Studies On Multicenter Bonding In Boron/Palladium Clusters

Clusters composed of atoms display unique physical,chemical and electronic characteristics that have gained extensive attention in recent years.Metal and nonmetal clusters are two important branches in the field of cluster science,which are also the starting point for clusters research.Until now,the electronic structures of Closo-BnHn2-cannot be explained by classical Lewis structure due to their electron-deficient feature and complicated structure.Meanwhile,a lot of experimental and theoretical investigation are needed to put on metal clusters.In this thesis,a series of borane dianions BnHn2-(n = 2-17),Pd4(μ3-SbH3)4(SbH3)4,Pd3(PH3)3(μ2-CO)3 and Pd4(PH3)4(μ2-CO)5 clusters are systematically studied using the density functional theory,including their geometrical structure,electronic structure,chemical bonding,stability and aromaticity.The main works are listed as follows:1.Study on structure and stability of borane dianions BnHn2-(n = 2-17)As multicenter bonding is a popular language in the community of chemistry which is wildly used to interpret the electronic structure,stability and aromaticity of boron clusters,in this part,we try to reveal the nature of electronic structure in borane dianions BnHn2-series in the view of chemical bonding.The geometrical and electronic structures of BnHn2-(n = 2-17)are investigated at DFT-TPSSh level.The large HOMO-LUMO gaps and negative NICS values indicate their close-shell electronic structure.Further chemical bonding analysis reveals that all 2n + 2 delocalized valence electrons in closo-DnWi1n are participated in various delocalized σ and π multicenter bonding systems on the surface of borane cage.The multicenter bonding in this system show five types,including open 3c-2e BBB bond,close 3c-2e BBB bond,4c-2e bond,8c-2e bond and fully delocalized bond,which are delocalized on B3 broken-line,B3 triangle,B4 rhombus,B8 double-ring and the whole surface of boron cage,respectively.The flexibility of multicenter bonding can adapt well to all borane cages with different configurations,making their stable and aromatic.2.Electronic Stability of the Eight-electron Tetrahedral Pd4 ClustersMotivated by the unusual structure of the Pd4(μ3-SbMe3)4(SbMe3)4 cluster,which is composed of a tetrahedral Td Pd(0)core with four terminal SbMe3 ligands and four triply bridging SbMe3 ligands capping the four triangular Pd3 faces.A DFT computational study is performed to investigate the structure and bonding characteristics of Td Pd4(μ3-SbH3)4(SbH3)4 cluster and a series of its analogues.The Td structure of the Pd4(μ3-SbH3)4(SbH3)4 cluster can be explained by the cluster electron-counting rules based on the 18-electron rule for transition-metal centers,the empty 5s and 5p orbitals of Pd(40d10)are occupied with eight valence electrons which are from one terminal SbH3 and three bridging μ3-SbH3 ligands.There are two bonding patterns of the eight delocalized electrons between the four capping Sb atoms and the Pd4 core.The first pattern is on account of the superatom-network(SAN)model,whereby the palladium cluster can be viewed as a network of four 2e superatoms.The second pattern is based on the spherical jellium model,whereby the cluster can be rationalized as an 8e[Pd4(μ3-SbH3)4]superatom with 1S21P6 electronic configuration.Meanwhile,the d10…d10 interaction plays a critical role in stabilizing the Pd4 tetrahedron structure,which is similar to the aurophilicity in Au-Au clusters.The calculations show that Td Pd4(μ3-SbH3)4(SbH3)4 cluster hsa a large HOMO-LUMO energy gap(2.84 eV)and a NICS value(-12 ppm)at the center of the Pd4(μ3-SbH3)4(SbH3)4 cluster,indicating its high chemical stability and aromaticity.In addition,the designed analogues are recognized to have the similar structure and bonding pattern to Pd4(μ3-SbH3)4(SbH3)4,which are possible to get in labs.3.Pd4 Cluster with Co-AromaticityThe Pd4(PH3)4(/U2-CO)5 cluster composed of two Pd3(PH3)3(μ2-CO)3 by sharing two Pd atoms and one CO molecule with seashell-like open structure has a large HOMO-LUMO energy gap and a negative NICS value,indicating its high stability and aromaticity.Chemical bonding analysis reveals that Pd4(PH3)4(μ2-CO)5 cluster has delocalized 3c-2e PdCPd σ bonds formed by two Pd atoms and C atom of CO,meanwhile,there are part of d electrons flow from Pd to the π*orbitals of CO.The multicentre bonding and π backdonation ensure the stability of Pd4(PH3)4(μ2-CO)5 cluster.Even more exciting,we find a unique aromaticity"co-aromaticity" in Pd4(PH3)4(μ2-CO)5 cluster,by sharing two delocalized electrons of C in CO,the 10e system can be viewed as two 6e system that each two Pd3(μ2-CO)3 units is even more aromatic than Pd3(PH3)3(μ2-CO)3 cluster,making double σ-aromaticity.