Eclipsed configurations

Figure 19.28 The eclipsed (De ) structure of [Cr( ) -C6Hg)2] as revealed by X-ray diffraction, showing the two parallel rings 323 pm apart. Neutron diffraction shows the H atoms are tilted slightly towards the Cr, and electron diffraction on the gaseous compound shows that the eclipsed configuration is retained without rotation.

Another set of terms considered by Eyring, et al. consisted of structures having a double bond between carbons. These should be expected to be involved to some small extent—small because they have one less real bond. This is essentially invoking hyperconjugation. These authors did not reach a definite conclusion regarding the importance of these structures and decided that they probably would stabilize the eclipsed configuration, which was not known at that time to be the incorrect one. It is still not simple to make a reliable estimate of the importance of this contribution. 

A further complication is that an icosahedron sharing four faces loses its symmetry operations of the second sort and becomes chiral, D or L. The eclipsed configuration described above requires that D and l alternate. Hence, in a ring of five icosahedra one DD or LL bond must occur, with a strain in the ring of 40°, which is a rotation of 4 per shared face in each icosahedron. The space-group symmetry is thus changed from that of diamond to a subgroup. 

Figure 1 Molecular orbital diagram for metallocenes (in the eclipsed configuration)...

The experimental deformation density features, including the displacement of the bond peaks from the bond axes, are well reproduced in theoretical maps on the molecule H6Si207, which contains two linked Si04 tetrahedra in a doubly eclipsed configuration. The results for different values of the central SiOSi angle are shown in Fig. 11.4. 

The two C5H5 rings in this molecule have a cis-eclipsed configuration... 

In this model, when the total rotation is only 30° (midway through the third process), one of the points of highest energy is reached, corresponding to a 30°-TR barrier. If this energetic barrier is too high or the final (eclipsed) configuration is unfavourable, the... 

The bent-bond model of the triple bond gives this bond a threefold symmetry axis and leads to the prediction that the two methyl groups in dimethylacetjdeno should be somewhat restricted in their mutual rotation, with the eclipsed configuration stable. Restriction of rotation about the single bonds in conjugated systems is also expected, and the nature of the stable configurations can be predicted from the theory described above. These systems are discussed in Chapters 6 and 8. 

The most interesting aspect of these compounds is their structure (Fig. 16.57), which possesses two unusual features. The first is the extremely short Re—Re distance of 224 pm compared with an average Re—Re distance of 275 pm in rhenium metal and 248 pm in Re,Cl. The second unexpected feature is the eclipsed configuration of the chlorine atoms. One might have supposed that since the short Re—Re bond requires that the chlorine atoms lie at distances ( 330 pm) which are less than the sum of their van der Waals radii (—340-360 ppm), the staggered configuration would be preferred (the chlorine atoms would then form a square antiprism rather than a cube). Cotton explained both phenomena by invoking a quadruple bond.147... 

The reaction of Mo(CO)6 and acetic acid is still the most convenient preparative route known for condensing two monomeric units into a compound containing a quadruple bond. In fact, facile conversion of Mo2(02CCH3)4 to chloro species occurs in concentrated hydrochloric acid to form the Mo2 Clj ion (36), which is isostructural with the Re2Clg ion. The eclipsed configuration of Mo2Clg" and the molybdenum atom separation of 2.14 A are entirely consistent with a quadruple bond in the anion as expected (33). 

The ability of the cations [M(CNR)4]+ (M = Rh, Ir) to self-associate is a function of steric crowding in the molecule, and for bulky R groups monomeric species predominate. An estimate has been made of the steric size of isocyanides in terms of fan-shaped angles and as part of this study the structure of RhCl(CNC6H2Bu -2,4,6)3 has been elucidated (126). The structural determinations of a series of dimeric rhodium(I) isocyanide salts have been completed. An eclipsed configuration was found for [Rh2 CN (CH2)3NC)4](BPh4)2 NCMe (42)2 (287), whereas [Rh2(CNPh)8]-... 

Inversion doubling has been observed in microwave spectrum of methylamine CH3NH2. This splitting depends on the quantum numbers of rotation and torsion vibrations [Shimoda et al., 1954 Lide, 1957 Tsuboi et al., 1964]. Inversion of NH2 alone leads to the eclipsed configuration corresponding to the maximum barrier for torsion. Thus, the transition between equilibrium configurations involves simultaneous NH2 inversion and internal rotation of CH3 that is, inversion appears to be strongly coupled with internal rotation. The inversion splits each rotation-vibration (n, k) level into a doublet, whose components, in turn, are split into three levels with m = 0, 1 by internal rotation of the... 

Figure 8.5. The methylcyclopropyl radical in various conformations, (a) The non-planar-bisected conformation is stable, (b) The non-planar-straddled conformation corresponds to methyl group rotation by 60°. (c) The planar eclipsed configuration is the transition state, (d) The planar-bisected structure corresponds to conversion without rotation. (From Zebretto et al. [1989].)...

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