Molecular orbital calculations, transition metal

Eor transition metals the splitting of the d orbitals in a ligand field is most readily done using EHT. In all other semi-empirical methods, the orbital energies depend on the electron occupation. HyperChem s molecular orbital calculations give orbital energy spacings that differ from simple crystal field theory predictions. The total molecular wavefunction is an antisymmetrized product of the occupied molecular orbitals. The virtual set of orbitals are the residue of SCE calculations, in that they are deemed least suitable to describe the molecular wavefunction. 

Molecular orbital calculations for transition metal compounds. D. R. Davies and G. A. Webb, Coord. Chem. Rev., 1971, 6, 95-146 (267). 

Dioxygen activation in transition metal complexes in the light of molecular orbital calculations. R. Boca, Coord. Chem. Rev., 1983, 50,1-72 (245). 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

The bonding capabilities of transition metal clusters (no nonmetals in the framework), based on molecular orbital calculations, has been nicely summarized by Lauher14 (Table 16.3). Within this table we see three structures (tetrahedron, butterfly, and square plane) for tetranuclear metal clusters. The tetrahedron is a 60-electron cluster, while the butterfly and square plane clusters have 62 and 64 electrons. respectively. When we go from a tetrahedron to a butterfly, one of the edges of the tetrahedron is lengthened corresponding to bond breaking. 

The valence orbitals taken for a molecular-orbital calculation of a transition metal complex are the nd, (n + l)s, and (n + l)p metal orbitals and appropriate a and n functions of the ligands. Many of these valence orbitals are not individually basis functions for an irreducible representation in the symmetry under consideration. Symmetry basis functions transforming properly must be constructed, by methods analogous to those used throughout this volume. The results for a number of important symmetries are tabulated in this volume in various places, as follows ... 

Tennakone et al.%S) used triphenylmethane type (metallochromic) organic dye (Dye 21, 22), both of which show a large bathochromic shift on complexing with metal ions. The molecular orbital calculation of these dyes in chelating condition with the Tilv ion revealed that the LUMOs of these dyes are localized on the TiIV ion, but the HOMOs are delocalized in the whole dyes. Such MO distribution similar to the LMCT transition in transition metal complexes should contribute to the vectorial electron transfer (high rj ) from the excited dye to Ti02. 

In so far as the decrease in chemical reactivity is an indication of diminished transition metal basicity, it was proposed143 that the thiocarbonyl complex is less basic than its carbonyl analogue. This conclusion is substantiated by the spectral shifts in Table 22 and is also in agreement with molecular orbital calculations which predict the thiocarbonyl complex to be less basic than the carbonyl complex155,156. 

Its absorption spectrum shows one band at 320 nm (e = 2900 M 1cm 1), assigned to the cti - ct2 transition localized in the Au-Tl moiety. The emission spectrum in the solid state at 77 K shows a band at 602 nm, which is attributable to a transition between orbitals that appear as a result of the metal-metal interaction. In this sense, Fenske-Hall molecular orbital calculations indicate that the ground state is the result of the mixing of the empty 6s and 6pz orbitals of gold(I) with the filled 6,v and the empty 6pz orbitals of thallium(I). In frozen solution, this derivative shows a shift of the emission to 536 nm, which has been explained by a higher aggregation of [AuT1(MTP)2] units in the solid state if compared to the situation in solution. 

Accurate molecular orbital calculations on transition metal... 

Density functional molecular orbital calculations suggest that a-agostic hydrogen interaction with the metal atom in a Zr-CH3 unit helps to stabilise it and align the methyl group of this unit for interaction with the ethylene n orbital [351,352]. The a-agostic transition state has one less rotational degree of freedom, which stabilises it and reinforces for the C C bond formation. 

It is gratifying that later molecular orbital calculations (3a) and structure correlations (49) for the addition of C—H a bonds to transition metals gave similar results. The results obtained by structure correlation appear more straightforward for the Si—H addition, because they are derived from a single type of complex and the correction of the silicon radius is more precise than the very crude correction of the metal radius, which was applied in the C—H case. 

Recent treatments have described the skeletal bonding64,68,71) contributions for ligated transition metal moieties in metalloboranes and in transition-metal clusters. Wade indicated the important similarity between boranes and transition-metal clusters, in particular the species [B6H6]2" and several octahedral transition-metal clusters and Mingo82,83), on the basis of detailed molecular orbital calculations for [Co6(CO)14]4, has confirmed the validity of these comparisons. 

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