (δ(L-Ag), au) are computed at the PBE0/VTZ/WTBS level where L is the coordinating center of the ligand. D0BSSE

Complexes

∆H

∆G

[Ag-(bipy)]+

∆EH-L

WBI(L-Ag)

q(L)

q(Ag)

H(rc)

δ(L-Ag)

0.83

5.00

[H2Ag-(bipy)]+

3.7

5.0

-2.3

5.17

0.16

0.06

0.70

-0.021

0.41

[N2Ag-(bipy)]+

12.1

13.0

5.3

5.17

0.30

0.08

0.70

-0.016

0.53

20.3

21.2

12.2

5.09

0.67

0.16

0.58

-0.044

0.94

0.9

1.0

-4.4

5.15

0.11

0.05

0.77

0.002

0.24

12.7

14.8

7.3

5.08

0.17

0.09

0.73

-0.003

0.37

18.4

19.0

10.8

5.23

0.45

0.21

0.57

-0.019

0.74

21.7

22.5

12.9

5.20

0.31

0.00

0.73

-0.021

0.56

23.8

24.6

14.2

5.19

0.31

0.01

0.73

-0.019

0.50

2.3

2.3

-4.2

5.23

0.12

0.08

0.72

-0.004

0.30

11.5

11.4

3.8

5.15

0.33

0.17

0.63

-0.009

0.52

3.6

3.3

-4.0

5.21

0.11

0.08

0.71

-0.004

0.29

[OCAg-(bipy)]

+

[CO2Ag-(bipy)]

+

[H2OAg-(bipy)] [H2SAg-(bipy)]

+

+

[C2H2Ag-(bipy)]

+

[C2H4Ag-(bipy)]

+

[CH4Ag-(bipy)]

+

[CH3ClAg-(bipy)]

+

[C2H6Ag-(bipy)]+

31



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Table 3. Both ZPE- and BSSE-corrected dissociation energy (D0BSSE, kcal/mol) of L-Au bonds, enthalpy change (∆H, kcal/mol) at 298 K, free energy change (∆G, kcal/mol) at 298 K for the dissociation process: [LAu-(bipy)]+ → L + [Au-(bipy)]+, HOMO-LUMO energy difference (∆EH-L, eV), NPA charges on Au and L (q, au), at the PBE0/VTZ level. The local energy density (H(rc)) at the BCP of L-Au bond and delocalization index (δ(L-Au), au) are computed at the PBE0/VTZ/WTBS level where L is the coordinating center of the ligand. D0BSSE

Complexes

∆H

∆G

[Au-(bipy)]+

∆EH-L

WBI(L-Au)

q(L)

3.79

q(Au)

H(rc)

δ(L-Au)

0.71

[H2Au-(bipy)]+

29.0

31.5

21.5

5.09

0.74

0.12

0.42

-0.137

0.81

[N2Au-(bipy)]+

24.6

25.9

17.7

4.71

0.52

0.09

0.57

-0.049

0.83

[OCAu-(bipy)]+

45.6

46.8

36.6

5.04

1.00

0.17

0.47

-0.100

1.25

[CO2Au-(bipy)]+

4.3

4.7

-3.1

4.71

0.20

0.11

0.58

-0.009

0.41

[H2OAu-(bipy)]+

20.9

24.2

15.4

4.80

0.29

0.17

0.53

-0.017

0.55

[H2SAu-(bipy)]+

34.8

35.7

26.1

4.74

0.65

0.33

0.38

-0.041

1.01

[C2H2Au-(bipy)]+

48.0

49.4

37.7

5.17

0.56

-0.06

0.66

-0.049

0.79

[C2H4Au-(bipy)]+

49.4

50.7

38.0

5.16

0.52

-0.04

0.67

-0.043

0.72

[CH4Au-(bipy)]+

10.3

10.7

3.1

4.76

0.26

0.11

0.58

-0.045

0.56

22.2

22.3

13.5

4.72

0.49

0.18

0.43

-0.023

0.75

11.7

11.8

3.2

4.79

0.22

0.13

0.56

-0.036

0.48

[CH3ClAu-(bipy)] [C2H6Au-(bipy)]

