Bonding extended Huckel calculations

CNDO/2 and extended Huckel calculations 74> of 13 (X = S) revealed a small difference in energy between the planar and nonplanar structure, both with bond alternation. These results can be translated into the valence bond structure corresponding to a cyclic thioether. 

The interactions between the [W(calix)] metal fragment and the ethylene moiety has been analyzed using the extended Huckel calculations (see Chart 4).22b As supported by the structural parameters of 31, the dxz is no longer available for -it donation from the calixarene oxygens in the xz plane, thus explaining the strong C2v distortion of the [ [ Bu -cal ix 14]-(0)4 W] unit observed in the X-ray structure of 31, with the two WO bonds in the metal-ethylene plane ca. 0.2 A longer than... 

Octahedral olefin-alkyne d4 complexes are characterized by a one-to-one match of each of the three metal dir orbitals with a ligand tt function as mentioned above. Three constructive two-center-two-electron metal-ligand tt bonds result. Extended Huckel calculations on W(H2C=CH2)-(HC=CH)(S2CNH2)2 produce the dv level ordering shown in Fig. 16... 

The extended Huckel calculations (45) start from an unbridged geometry where the OC—Mo—CO and Mo—Mo—C bond angles are both 90° while the Mo—Mo—Cp bond angle is set to 125.3°. This leads to a bond description including characteristic five below two frontier orbitals shown in Fig. 3. The five highest occupied molecular orbitals are 7r, 8, ir, 8, and a and allow a correlation with the triple bond assigned on the basis of the 18-electron rule. 

The third mechanism, suggested by R. Hoffmann as a result of extended Huckel calculations, also involves trimethylene but gives it special properties that would lead to correlated conrotation of the terminal methylenes. Specifically, the calculations suggested that the C-H a bonds of the central methylene could be used as a kind of pseudo-p -orbital... 

After establishing that the central bond is composed almost solely of p orbitals on each of the two carbons, it remains to establish the orientation of these orbitals with respect to each other. The two possible extremes are a overlap in which the two orbitals are oriented towards each other or n bonding for which they are parallel. Extended Huckel calculations performed by Yonezawa and coworkers assign a large n character to this bond. On the other hand, a 5 1 ratio of p-cr to p-7r bonding was reported on the basis of ab initio calculations. Other ab initio calculations (STO-3G) suggest that the bond is bent outward but differ as to the extent (45° and 30.8° Thus one may conclude that the central bond of bicyclobutane, although formally a o bond, contains appreciable n character. 

N.m.r., X-ray photoelectron studies, and extended Huckel calculations on HgCl2Py2 indicate a moderate amount of electron transfer to the metal from pyridine relative to ZnQ2py2 the filled mercury d-orbitals do not participate in bonding. Dipole moment studies on Cdl2py2 have been made, while stability constant studies for the formation of complexes between Cd and pyridine in water, water-MeOH, and water-DMF mixtures show the formation of the species [CdlpylJ (x = 1—3). A structural determination of Hg(N03)2(py)2(H20)2 reveals mercury in a distorted octahedral environment consisting of two water and two pyridine ligands (Hg—N =... 

Fig. 8.10. Migration of the ML2 fragment as mapped by four structures. The structure on the left closely resembles the ground state, the one on the right the transition state, as determined by extended HUckel calculations on H3CjNi(PH3)2. Note that the ML2 moiety rotates w.r.t. the cyclic ene, as M migrates from one C-C bond to the next...

The existence of a complex between H2 and a metal is one of the few notable examples in all of science of the nearly simultaneous and independent derivation of theory and fact. Neither the theoreticians nor the experimentalists involved were aware of the seminal research being carried out in their respective fields during 1979-1983 when H2 activation was initially studied computationally and W(CO)3(P Pr3)2(H2) was first prepared and later proven to contain aH2 ligand (see Chapter 2). This is marvelously exemplified by the classic theoretical paper by Saillard and Hoffmann in 1984 presenting extended Huckel calculations on the bonding of H2 and CH4 to metal fragments10 that appeared only months after the... 

Calculations support a similar bonding picture for silane complexes as to that for H2 complexes. Early extended Huckel calculations in 1987 by Saillard on binding of SiH4 to the CpMn(CO)2 fragment showed that the Si-H overlap population is reduced to 0.24 by the [Pg.342]

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