Geometry, energetically preferred

The preceding perturbation theory analysis is supported by extended Hiickel calculations by Cusachs and his co-workers (166, 167, 237) on model platinum(II)- and platinum(0)-olefin and -acetylene complexes and Hoffmann and Rossi s extensive analysis of five-coordinate transition metal complexes (194). By using similar arguments, Hoffmann and Rosch (190) predicted that the planar conformation would be energetically preferred for d10 M(C2H4)3 complexes. This geometry has now been established by Stone (214) and his co-workers for the platinum-olefin complex shown in Fig. 12. 

Systematic analyses of crystallographic data for hydrogen bonds have revealed a range of geometries and have led to proposals for rules to rationalize or predict hydrogen bonding patterns.18 An energetic preference for linear or near-linear... 

In MB 6 complexes based on the octahedron or square pyramid with B occupying an apical site, the linear and i/1 -planar geometries are energetically preferred. For MB 8 complexes the bent or til-pyramidal geometries are more stable. 

The energetic preference of the endo form was demonstrated by the first quantum chemical calculations of silatranes performed on 1-hydro- and 1-fluorosilatranes12,181. These calculations employed the CNDO/2 semiempirical method with spd basis set and geometries of the endo and exo forms corresponding to those obtained for 1-methylsilatrane by modified molecular mechanics method. The energy of the Si-e- N coordinative bonding was found to be about 20-25 kcalmol-1 and to be the main contributor to stabilization of the endo structure of silatranes. 

From the results obtained for the model species 3, it is concluded that the TBP, with oxygen atoms in the axial positions, is the energetically preferred geometry of the Si coordination polyhedra of the title compounds. As the pseudorotation according to path 1 needs only a small amount of energy, package effects in the crystal (which are individual for each compound) can easily cause distortions of the TBP toward the SP. This assumption is in agreement with the results obtained in experimental studies [1]. 

These results demonstrate that Mg + and Mn + show energetic preferences for different chelation sites on both the salicyUiydrazine and thiazolothiazepine structures. The Mg ion forms a more stable a complex in both salicylhydrazine structures. These complexes are, respectively, 83 and 31 kcal/mol more stable than the complexes formed when forms a n complex with the phenol or dirazolone ring. The Mn + ion, however, shows an 11-94-kcal/mol preference to form 7t complexes with the salicylhydrazines. Similar geometries are preferred for the thiazolothiazepine structures. These structures show an average 45-kcal/ mol preference for Mg to form o rather than n complexes. The opposite prefer-... 

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