+

+

32


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1 2 3 4 5 6 7 8 9 10 11 Table 4. EDA-NOCV results of the [LCu-(bipy)]+ complexes taking L as one fragment and [Cu12 (bipy)]+ as another, studied at the revPBE-D3(BJ)/TZ2P//PBE0/VTZ level. All the energy terms 13 14 are in kcal/mol. 15 16 Complexes ∆EPauli ∆Eelstat ∆Eorb ∆Edisp ∆Eint σ 1) ∆Eorb(σ σ 2) ∆Eorb(π π1) ∆Eorb(π π2) ∆Eorb(σ 17 + 71.9 -54.9(59.0) -36.2(38.9) -1.9(2.1) -21.1 -12.2{33.6} -8.7{23.9} -14.4{39.7} -1.1{3.1} 18 [H2Cu-(bipy)] 19 [N2Cu-(bipy)]+ 82.8 -58.1(54.8) -45.1(42.5) -2.9(2.7) -23.3 -13.8{30.5} -6.4{14.2} -14.8{32.8} -9.6{21.2} 20 + 118.1 -97.8(62.0) -57.2(36.2) -2.8(1.8) -39.7 -14.6{25.5} -9.7{17.0} -20.3{35.4} -12.9{22.5} 21 [OCCu-(bipy)] + 22 [CO2Cu-(bipy)] 38.0 -27.3(55.5) -19.5(39.7) -2.3(4.8) -11.1 -7.4{38.0} -4.0{20.5} -3.9{19.9} -3.3{16.9} 23 [H OCu-(bipy)]+ 51.3 -50.5(67.6) -21.8(29.2) -2.4(3.2) -23.4 -11.8{54.2} -2.5{11.6} -4.0{18.4} -1.6{7.3} 24 2 + 81.8 -73.2(63.5) -38.0(32.9) -4.1(3.6) -33.5 -16.9(44.5) -6.6{17.5} -8.4{22.2} -5.0{13.3} 25 [H2SCu-(bipy)] 26[C H Cu-(bipy)]+ 140.0 -110.6(58.8) -72.4(38.5) -5.2(2.8) -48.3 -15.8{21.9} -4.6{6.4} -44.9{61.9} -4.2{5.7} 2 2 27 + 126.6 -102.1(59.1) -65.0(37.7) -5.6(3.2) -46.1 -17.6{27.1} -3.5{5.3} -38.4{59.1} -3.3{5.0} 28[C2H4Cu-(bipy)] 29 [CH Cu-(bipy)]+ 56.0 -40.6(56.6) -27.1(37.8) -4.1(5.7) -15.8 -10.5{38.8} -4.6{16.8} -5.8{21.5} -3.2{11.8} 4 30 + 62.4 -52.2(59.0) -32.0(36.2) -4.3(5.8) -26.0 -13.6{42.5} -5.7{17.7} -7.6{23.8} -3.5{11.0} 31[CH3ClCu-(bipy)] 32[C2H6Cu-(bipy)]+ 56.7 -41.4(55.3) -28.4(37.9) -5.1(6.8) -18.1 -10.9{38.4} -4.4{15.6} -3.2{11.2} -6.1{21.5} 33 (The values within the parentheses show the percentage contribution towards the total attractive 34 interaction ∆Eelstat + ∆Eorb + ∆Edisp and the values within the braces show the contribution towards the 35 total orbital interaction, ∆Eorb.) 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 33 57 58 59 ACS Paragon Plus Environment 60


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+ Table 5. EDA-NOCV results of the [LAg-(bipy)] complexes taking L as one fragment and [Ag-

(bipy)]+ as another, studied at the revPBE-D3(BJ)/TZ2P//PBE0/VTZ level level. All the energy terms are in kcal/mol. Complexes

∆EPauli

∆Eelstat

∆Eorb

∆Edisp

∆Eint

∆Eorb(σ σ 1)

∆Eorb(σ σ 2)

∆Eorb(π π1)

[H2Ag-(bipy)]+

61.4

-46.0(63.3)

-25.0(34.4)

-1.7(2.4)

-11.4

-9.2{37.0}

-6.5{25.9}

-8.2{32.9}

-1.0{3.9}

+

52.6

-36.4(56.8)

-25.2(39.2)

-2.6(4.0)

-11.5

-10.1{40.3}

-3.4{13.3}

-6.3{25.1}

-4.4{17.6}

+

110.8

-89.6(65.7)

-44.1(32.4)

-2.6(1.9)

-25.5

-13.8{31.3}

-7.9{18.0}

-13.3{30.1}

-8.8{19.8}

+

19.7

-15.0(56.2)

-9.5(35.7)

-2.2(8.1)

-6.9

-4.1{43.1}

-1.4{14.4}

-1.7{17.8}

-1.8{19.0}

+

35.0

-35.9(69.1)

-13.8(26.6)

-2.3(4.4)

-16.9

-8.5{61.9}

-1.9{13.9}

-1.3{9.1}

-1.0{7.2}

+

[N2Ag-(bipy)]

[OCAg-(bipy)]

[CO2Ag-(bipy)]

[H2OAg-(bipy)] [H2SAg-(bipy)]

∆Eorb(π π2)

70.9

-63.4(66.2)

-28.7(29.9)

-3.7(3.9)

-25.0

-14.8{51.6}

-5.1{17.7}

-4.5{15.8}

-3.1{11.0}

[C2H2Ag-(bipy)]

