Insight into the Bonding Mechanism and the Bonding Covalency in

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Insight into the Bonding Mechanism and the Bonding Covalency in Noble Gas-Noble Metal Halides: An NBO/NRT Investigation Guiqiu Zhang, Lei Fu, Hong Li, Xuchan Fan, and De-Zhan Chen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b02047 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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The Journal of Physical Chemistry

Insight into the Bonding Mechanism and the Bonding Covalency in Noble Gas-Noble Metal Halides: An NBO/NRT Investigation Guiqiu Zhang,* Lei Fu, Hong Li, Xuchan Fan and Dezhan Chen

College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Wenhua East Road 88, Jinan, Shandong 250014, P. R. China

ABSTRACT: The bonding between noble gas and noble metal halide like hydrogen bonding (H-bonding) motivates us to investigate the bonding mechanism and the bonding covalency in NgMX (Ng = He, Ne, Ar, Kr, Xe, Rn; M = Cu, Ag, Au; X = F, Cl, Br, I) complexes using Natural Bond Orbital (NBO) and Natural Resonance Theory (NRT) methods. In this study, we introduce the new resonance bonding model in H-bonding into NgMX bonding. We provide strong evidence for resonance bonding involving two important resonance structures: Ng: M-X ↔ Ng+-M :X- in each of NgMX complexes, originating in the nNg→σ*MX hyperconjugative interaction. The covalency of the bonding could be understood by the localized nature of Ng-M bonds in these two resonance structures, and the degree of Ng-M covalency can be quantitatively described by calculated NRT bond orders bNgM. Furthermore, we find that the bond order satisfies conservation of bond orders, bNgM + bMX = 1 for all of the studied complexes. On the basis of the conservation of bond orders and some 1

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statistical correlations, we also reveal that the Ng-M bond (except He-Ag and Ne-Ag bonds) can be tuned by changing auxiliary ligand X. Overall, the present studies provide new insight into the bonding mechanism and the covalency of the bonding in noble gas-noble metal halides, and develop one resonance bonding model.

1. INTRODUCTION Noble gases are extremely reluctant to be involved in chemical bonding due to their completely filled valence shell electronic configurations. And likewise, the coinage metals Cu, Ag and Au are in low reactivity when reacting with other substance, which in fact has resulted in crowning them as “noble metals”. Consequently, the preparation of noble gas-noble metal complexes has always been an enormous challenge for experimental chemists. During 2000 to 2006, Gerry’s group reported the preparation and characterization of a series of heavier noble gas-noble metal complexes NgMX (Ng = Ar, Kr, Xe; M = Cu, Ag, Au; X = F, Cl, Br) using Fourier transform microwave spectroscopy.1-11 Their work opened an important new chapter in noble gas chemistry. Subsequently, some new members of NgMX complexes continue to be discovered. In 2012 and 2013, Andrews’s group produced and characterized NeAuF by using matrix infrared spectroscopy.12,13 A latest report is about the new finding of ArAgI, just published by Legon et al.14

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These experimental studies have provided opportunities for theoretical chemists to develop new bonding models in chemical science and triggered intensive research on the bonding of NgMX complexes. Earlier computational studies focused on the Ar/Kr/Xe complexes by Gerry’s,10,11 Belpassi’s15 and Haiduke’s16 groups using different methods. Their computational analyses led to the same conclusion that the noble gas-noble metal bonding is predominantly covalent in nature. Analogous complexes containing He or Ne have been investigated by Evans and co-workers, using QTAIM analyses.17 QTAIM analyses of the He-AgF and Ne-MF (M = Cu, Ag, Au) complexes show that the Ng-M bonding is more electrostatic in nature. Recently, Boggs et al. investigated the series (Ng = He, Ne, Ar, Kr, Xe) of noble gas-noble metal complexes using QTAIM, NBO and other analysis methods, and proposed different viewpoints on the bonding between Ng and MX.18 They pointed out that the noble gas bond is a new kind of weak interaction, instead of van der Walls or covalent ones, like the hydrogen bond. Regarding to H-bonding, it is worth mentioning two latest papers. One is Grabowski’s review titled “What Is the Covalency of Hydrogen Bonding?”. Grabowski pointed out in this review, “Because for all hydrogen bonds the stabilizing role of charge transfer interaction may be found, thus one may state that all hydrogen bonds are in any way covalent in nature”.19 The other is about a latest definition put forward by Weinhold and Klein.20,21 They proposed that H-bonding can be described as resonance bonding, commonly originating in the n→σ* donor-acceptor interaction. More recently, our group extended this resonance

