EHMO calculations

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Ti2. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

In order to explain why the bridging vinylidene group of MoCo2(/r 3-)F-CCH2) (COlsI / -CyHOr" (Fig. 83) leans toward the molybdenum atom rather than a cobalt atom, EHMO calculations have been employed. Results showed that the... 

Extended Hiickel molecular-orbital (EHMO) calculations using structural parameters from the X-ray determination of Cp(CO)3Mo 3In (Fig. 29) and ideal symmetry Cih were carried out for this compound in order to investigate the extent of the indium-molybdenum ir-bonding.122 The HOMO of the compound is the 3e molecular orbital, which is In—Mo [Pg.54]

Since the g-matrix has only three principal values and there are almost always many potentially interacting molecular orbitals, there is rarely sufficient information to interpret a g-matrix with complete confidence. When a well-resolved and reliably assigned optical spectrum is available, the energy differences, E0—Em, are known and can be used in eqn (4.11) to estimate the contribution of the corresponding MOs to the g-matrix. Extended Hiickel MO (EHMO) calculations can be useful (but do not trust EHMO energies ), but one is most commonly reduced to arguments designed to show that the observed g-matrix is consistent with the interpretation placed on the hyperfine matrix. 

The fact that the electrochemical oxidation of ds-[M2(cp)2(/r-SR)2(CO)4] and of [M2(/r-SR)2(CO)8] is reversible despite the amount of structural reorganization involved suggests that these changes require low activation energy. Extended Htlckel Molecular Orbital (EHMO) calculations on the model complex ds-13 (R = H) indicated that the Lowest Unoccupied Molecular Orbital (LUMO) was the level [47]. Weakening of the... 

EHMO calculations performed on the reactants, products, and possible intermediates of the reactions shown in Sch. 15 supported the proposed mechanism [58]. Thus, steric and electronic factors combine to make 23 a good substrate-binding site it has an uncongested M02S2 face and a low-lying LUMO available to hold an incoming nucleophile. The proposed mechanism remains valid if the CO substrate is replaced by other two-electron donor ligands such as isocyanide [56] or cyanide [59]. 

No photoreactions, except incidental photodestruction are known for hexa- and higher helicenes. As already mentioned in 2.1.1. pentahelicene forms benzo[g,h,i]perylene on irradiation in the presence of iodine as an oxidizing agent. Remarkably, the symmetrical benzoderivative (111) does not yield a similar cyclization product. This is ascribed to the antibonding character of the ji-orbitals at C-(l) and C-(14) involved in the expected photoreaction as appeared from EHMO calculation 17b). 

From iterative extended Hiickel molecular orbital (EHMO) calculations, [VX]"+ (X = S, Se n = 0, 1, 2) ions and their tetrachloro complexes [VXCl4]m- (m = 4, 3, 2) decrease in stability 0>S>Se and with increasing charge of the anion.695 The [VXC14]2- and [VXCI4]4-complexes are expected to be S = systems while [VXC14]3- species are expected to be diamagnetic.695... 

The results of CNDO/2 calculations for [Zn(Pc)] have been reported, and compared with the PPP and EHMO calculations for the same species.1183... 

Nucleophilic reaction of Et3N with diallyl disulfide has been found to differ from that with diallyl sulfide in being very much more exothermic and also autocatalytic.14 Possible mechanisms were discussed. EHMO calculations have been performed for... 

Fig. 3. Pictorial representation of the M—M 5 interaction together with carbon p mixing of the HOMO predicted for [X(Me,SiCH2)2W(/r-CSiMe3)]2 compounds, where X - Cl, Br, or I, on the basis of EHMO calculations, using weighted H s (R. HofTmann, J. Chem. Phys. 39,1397,1963) and previously published parameters for W (R. H. Sommerville and R. Hoffmann, J. Am. Chem.

The iron complex of 1,2-cycloheptadiene was synthesized to study the relationship between 312 and a valence isomeric form (316) that has been referred to as the allyl cation form (Scheme 39).119 Because of ring strain, the latter had been predicted by EHMO calculations to lie rather close in energy to an allene ground state.126 The NMR of 312 confirmed the allene form as the ground state and fluxionality experiments on the terminal allene... 

Somewhat more loosely related to platinum antitumor drugs were EHMO calculations performed on heterobimetallic Pt-Pd complexes with a bridging methylcytosinate anion by Mealli, Randaccio, Lippert, and coworkers [31][32], The calculations served to elucidate the metal-metal and metal-ligand binding interactions, and yielded a qualitative interpretation of the 195Pt-NMR chemical shifts. 

A secondary feature of the cis-M(CO)(RC=CR) molecular orbital description is the three-center-two-electron bond built from carbonyl tt, alkyne 77, and metal dyz (Fig. 15) (45). EHMO calculations locate the completely in-phase bonding combination, dominated by dyz, approx-... 

Dynamic NMR studies of W(CO)(HC=CH)(S2CNEt2)2 and related complexes (58) indicate that the barrier to alkyne rotation is around 11-12 kcal/mol (Table IV). Discordant metal-ligand 77 interactions anticipated as the alkyne rotates away from the ground state orientation are evident in EHMO calculations (Fig. 23). When the alkyne is orthogonal to the M—CO axis the dyz orbital is stabilized by CO it but destabilized by alkyne ir while dxy becomes the lowest lying dir orbital due to... 

Fig. 15. Frontier orbitals of Dicyd2- from EHMO calculations. Minor orbital corrections are omitted for clarity. [Reproduced with permission from Ref. (78). Copyright (1992) American Chemical Society.]...

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