Theoretical Study On The Resonance Character And Bonding Model Of Cu/Ag/Au-Bonding

The Cu/Ag/Au-containing complexes have been widely used in catalysis, medicine andnanophase materials. The study on bonding nature and character of Cu/Ag/Au-bondedcomplexes is very important for their application on catalysts, medicines andnanophase materials. The Cu/Ag/Au-bonding is a new concept proposed in recent years. Just asthe hydrogen-and halogen-bonding, the Cu/Ag/Au-bonding complexes are generally presentedin linear structures. At present, the researchs on the Cu/Ag/Au-bonding are mainly concentratedon whether electrostatic character or covalent character play the dominant role as well as whatkind of covalent character is the dominate component. Recently, Weinhold and Klein proposedthat the hydrogen bonding can be a fractional chemical bonding due to partial intermolecular B:H–A B+–H:A-resonance delocalization arising most commonly from quantum mechanicalnBâ†'σ*H–Ahyperconjugation interaction, i.e. a specific case of three-center-four-electron (3c/4e)hyperbonding interaction. Thus, whether the Cu/Ag/Au-bonding, which is much stronger thanthe H-bonding, can also be understood from the3c/4e hyperbonding concept? Additionally,Shahi and Arunan proposed that the bond order that includes ionic and covalent contributions isconserved in the hydrogen bonding, lithium bonding and chlorine bonding. Naturally, whetherthe bond order is conserved in the Cu/Ag/Au-bonding?Legon et al. have generated and characterized the complexes of OC···M–X (M=Cu, Ag, Au),H2O/H2S···M–Cl (M=Cu, Ag) and C2H2/C2H4···M–Cl (M=Cu, Ag), C2H2···Ag–CCH by therotational spectroscopy. In addition, Wang et al. investigated the gold(I)–alkynyl complexes(LAuCCH-) using the photoelectron spectroscopy and theoretical calculations, finding the resultthat the single Au-C bond is more stronger than the AuC multiple bonds and significantlyaffected by the the ancillary ligand. On the basis of these experimental studies, we choose theneutral B···M–X complexes and anionic LMCCH-(M=Cu, Ag, Au) complexes to investigate theresonance character and whether the bond order is conservative, trying to establish theCu/Ag/Au-bonding model. To finish this study target, we have done this work in the following two aspects:(1) Firstly, we studied the B···M–X (B=CO, H2O, H2S, C2H2, C2H4,M=Cu, Ag, Au, X=F, Cl,Br, CH3, CF3) complexes by using the natural bond orbital (NBO) method. The NBO resultsshow that strong hyperconjugation interactions take place in the B···M–X (B=H2O, H2S)complexes, and for the complexes of B···M–X (B=CO, C2H2, C2H4) there exists the additionalnon-negligible Ï€-back donation. The following results of natural resonance theory (NRT)analysis show that all the complexes can be regarded as the resonance hybrid of severalresonance structures. The B···M–X (B=H2O, H2S) complexes each can be regarded as theresonance hybrid of B: M–X B+–M:X-due to nBâ†'σ*M Xhyperconjugation interaction. Incontrast for the B···M–X (B=CO, C2H2, C2H4) complexes, besides the two resonance structuresof B: M–X and B+–M:X-, there exists additional resonance structures generated from Ï€-backdonation. The variety of resonance structures as well as the nearly-equal relative weightingsindicates that the intermolecular resonance effects in these Cu/Ag/Au-bonded complexes are noless important and indeed more important than that in H-bonding. To sum up, the NBO/NRTresults strongly support the general resonance-type3c/4e picture of Cu/Ag/Au-bonding becauseof the resonance hybrid of B: M–X B+–M:X-arising mainly from hyperconjugationinteraction. In addition, the Cu/Ag/Au-bonding contains varying degrees of Ï€-back bonding.On the basis of such resonance-type bonding nature, we investigate the ligand effects andbond order-conservation. The Cu/Ag/Au-bonding is found to have distanct ligand effects and theorigin of such ligand effects can be traced to the resonance nature of the Cu/Ag/Au-bonding. Theresults of NRT bond order indicates that the pure Cu/Ag/Au-bonding is satisfied as the idealizedrelation bB···M+bMX=1. When there exists additional Ï€-back bonding, because of contributionfromÏ€-back donation, the sum of bond order of B···M and MX is larger than one.(2) Secondly, we performed the NBO/NRT analysis for the complexes of LMCCH (L=F-, Cl-,Br-, CCH-, C2H2, C2H4)(where M is the coinage metal Cu, Ag, and Au). The NBO/NRT resultsshow that these Cu/Ag/Au complexes can also be understood from the3c/4e hyperbondingarising mainly from the nLâ†'σ*M–CCHhyperconjugation with varying degrees of Ï€-back bonding.That is to say, the Cu/Ag/Au-bonding has significant resonance character. Because of suchsignificant bonding characetr, two ligands bound to M present a distanct competitive role. Fromthis view of point, it can be easily to explain the ligand effects on the M–CCH bonding. In addition, the sum of bond order of L–M and M–CCH in these complexes is slightly larger thanone. This can be explained by the existence of Ï€-back donation which can contribute to the bondorder. Moreover, the larger the Ï€-back donation, the larger the sum of bond order of L–M andM–CCH.Considering the theoretical study on these complexes, we established the resonance bondingmodel for the Cu/Ag/Au-bonding. The Cu/Ag/Au-bonding network can be regarded as theresonace picture of3c/4e hyperbonding with varying degrees of Ï€-back bonding.