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bonding model into the noble gas hydrides22 in addition to small molecule-MX complexes.23 In this study, we focus on the bonding aspects of NgMX complexes. We raise two questions: 1). Is it resonance bonding for noble gas-noble metal halides? 2). If the answer is yes, how to understand the covalent character of Ng-M bonds? We carry out NBO/NRT analyses and focus on the resonance bonding and the covalent character of Ng-M bonds in the whole series of NgMX (Ng = He, Ne, Ar, Kr, Xe, Rn; M = Cu, Ag, Au; X = F, Cl, Br, I) complexes. We provide new insight into the bonding and the bonding covalency in noble gas-noble metal halides. The results of this research are of important for the full understanding of structures and properties of this new class of complexes. The paper is organized as follows. Firstly, we summarize computational details and compare optimized structures of our chosen complexes with available experimental results. Secondly, we discuss the bonding mechanism involving two important resonance structures: Ng: M-X ↔ Ng+-M :X-. Thirdly, in the localized NBO/NRT framework, we discuss the covalency of the Ng-M bond in NgMX complexes. Fourthly, we analyze whether the calculated NRT bond order satisfies the conservation of bond order or not. Finally, on the basis of this resonance bonding model and some statistical correlations, we analyze how to modify the Ng-M bond by changing auxiliary ligand X.

2. COMPUTATIONAL DETAILS 4


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All calculations were carried out with the Gaussian 09 program package24 at the coupled-cluster CCSD(T) level of theory.25 The convergent triple-ζ basis sets (aug-cc-pVTZ-PP),26 which include small-core energy-consistent relativistic pseudopotentials (PP) to account for relativistic effects, were adopted for heavy atoms : Xe, Rn, I and noble metal atoms, whereas the systematically augmented correlation-consistent triple-ζ Dunning basis sets (aug-cc-pVTZ)27 were used for other atoms. Experimental and previous theoretical data1-14 were also exhibited in this paper for comparison. Frequency and dissociation energy calculations were performed at the same level of theory and basis sets, and the absence of imaginary frequencies confirms that all of the optimized structures are at the energy local minima. Bonding analyses were implemented for each optimized system by Natural Bond Orbital (NBO)28 and Natural Resonance Theory (NRT)29-31 with the GENNBO 6.0W program.32 We read the hyperconjugative interaction and the corresponding second-order perturbation energy from NBO analysis results. The weightings of the resonance structures and bond orders were evaluated by NRT analyses.

3. RESULTS AND DISCUSSION 3.1. Geometry, Vibrational Frequency and Bond Dissociation Energy.

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Figure 1. Generic optimized geometry for NgMX complexes (Ng = He, Ne, Ar, Kr, Xe, Rn; M = Cu, Ag, Au; X = F, Cl, Br, I).

The optimized most stable structures of NgMX complexes are presented in Figure 1. Each of the species possesses linear equilibrium structures with C∞v symmetry. All of the noble gas-noble metal halides have a linear triad with the noble metal atom (M = Cu, Ag, Au) as the centre, which is consistent with the experimental results,1-11 as well as the calculation results in different levels.33 The bond length and the dissociation energy of the Ng-M bond in NgMX complexes are listed in Table 1. The bond length and the vibrational frequency of Ng-M and M-X with corresponding experimental and previous theoretical values in the parentheses for comparison are collected in Table S1. Because of the obvious coupling, we did not display the frequency of HeMX complexes. To validate the computational method and basis sets, we take ArAgX as one example to compare with available experimental data.14 The calculated bond length of Ar-Ag in our work is 2.594, 2.635, 2.648 and 2.692 Å for ArAgF, ArAgCl, ArAgBr and ArAgI, respectively. And the experimental data are 2.558, 2.612, 2.637, and 2.676 Å, respectively. The differences are not more than 0.04 Å in ArAgX complexes. The calculated bond lengths exhibit differences in a reasonable range from the available experimental values. Therefore, the theoretical method and basis sets are appropriate for describing these complexes. Table 1. Calculated bond lengths RNgM (Å) and Ng-M bond dissociation energies, ENgM = ENg + EMX – ENgMX (kcal/mol), of NgMX complexes at the CCSD(T) level of theory. Complexes

