Theoretical Studies On The Structure Of Group14and Some Low Valence State Bimetallic Cluster Compounds

The cluster compounds which contain metal-metal bond have been one of the mostactive regions of the current chemistry science because its special physicochemical property.Researching the structure of bimetallic cluster compounds is of great importance because itprovides great value to fossil fuels, catalysis field and the region of biological activity, etc.In this thesis, we studied the nature of chemical bond in low valence state metallocenesand group14cluster compounds.We also research the reaction mechanisms and the change ofthe chemical bonds in the reaction of CH3M≡MCH3(M=C, Si, Ge) with C2H4. All the workhas been studied within the framework of the ‘atoms in molecules’(AIM) theory and using‘electron localization function’(ELF) analysis. All calculations are carried out using theGaussian03program package. The topological analysis of electron density of molecule havebeen studied using AIMALL program, AIM2000program and TOPMOD program. The maincontents are the following aspects:1. The metal-metal and metal-ligand bonding in a series of binuclear metallocenes (η5-C5H5)2M2(M=Be, Mg, Ca, Ni, Cu, Zn) are characterized within the framework of theatoms in molecules (AIM) theory, electron localization function (ELF), and molecularformation density difference (MFDD). The calculated results show that the metal-metalbonding in the binuclear main-group metal metallocenes are different from those in binucleartransition metal metallocenes. In binuclear main-group metal metallocenes, the metal-metalbondings are linked by two metal-“non-nuclear attractor (NNA)” bonds, while such NNAsdo not exist in the binuclear transition metal metallocenes. In addition, the transitionmetal-metal bonding is stronger than that of the overall main-group metal-metal bonding. Themain group metal-metal bonding shows covalent characteristics while the transitionmetal-metal bonding displays ‘closed shell’ ionic characteristics. The metal-ligand bondingis mainly ionic. There are both σ and π characteristics in the metal-ligand interactionsand the π interaction is predominant.2. The nature of the E-E bonds in heavier ditetrel alkyne analogues ArE≡EAr (Ar=C6H3-2,6(C6H2-2,6-Pri2)2; E=Si, Ge, Sn, Pb) have been studied using a topological analysisof the electron localization function (ELF), the delocalization index from the atoms inmolecules (AIM) theory and bond orbital analysis. Both the topological and the orbitalanalysis show that the nature of the E-E bonds in ArE≡EAr are different to that of theclassical covalent triple C≡C bond of C2H2. In ArE≡EAr, more and more electrons aretransferred from the valence basins of the E-E bond to the lone pair basin of the E atoms. Hence, the E-E bonds are all weaker than the C≡C bonds and their strength decreases in thesequence E=Si, Ge, Sn, Pb. The Si-Si bond can be described as a slipped covalent triplebond. The Ge-Ge and Sn-Sn bonds are, for the most part, not covalent bonds but another(much less recognized type) known as the charge-shift bond, in which both contain oneslipped σ orbital, one normal π orbital and one slipped π orbital. In particular, the ELF valencebasin centroids of the E-E (E=Ge, Sn) bond does not occupy the central region between thetwo atoms, but has, in effect, fled that region and split into two kinds. The bond ordersdecrease to values that almost represent double bonds for the Ge-Ge and Sn-Sn bonds and asingle bond for the Pb-Pb metal bond.3. The mechanism of the cycloaddition reaction CH3M≡MCH3(M=C, Si, Ge) withC2H4has been studied at the CCSD(T)/6-311++G(d, p)//MP2/6-311++G(d, p) level.Vibrational analysis and Intrinsic Reaction Coordinate (IRC), calculated at the same level,have been applied to validate the connection of the stationary points. The breakage andformation of the chemical bonds of the titled reactions are discussed using the topologicalanalysis method for the electron density. The calculated results show that, of the three titledreactions, the CH3Si≡SiCH3+C2H4reaction has the highest reaction activity because it hasthe lowest energy barriers and the products with the lowest energy. The CH3C≡CCH3+C2H4reaction occurs only with difficulty since it has the highest energy barriers. The reactionmechanisms of the title reactions are similar. A three-membered-ring is initially formed, thenit changed to a four-membered-ring structure. This means that these reactions involve a [2+1]cycloaddition as the initial step, instead of a direct [2+2] cycloaddition.The innovations in the thesis:1. Non-nuclear attractor (NNA) has been discovered in binuclear main-group metalmetallocenes. It has been illustrated in detail from the angle of theory study.2. In the study of E≡E bond in ArE≡EAr (Ar=C6H3-2,6(C6H2-2,6-Pri2)2; E=Si,Ge, Sn, Pb) compound, a new type of chemical bond, we call it CS (charge-shift)bond, has been discovered. Ge-Ge and Sn-Sn bond belong to CS bond.3. The mechanism of the cycloaddition reaction CH3M≡MCH3(M=C, Si, Ge)with C2H4has been studied. These reactions involve a [2+1] cycloaddition asthe initial step, instead of a direct [2+2] cycloaddition.