Structures,Chemical Bonding,and Aromaticity Of Transition Metal Based Heterocyclic Clusters

Quantum chemistry has become a powerful means to investigate the structures,electronic properties,and chemical bonding of nanoclusters.The concepts of aromaticity and antiaromaticity has long been utilized to understand the unique properties of planar,cyclic,and conjugated hydrocarbon compounds.However,in recent years,aromaticity has gradually expanded its territory towards inorganic clusters and all-metal clusters.For transition metal cluster systems,d-orbitals further contribute to the complexity,diversity,and uniqueness in chemical bonding,which leads to the new concepts of d-orbital(?/?)aromaticity and even ?-aromaticity.The latter is a completely new mode of chemical bonding,which is not possible for classical aromatic hydrocarbon systems.In this thesis work,we report on quantum chemical studies of the geometric structures,chemical bonding,and aromaticity of transition metal nitride amd fluoride clusters: Os3N3+ and Re3F32+.The computations and analyses are accomplished using density-functional theory,with the aid of an array of modern tools for chemical bonding analyses.Basing on concerted computational data,the heterocyclic Os3N3+ cluster,as a model system,is shown to be the first transition metal-based inorganic benzene,featuring globally delocalized 6? electrons on the heterocyclic ring.It also has ?-aromaticity,owing to a unique ?2?*1?*1 framework,which follows the revised 4n Hückel rule for aromaricity in a triplet system.In contrast,prelimarily results for the Re3F32+ cluster reveal d-orbital antiaromaticity with delocalized ?/? frameworks,which is also a rare case in the cluster literature.The main contents of this thesis are briefly described as follows:1.Heterocyclic transition metal nitride Os3N3+ cluster: 6? inorganic benzene and ?2?*1?*1 aromaticity.Here we present a density-functional theory study on the structure,chemical bonding,and aromaticity of a binary,heterocyclic D3h(7A2?)Os3N3+ cluster.The transition metal based cluster assumes a perfectly planar,heteroatomic,hexagonal geometry.Detailed canonical molecular orbital(CMO)analyses,adaptive natural density partitioning(Ad NDP),natural bond orbital(NBO)analysis,and orbital composition and nucleus-independent chemical shift(NICS)calculations reveal that the cluster supports two-fold ?/? aromaticity: 6? electrons delocalized over all Os/N centers versus an Osbased 4? framework in unique ?2?*1?*1 configuration.The ? sextet renders this heteroatomic cluster an inorganic analog of benzene.Transition metal-based inorganic benzenes are unknown in the literature,to our knowledge.The triplet 4? electron-counting is a rare case of d-orbital aromaticity and ?-aromaticity,following the reversed 4n Hu?ckel rule for aromaticity in a triplet system.2.A preliminary study on the structure and bonding of transition metal fluoride Re3F32+ cluster.Based on first-principles calculations,we present herein a preliminary exploration of the structural and bonding properties of transition metal fluoride Re3F32+ cluster.The global-minimum structure of the cluster is searched using the CoalescenceKick(CK)method,aided by manual structural building.Candidate structures are then fully reoptimized at the density-functional theory level.Frequency analyses are performed at the same level.The global-minimum structure is Re3F32+ C2v(5B2).Chemical bonding analyses show that the cluster possesses d-orbital antiaromaticity,featuring double delocalization of ? and ? bonds over three Re centers.Their electron counting follows the 4n Hu?ckel rule for antiaromaticity,which is in line with the distorted Re3 core relative to an equilateral triangle.In addition,terminal F-Re bonds are two-center in nature and localized,whereas bridging F-Re bonding is three-center Re2 F delocalized.d-Orbital antiaromaticity is relatively new in chemistry,as is two-fold ?/? antiaromaticity.