RNgM

ENgM

Complexes

RNgM

ENgM

Complexes

RNgM

ENgM 6


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HeCuF

1.655

6.43

HeAgF

2.221

1.02

HeAuF

1.841

6.28

HeCuCl

1.729

4.27

HeAgCl

2.300

0.88

HeAuCl

1.977

3.32

HeCuBr

1.756

3.60

HeAgBr

2.278

0.84

HeAuBr

2.042

2.48

HeCuI

1.807

2.54

HeAgI

2.411

0.70

HeAuI

2.256

1.32

NeCuF

2.179

2.68

NeAgF

2.700

0.91

NeAuF

2.445

2.54

NeCuCl

2.260

1.95

NeAgCl

2.719

0.94

NeAuCl

2.569

1.77

NeCuBr

2.320

1.77

NeAgBr

2.676

0.95

NeAuBr

2.605

1.56

NeCuI

2.444

1.37

NeAgI

2.758

0.90

NeAuI

2.771

1.20

ArCuF

2.224

10.50

ArAgF

2.594

4.75

ArAuF

2.406

12.19

ArCuCl

2.271

8.69

ArAgCl

2.635

4.45

ArAuCl

2.489

9.08

ArCuBr

2.308

8.10

ArAgBr

2.648

4.36

ArAuBr

2.515

8.07

ArCuI

2.333

6.99

ArAgI

2.692

3.97

ArAuI

2.586

6.32

KrCuF

2.298

14.22

KrAgF

2.603

8.04

KrAuF

2.466

17.93

KrCuCl

2.358

12.27

KrAgCl

2.648

7.48

KrAuCl

2.527

14.15

KrCuBr

2.376

11.66

KrAgBr

2.665

7.31

KrAuBr

2.546

12.90

KrCuI

2.395

10.41

KrAgI

2.698

6.71

KrAuI

2.599

10.65

XeCuF

2.435

17.67

XeAgF

2.684

11.92

XeAuF

2.565

24.57

XeCuCl

2.473

15.63

XeAgCl

2.727

11.01

XeAuCl

2.620

20.12

XeCuBr

2.487

14.98

XeAgBr

2.738

10.71

XeAuBr

2.638

18.66

XeCuI

2.512

13.63

XeAgI

2.771

9.87

XeAuI

2.677

15.96

RnCuF

2.509

18.96

RnAgF

2.746

13.81

RnAuF

2.634

27.03

RnCuCl

2.550

17.24

RnAgCl

2.783

13.06

RnAuCl

2.686

22.79

RnCuBr

2.569

16.66

RnAgBr

2.800

12.82

RnAuBr

2.704

21.41

RnCuI

2.586

15.40

RnAgI

2.825

11.97

RnAuI

2.737

18.75

As shown in Table 1, the dissociation energy is always below 25 kcal/mol. Note that the He/Ne-Ag bond dissociation energies in He/Ne-AgX complexes are always significantly lower than those of other complexes studied here. The calculated dissociation energy of the Ng-M bond decreases in the order of F > Cl > Br > I, which is in accord with the atomic number of halogen. In XeAuX (X = F, Cl, Br, I), for example, the EXeAu (in kcal/mol) is 24.57, 20.12, 18.66 and 15.96 for XeAuF, XeAuCl, XeAuBr and XeAuI, respectively. The calculated Ng-M bond lengths in our studied 7



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complexes range from 1.655 to 2.825 (Å) in the order of F < Cl < Br < I. Again, taking XeAuX (X = F, Cl, Br, I) for example, the RXeAu (in Å) is 2.565, 2.620, 2.638, and 2.677 for XeAuF, XeAuCl, XeAuBr and XeAuI, respectively, showing the monotonic increase in the vertical order F < Cl < Br < I. Note that HeAgX and NeAgX are the exceptional series. We can find that the Ng-M bond length and the bond dissociation energy do not follow above trends. The irregularity maybe has something to do with the much lower dissociation energy. The reasons for the apparent trends of the bond length and the bond dissociation energy in NgMX complexes will be discussed in detail in section 3.5. Also note that all of the Ng-M bond lengths in studied complexes are very close to the covalent limit but well below the van der Waals radii limit.34 The calculated frequencies are all in a larger value which illustrates that all of the studied complexes are rigid. These basic structural and energetic trends provide the backdrop for ensuing NBO/NRT analyses. 3.2. Resonance Bonding in NgMX Complexes. NBO analyses are particularly useful for understanding the chemical bonding in complexes, because it can provide the best natural Lewis structure (NLS), identify the donor-acceptor orbital interactions, and provide the corresponding second-order perturbation energy (E(2)) (E(2) indicates the reduction of the energy arising from electron delocalization effects). The NBO-based natural resonance theory (NRT) can provide resonance structures and the weightings (wI, wII, …) of contributing resonance structures I, II, …, as well as the