+

112.3

-90.0(62.9)

-48.6(33.9)

-4.5(3.1)

-30.7

-14.6{30.0}

-3.6{7.3}

-25.1{51.6}

-2.5{5.1}

[C2H4Ag-(bipy)]

+

107.9

-88.2(62.9)

-47.1(33.6)

-4.9(3.5)

-32.3

-15.7{33.4}

-3.2{6.7}

-23.0{48.9}

-2.4{5.2}

+

34.8

-24.4(55.0)

-16.3(36.8)

-3.7(8.2)

-9.6

-7.7{47.5}

-2.1{13.0}

-3.1{19.2}

-1.7{10.1}

45.2

-38.4(60.4)

-21.4(33.7)

-3.8(5.9)

-18.3

-10.9{51.0}

-3.2{15.0}

-3.8{17.7}

-2.0{9.5}

35.1

-24.8(53.2)

-17.4(37.2)

-4.5(9.6)

-31.9

-8.1{46.8}

-2.0{11.7}

-3.3{19.0}

-1.7{9.6}

[CH4Ag-(bipy)]

[CH3ClAg-(bipy)] [C2H6Ag-(bipy)]

+

+

(The values within the parentheses show the percentage contribution towards the total attractive interaction ∆Eelstat + ∆Eorb + ∆Edisp and the values within the braces show the contribution towards the total orbital interaction ∆Eorb.)

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1 2 3 4 5 6 7 8 9 10 + Table 6. EDA-NOCV results of the [LAu-(bipy)] complexes taking L as one fragment and [Au11 12 (bipy)]+ as another, studied at the revPBE-D3(BJ)/TZ2P//PBE0/VTZ level. All the energy terms 13 are in kcal/mol. 14 15 Complexes ∆EPauli ∆Eelstat ∆Eorb ∆Edisp ∆Eint σ 1) ∆Eorb(σ σ 2) ∆Eorb(π π1) ∆Eorb(σ 16 + 275.4 -269.4(64.5) -145.9(34.9) -2.3(0.6) -142.2 -40.8{28.0} -1.6{1.1} -96.4{66.0} 17[H2Au-(bipy)] 18 + [N2Au-(bipy)] 117.4 -79.7(54.7) -63.1(43.3) -2.9(2.0) -28.3 -26.0{41.3} -8.2{12.9} -16.7{26.4} 19 [OCAu-(bipy)]+ 213.0 -169.3(62.9) -96.9(36.0) -2.8(1.1) -56.1 -36.7{37.9} -14.7{15.2} -27.1{28.0} 20 + 21 [CO2Au-(bipy)] 48.1 -32.8(52.4) -27.0(43.2) -2.8(4.4) -14.4 -15.3{56.8} -5.8{21.4} -2.9{10.7} 22 + [H2OAu-(bipy)] 70.1 -64.1(63.0) -35.0(34.4) -2.6(2.6) -31.6 -24.5{70.0} -4.2{12.1} -2.4{6.7} 23 24 [H2SAu-(bipy)]+ 132.4 -113.1(63.1) -62.9(34.6) -4.1(2.3) -46.9 -35.9{57.8} -8.2{13.2} -9.5{15.3} 25 + [C2H2Au-(bipy)] 218.0 -117.4(50.1) -112.1(47.8) -4.9(2.1) -70.5 -35.7{31.9} -7.1{6.3} -58.7{52.3} 26 + 27 [C2H4Au-(bipy)] 205.7 -165.4(60.3) -103.4(37.7) -5.4(2.0) -68.5 -38.1{36.8} -5.2{5.0} -51.2{49.5} 28 + [CH Au-(bipy)] 95.3 -70.3(56.8) -49.0(39.6) -4.4(3.6) -28.3 -25.7{52.4} -6.3{12.8} -12.0{24.6} 29 4 + [CH 85.6 -68.5(57.0) -47.4(39.4) -4.3(3.6) -34.6 -28.6{60.3} -5.9{12.5} -6.6{13.9} 30 3ClAu-(bipy)] + 31 [C2H6Au-(bipy)] 82.3 -60.6(54.7) -44.4(40.1) -5.7(5.1) -28.3 -24.9{56.1} -5.6{12.5} -7.9{17.7} 32 (The values within the parentheses show the percentage contribution towards the total attractive 33 34 interaction, ∆Eelstat + ∆Eorb + ∆Edisp and the values within the braces show the contribution towards the 35 total orbital interaction, ∆Eorb.) 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 35 57 58 59 ACS Paragon Plus Environment 60

∆Eorb(π π2) -5.0{3.4} -10.9{17.2} -17.8{18.4} -1.8{6.8} -2.3{6.5} -6.4{10.3} -4.3{3.9} -4.6{4.5} -2.5{5.1} -4.0{8.5} -2.7{6.0}


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