Molecular orbital calculations titanium

The methylidyne cubane [Cp Ti(/x-CH)]4 (Scheme 202) is obtained as a dark brown crystalline solid by thermolysis of Cp TiMe in toluene with methane elimination. This transformation was monitored by NMR and no intermediates are observed. The signals assignable to the methylidyne groups appear as singlets at 6 17.75 in the NMR and at 6 490.8 in the 13C NMR spectra. In order to analyze the interaction between the /i3-ligand and the titanium centers, extended Huckel molecular orbital calculations have been carried out. In contrast, the thermolysis of the trinuclear oxo alkyls [Cp Ti(/i-0)(CH2R)]3 (R = H, Me) affords the //3-alkylidyne derivatives [Cp Ti(//-0)]3-(/x3-CR).505-507... 

The molecular and electronic structures of (CpM)4(//2-E)6 and (CpM)4(/x2-E)3(//3-E)3 (E = O, Se) for titanium as well as group 5 and 6 transition metals have been compared using extended Hiickel molecular orbital calculations. The conclusions indicate that the M4(/x2-0)3(/x3-0)3 structure cannot exist unless the M4-tetrahedron is severely distorted by lengthening of the M(apical)-M(basal) distance. The M4(/x2-Se)3(/x3-Se)3 structure can exist with a small distortion. The M4(/x2-E)6 structure is preferred over M4(//2-E)3(//3-E)3 when M-E multiple bonding is important, that is, when E = O. There is little M-M interaction in any of the cluster orbitals.830... 

Even though the original catalyst A is depicted as an octahedral complex, the self-consistent all-valence-electron molecular orbital calculation shows that the most stable complex is a trigonal-bipyramidal one with titanium ethyl at an intermediate position between two octahedral sites [71-73]. 

The Cossee-Arlman mechanism for the polymerization of olefins is the most widely accepted theory but as yet it is not complete. Cossee developed his early ideas of polyethylene growth at a titanium-carbon bond and supported the theory by molecular orbital calculations. The role of the alkyl aluminium co-catalyst was in the generation of the active species, via the alkylation of the titanium chloride bonds, and to remove impurities in both the gas stream and catalyst preparative procedure. There was also the suggestion that it might be involved in the insertion of each monomer molecule, and also in the regeneration of dormant sites or the formation of new active sites. 

The electronic configuration of titanium is [Ar] 3d24s2, which means that Ti(IV) compounds are d° species with free coordination sites 1-27,28). H-NMR and 13C-NMR data are known and have been occasionally discussed in terms of bond polarity 19), but such interpretations are obviously of limited value. The electronic structure of methyltitanium trichloride 17 and other reagents have been considered qualitatively 52) and quantitatively S3 56> using molecular orbital procedures. It is problematical to compare these calculations in a quantitative way with those that have been carried out for methyllithium 57> since different methods, basis sets and assumptions are involved, but the extreme polar nature of the C—Li bond does not appear to apply to the C—Ti analog. Several MO calculations of the w-interaction between ethylene and methyltitanium trichloride 17 (models for Ziegler-Natta polymerization) clearly emphasize the role of vacant coordination sites at titanium 58). 

XX is generally considered as the active species formed from titanium chloride and alkyla-luminum components. The in the structure represents an unoccupied (vacant) site of the octahedral titanium complex. XX represents an active titanium site at the surface of a TiCls crystal after modification by reaction with the alkylaluminum component. The titanium atom shares four chloride ligands with its neighboring titanium atoms and has an alkyl ligand (incorporated via exchange of alkyl from the alkylaluminum for chloride) and a vacant orbital. There are molecular mechanics calculations that indicate dimeric Ti2Cl6 may be the active species instead of monomeric TiCls [Monaco et al., 2000]. Other proposals for the active species include bimetallic species that contain both titanium and aluminum [Liu et al., 2002]. To simplify matters, our discussions will center on the monomeric and monometallic titanium species, especially since the mechanistic details of stereoselectivity and activity are essentially the same for both monomeric and dimeric titanium species as well as titanium-aluminum species. 

Figure 2.9. The qualitative behavior of the ground state energy of titanium oxide and nickel oxide calculated with a VB model that includes some ionic contribution (amount X). A pure valence bond wavefunction has = 0 and a molecular orbital state has A = 1. For titanium oxide the MO model predicts an energy closer to the real value than a VB calculation. For nickel oxide the valence bond model is more realistic.

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