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composite bond order (b) that best expresses the strength of resonance-weighted chemical bonding between any atom pair. Here, we study the bonding in NgMX complexes by means of these NBO/NRT descriptors. Table 2. Second-order perturbation stabilization energy (E(2), kcal/mol) due to the hyperconjugative interaction nNg→σ*MX in NgMX. Complexes

nNg→σ*MX

Complexes

nNg→σ*MX

Complexes

nNg→σ*MX

HeCuF

24.66

HeAgF

4.30

HeAuF

41.88

HeCuCl

30.32

HeAgCl

7.04

HeAuCl

36.10

HeCuBr

30.63

HeAgBr

8.69

HeAuBr

30.46

HeCuI

29.04

HeAgI

6.72

HeAuI

14.88

NeCuF

8.79

NeAgF

1.31

NeAuF

10.03

NeCuCl

12.33

NeAgCl

3.27

NeAuCl

9.12

NeCuBr

11.74

NeAgBr

4.62

NeAuBr

8.59

NeCuI

9.42

NeAgI

4.40

NeAuI

5.09

ArCuF

40.81

ArAgF

15.93

ArAuF

67.07

ArCuCl

55.76

ArAgCl

24.95

ArAuCl

66.07

ArCuBr

56.19

ArAgBr

27.05

ArAuBr

62.23

ArCuI

57.46

ArAgI

26.75

ArAuI

49.19

KrCuF

54.60

KrAgF

28.05

KrAuF

92.92

KrCuCl

72.65

KrAgCl

40.95

KrAuCl

98.79

KrCuBr

76.13

KrAgBr

43.79

KrAuBr

95.16

KrCuI

78.35

KrAgI

44.13

KrAuI

79.60

XeCuF

65.28

XeAgF

41.45

XeAuF

121.68

XeCuCl

93.90

XeAgCl

59.36

XeAuCl

133.95

XeCuBr

99.38

XeAgBr

63.54

XeAuBr

129.19

XeCuI

101.23

XeAgI

63.87

XeAuI

112.14

RnCuF

65.51

RnAgF

43.10

RnAuF

121.85

RnCuCl

95.14

RnAgCl

63.19

RnAuCl

136.13

RnCuBr

99.59

RnAgBr

67.05

RnAuBr

131.87

RnCuI

103.64

RnAgI

68.52

RnAuI

116.52

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From NBO analyses, we know that the best natural Lewis structure (NLS) of NgMX is Ng: M-X, and one donor-acceptor interaction (hyperconjugative interaction) takes place between the lone pair electrons of Ng (nNg) and the antibonding orbital of the M-X moiety (σ*MX), expressed as nNg→σ*MX with associated E(2) (in kcal/mol) stabilization energy (as shown in Table 2). Due to the hyperconjugation nNg→σ*MX, electron density transfers from Ng lone pair to the antibonding orbital of M-X, making the Ng atom carry a little positive charge and the X atom get slightly negative. This leads to the other resonance structure Ng+-M :X-. Obviously, for each of the studied complexes there are two major resonance structures Ng: M-X and Ng+-M :X-, originating in the nNg→σ*MX hyperconjugative interaction. Similar resonance structures for the H-NgF (Ng = He, Ar and Kr) complexes originating from the n→σ* hyperconjugative interaction were proposed by Alabugin and co-workers in 2004.35 Taking XeAuF for one representative instance, the best natural Lewis structure (NLS) is Xe: Au-F. The hyperconjugative interaction takes place in this case: from lone pair electrons of Xe to the antibonding orbital of the Au-F moiety (nXe→σ*AuF) with E(2) = 121.68 kcal/mol. The 3-D surface view of the hyperconjugative interaction is exhibited in Figure 2. This nXe→σ*AuF hyperconjugative interaction induces resonance mixing: Xe: Au-F ↔ Xe +- Au :F -.

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Figure 2. 3-D surface view of hyperconjugative interaction nXe→σ*AuF in XeAuF.

Subsequently, in terms of this bonding picture, we carry out the NRT analysis for all of the studied systems. It should be noted that we also identify one long-bond structure Ng^X, similar to the H^Y in HNgY,22 besides the two primary resonance structures. However, the weighting of the long-bond structure is so small that it can be ignored. In order to insure the accuracy of the resonance weightings, the two key structure types are specified as reference structures (in $NRTSTR keylist input) for each complex. Table 3. The NRT weightings of resonance structures (wI and wII for Ng: M-X and Ng+-M :X-, respectively, %), bond order (bAB) for M-X and Ng-M bonds and their sum. Complexes

wI

wII

bMX

bNgM

Sum

HeCuF

73.6

25.8

0.74

0.26

1.00

HeCuCl

82.8

16.7

0.83

0.17

1.00

HeCuBr

85.0

14.5

0.85

0.14

0.99

HeCuI

87.9

11.6

0.88

0.12

1.00

NeCuF

86.7

13.0

0.87

0.13

1.00

NeCuCl

91.7

8.1

0.92

0.08

1.00

NeCuBr

93.1

6.7

0.93

0.07

1.00

NeCuI

95.0

4.8

0.95

0.05

1.00 11



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ArCuF

63.4

36.0

0.63

0.36

0.99

ArCuCl

74.4

25.2

0.74

0.25

0.99

ArCuBr

77.6

21.9

0.78

0.22

1.00

ArCuI

81.1

18.3

0.81

0.18

0.99

KrCuF

56.5

42.9

0.57

0.43

1.00

KrCuCl

68.9

30.7

0.69

0.31

1.00

KrCuBr

72.1

27.5

0.72

0.27

0.99

KrCuI

76.0

23.6

0.76

0.24

1.00

XeCuF

49.7

49.8

0.50

0.50

1.00

XeCuCl

62.6

37.0

0.63

0.37

1.00

XeCuBr

65.9

33.7

0.66

0.34

1.00

XeCuI

70.3

29.2

0.70

0.29

0.99

RnCuF

49.4

50.1

0.49

0.50

0.99

RnCuCl

61.4

38.2

0.61

0.38

0.99

RnCuBr

64.8

34.8

0.65

0.35

1.00

RnCuI

69.1

30.4

0.69

0.30

0.99

HeAgF

87.5

12.3

0.87

0.12

0.99

HeAgCl

92.1

7.6

0.92

0.08

1.00

HeAgBr

92.6

7.1

0.93

0.07

1.00

HeAgI

94.7

5.1

0.95

0.05

1.00

NeAgF

91.9

8.0

0.92

0.08

1.00

NeAgCl

94.9

5.0

0.95

0.05

1.00

NeAgBr

95.2

4.7

0.95

0.05

1.00

NeAgI

96.2

3.7

0.96

0.04

1.00

ArAgF

72.6

26.7

0.73

0.27

1.00

ArAgCl

81.9

17.5

0.82

0.17

0.99

ArAgBr

84.0

15.4

0.84

0.15

0.99

ArAgI

87.0

12.3

0.87

0.12

0.99

KrAgF

63.0

36.4

0.63

0.36

0.99

KrAgCl

74.8

24.7

0.75

0.25

1.00

KrAgBr

77.6

21.9

0.78

0.22

1.00

KrAgI

81.3

18.1

0.81

0.18

0.99

XeAgF

53.5

46.0

0.53

0.46

0.99

XeAgCl

67.1

32.6

0.67

0.33

1.00 12


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XeAgBr

70.3

29.3

0.70

0.29

0.99

XeAgI

74.7

24.8

0.75

0.25

1.00

RnAgF

49.9

49.5

0.50

0.50

1.00

RnAgCl

64.9

34.7

0.65

0.35

1.00

RnAgBr

68.4

31.2

0.68

0.31

0.99

RnAgI

72.8

26.7

0.73

0.27

1.00

HeAuF

79.4

19.9

0.79

0.20

0.99

HeAuCl

89.2

10.1

0.89

0.10

0.99

HeAuBr

91.6

7.8

0.92

0.08

1.00

HeAuI

95.4

4.2

0.95

0.04

0.99

NeAuF

92.5

7.2

0.93

0.07

1.00

NeAuCl

96.1

3.6

0.96

0.04

1.00

NeAuBr

96.8

3.0

0.97

0.03

1.00

NeAuI

98.0

1.9

0.98

0.02

1.00

ArAuF

69.3

30.3

0.69

0.30

0.99

ArAuCl

80.9

18.5

0.81

0.19

1.00

ArAuBr

83.6

15.8

0.84

0.16

1.00

ArAuI

87.6

11.7

0.88

0.12

1.00

KrAuF

61.4

38.2

0.61

0.38

0.99

KrAuCl

74.0

25.4

0.74

0.25

0.99

KrAuBr

77.0

22.4

0.77

0.22

0.99

KrAuI

81.6

17.7

0.82

0.18

1.00

XeAuF

52.9

47.1

0.53

0.47

1.00

XeAuCl

66.6

32.8

0.67

0.33

1.00

XeAuBr

69.9

29.7

0.70

0.30

1.00

XeAuI

74.8

24.6

0.75

0.25

1.00

RnAuF

50.6

49.5

0.51

0.49

1.00

RnAuCl

65.0

34.4

0.65

0.34

0.99

RnAuBr

68.4

31.2

0.68

0.31

0.99

RnAuI

73.2

26.4

0.73

0.26

0.99

The NRT analysis gives the corresponding weightings of two main resonance structures. This NRT result further demonstrates that both of the two primary 13



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resonance structures in the NgMX complexes (Ng: M-X ↔ Ng+-M :X-) play a significant role in the bonding. Thus, all such forms of resonance mixing are intrinsically stabilizing and mutually cooperative. Table 3 displays the calculated NRT weighting and bond order for each complex. The data show that the two main resonance weightings add up to approximate 100%, wI + wII ≈ 100%. The resonance weighting of Xe: Au-X is 52.9%, 66.6%, 69.9% and 74.8% if X is F, Cl, Br and I, respectively. Whereas the corresponding resonance weighting of the other resonance structure Xe+-Au :X- is 47.1%, 32.8%, 29.7% and 24.6%, respectively. Such a change is displayed in Figure 3.

Figure 3. Resonance weightings (%) of Xe: Au-X and Xe+-Au :X- (X = F, Cl, Br, I), as well as their sum.

Then let us turn to the NRT bond order of Ng-M and M-X. For XeAuF complex, the resonance weighting of Xe: Au-F is 52.9%, and the bond order bAuF = 0.53, while for Xe+-Au :F-, the resonance weighting is 47.1%, and the bond order bXeAu = 0.47. As 14


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table 3 shows, for all of the studied complexes, NRT bond orders display consistency with resonance weightings. The resonance weightings and NRT bond orders further support the resonance bonding in NgMX complexes. Detailed analyses of NRT bond orders are in section 3.3 and section 3.4. Overall, each of the NgMX complexes should be better described as a resonance hybrid: Ng: M-X ↔ Ng+-M :X-, originating in the nNg→σ*MX hyperconjugative interaction. Note that this resonance bonding picture is the direct analog of that in H-bonding. 3.3. Covalency of the Bonding in NgMX Complexes. The previous section has addressed the bonding mechanism question in NgMX complexes. This section focuses on the further understanding of the covalency of the bonding in NgMX complexes. Before we analyze the covalency of the bonding, it is worth mentioning the definition of covalent bonds. As the quantum theory of chemical bonds advances, the covalency concept is advocated in the varied definitions of “the sharing of a pair of electrons by the two bonded atoms” by Pauling36 and “a region of relating high electron density between nuclear which arises at least partly from sharing of electrons and gives rise to an attractive force and characteristic internuclear distance”, one more recent definition recommend by IUPAC.37 Prior to these two definitions of covalent bonds by Pauling and by IUPAC, there is Lewis’ electron-pair bond theory of chemical covalency.38, 39 Here, the electron-pair bonds40 include electron-sharing bonds and dative bonds (donor-acceptor).

15



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NBO/NRT analyses in this study provide a resonance bonding picture for the bonding in NgMX complexes. Such a bonding corresponds, in resonance terms, to an admixture of two resonance structures. Here, we analyze these two Lewis structures in conjunction with NRT calculations. For the individual Ng-M bond, it is a dative (donor-acceptor) electron-pair bond in the first resonance structure (Ng: M-X), while in the other one (Ng+-M :X-), it is a electron-sharing electron-pair bond. In other words, the Ng-M bond is related to two types of electron-pair bond, i.e. the electron-shared bond and the dative bond. Thus, the localized nature (dative and electron-shared) of the Ng-M bond in these two resonance structures determines its covalent character. Apparently, the covalent character of the Ng-M bond in NgMX complexes possesses duality of dative and electron-shared bonds. The similar conclusion can be drawn for M-X bonds in our studied complexes. It becomes clear that the covalent character of the bonding in NgMX complexes could be understood by the localized nature of two resonance structures. For Ng-M bonds in our studied complexes, the dative bonding plays a primary role, and the Ng-M bonds become more and more electron-shared with Ng being heavier. For M-X bonds, electron-shared bonding usually dominates, especially in heavier X cases. When the halogen atom is getting lighter and the noble gas atom getting heavier, both dative and electron-shared bondings trend equal such as RnMF cases. It is worthwhile to mention one latest study on the [NgAuNg]+ and [XAuX]- series by Grabowski and co-workers.41 They, from a new perspective, carried out the EDA for the Ng-Au bond and the X-Au bond in these two series, respectively. The results based on their EDA and our present NBO/NRT bonding analyses on Ng-Au and Au-X bonds have good

16


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consistency. Such a result suggests that our proposed understanding in covalency of Ng-M and M-X bonding is reasonable. Furthermore, the degree of Ng-M covalency can be quantitatively described by calculated NRT bond orders. It is important to note that the Ng-M bond studied here is the fractional chemical bond. In this aspect, it is similar to H-bonds. For Ar/Kr/Xe/Rn and some He-containing complexes, the Ng-M bond orders range from 0.1 to 0.5. Whereas all of the Ne cases and most of the He cases, the bond orders are less than 0.1. These results show that the complexes (except He/Ne cases) possess significant covalent character, and that He/Ne cases are slightly covalent in nature. AIM analyses support such a result. The details of AIM analyses (in Table S2) and the related discussions are presented in supporting information. Additionally, as shown in table 3, the degree of covalency for Ng-M bonds decreases in the order F > Cl > Br > I, while there is an increase according to Ne < He < Ar < Kr < Xe < Rn in the entire family. These results agree with our expected ones. Here, we want to point out that there are also examples of helium and neon complexes that features this reversed order, such as HNgY,22 NgBeCN2 and NgBeNBO complexes.42 3.4. Conservation of Bond Orders. The concept of conservation of the bond order is very old and it can be traced back to the time of Pauling.36,43 It was applied successfully in the bond-energy-bond-order (BEBO) method, developed by Johnston and Parr in the 1960s.44-46 Subsequently, this concept was further applied by Bürgi and Dunitz and others to different interactions.47-51 Recently, Weinhold and Landis nicely described the conservation of bond orders for H-bonds in NBO donor-acceptor 17



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perspective.52 Quite recently, Shahi and Arunan analyzed the NRT bond order in DX···A complexes and found that the sum of bond orders for D-X and X···A are conserved in X-bonded (X = H/Cl/Li) complexes.53 On the basis of this result, they proposed this finding can be extended into any atom (X) in the periodic table that involves intermolecular bonding of D-X···A complexes. Latest studies in our group extend this conservation concept into Cu/Ag/Au-bonding complexes23,54 and noble-gas hydrides.22 Here, we discuss the conservation of the bond order in the noble gas-noble metal halides. As seen in Table 3, NRT bond orders in these NgMX systems with two kinds of noble atoms conform to bond order conservation, as depicted in the below equation: bMX + bNgM = 1

(1)

For instance, there are bAuF = 0.53 and bXeAu = 0.47 for the XeAuF complex. When it comes to XeAuI, bAuI = 0.75 and bXeAu = 0.25 are obtained. When we change the auxiliary ligand from F to I, two kinds of bond orders (bMX and bNgM) display a reverse change. One increases, the other one decreases necessarily. The calculated values intuitively illustrate that NgMX complexes meet the conservation of the bond order. This implies that the Ng-M bonding can be tuned by the auxiliary ligand X (X = F, Cl, Br, I). We will make a detailed discussion in next section. 3.5. Modification of Ng-M Bonds by Changing Auxiliary Ligand X. As discussed above, the linear alignment of the Ng-M-X triad is associated with strong resonance bonding Ng: M-X ↔ Ng+-M :X- and the covalency of Ng-M bonding in

18


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NgMX complexes. On the basis of such a resonance bonding picture, we investigate the influence of auxiliary ligand X on the Ng-M bonding in all of the complexes studied here. As shown in Table 3, the calculated bNgM in NgMX is in the order of F > Cl > Br > I. Here, we consider the XeAuX complexes with the same XeAu. When X varies from F to I, the bond order of Xe-Au varies from 0.47 to 0.25 accordingly. With the halogen electronegativity getting greater, the Ng-M bond orders as well as the weightings of Ng+-M :X- are obviously increasing. Just as Table 3 presents, for RnCuF, bRnCu reaches 0.50. It is the largest of bNgM for all of the studied systems. Therefore, we can modify the resonance bonding of the Ng-M in NgMX complexes and the degree of covalency of Ng-M bonds by changing auxiliary ligand X. Can we tune the properties of Ng-M bonds by changing auxiliary ligand X? To answer this question, we firstly draw the correlations among the calculated bond order (bNgM), the bond length (RNgM) and the bond dissociation energy (ENgM). The Pearson χ2 coefficients55 are all above 0.85 for NgMX complexes (except for the He/Ne-AgX complexes), in Figure S1-S3. Therefore, the experimental parameters (RNgM, ENgM) of NgMX complexes are in significant linear correlation with the theoretical parameter (bNgM). On the basis of the excellent liner correlation among bNgM, RNgM and ENgM, we can easily explain the bond length and the bond dissociation energy trends that we mentioned in section 3.1. Obviously, these linear correlations further illustrate that we can tune the properties of bond between Ng and MX by modifying auxiliary ligand X.

19



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As with the exceptional HeAgX and NeAgX series, there are no good correlations among bond orders, bond lengths and dissociation energies. Such a result suggests that the bonding in these complexes is slightly covalent in nature. As shown in Evans and co-workers’ study, this bonding is more electrostatic in nature.17 Thus, this present study could not provide a reasonable explanation on the exception of HeAgX and NeAgX series.

4.

CONCLUSIONS

Experimental preparation and characterization of a series of noble gas-noble metal halides have provided us a wonderful opportunity to develop new bonding models. The present study shows that each of the NgMX complexes should be better described as a hybrid of two resonance structures, Ng: M-X ↔ Ng+-M :X-, originating in nNg→σ*MX hyperconjugative interaction. This result strongly supports that the bonding in NgMX complexes is resonance bonding. In the localized NBO/NRT framework, the covalency of the bonding between Ng and MX could readily be understood by the dative and electron-shared bonding in two resonance structures. The degree of covalency of the Ng-M bond could be quantitively described by the calculated bond order bNgM. The Ng-M bond orders range from 0.1 to 0.5 for most of the studied complexes (except He/Ne complexes), showing the Ng-M bond is significantly covalent in nature. The conclusion is that the bonding between Ng and MX is similar to H-bonding in three aspects: it is resonance bonding, originating in the nNg→σ*MX hyperconjugative interaction; it is a fractional chemical bond; and it 20


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satisfies the conservation of bond orders principle, bNgM + bMX = 1, throughout the NgMX series. Furthermore, we find that Ng-M bonds can be tuned by changing auxiliary ligand X. We believe that such a resonance bonding model may be suitable for NHC-MX and CR2-MX complexes, which are being further investigated by our group.

ASSOCIATED CONTENT Supporting Information

The full author list of refd 24; structural parameters (bond lengths, stretching frequencies) and Ng-M bond dissociation energies of all studied NgMX (Ng = He, Ne, Ar, Kr, Xe, Rn; M = Cu, Ag, Au; X = F, Cl, Br, I) complexes at the CCSD(T) level of theory, as well as experimental and previous calculated values in parentheses (Table S1); AIM data (Table S2) and detail analyses of AIM; correlation plots for bond order-bond dissociation energy (Figure S1), bond order-bond length (Figure S2), and dissociation energy-bond length (Figure S3) of Ng-M bond in NgMX complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *(G.Z.) E-mail: [email protected]. Tel: +86 15169053602. 21



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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2015BM025). We thank Professor Frank Weinhold of the University of Wisconsin-Madison for his continuous help. We also thank the reviewers for their comments and suggestions, which helped to improve the manuscript significantly. Finally, we thank UW-Madison Department of Chemistry research cluster as the computational facilities in part in our work.

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(48) Lendvay, G. Bond Orders from ab Initio Calculations and a Test of the Principle of Bond Order Conservation. J. Phys. Chem. 1989, 93, 4422-4429. (49) Shustorovich, E. The Bond-Order Conservation Approach to Chemisorption and Heterogeneous Catalysis: Applications and Implications. Adv. Catal. 1990, 37, 101-163. (50) Blowers, P.; Masel, R. Conservation of Bond Order during Hydrogenolysis and Dehydrogenation Reactions. Surf. Sci. 1998, 417, 238-246. (51) Majerz, I.; Olovsson, I. Comparison of the Proton-Transfer Paths in Hydrogen Bonds from Theoretical Potential-Energy Surfaces and the Concept of Conservation of Bond Order III. O–H–O Hydrogen Bonds. Phys. Chem. Chem. Phys. 2010, 12, 5462-5467. (52) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective; Cambridge University Press: New York, 2005. (53) Shahi, A.; Arunan, E. Hydrogen Bonding, Halogen Bonding and Lithium Bonding: An Atoms in Molecules and Natural Bond Orbital Perspective towards Conservation of Total Bond Order, inter- and intra-Molecular Bonding. Phys. Chem. Chem. Phys. 2014, 16, 22935-22952. (54) Zhang, G.; Wang, H.; Yue, H.; Li, H.; Zhang, S.; Fu, L. Ligand Effects Due to Resonance Character in LAuCCH−(L = F, Cl, Br, I, CCH) Complexes: An NBO/NRT Analysis. J. Mol. Mod. 2015, 21, 159. (55) Sachs, L. Applied Statistics: A Handbook of Techniques; Springer Science & Business Media, 2012.

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TOC Graphic

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Insight into the Bonding Mechanism and the Bonding Covalency in

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