Interactions and Crystal Packing Effects

While the rotation barriers for highly symmetric complexes such as Mo(CO)(H2)(dppe)2 (7) can be extremely low (0.6 kcal/mol), they are not in fact zero. This is because X-ray data reveal that the structure of the MP4 fragment is distorted with at least one of the P-M-P axes bent back away from the H2 (with which it is nearly parallel). The distortions rehybridize M orbitals such that the overlap with r (H2) does show a variation as the H2 rotates in the plane parallel to that formed by the MP4 fragment. In fact, the degree to which the P-M-P angle 

This distortion about M is sensitive to steric effects or crystal-packing forces, e.g., the same complex can have slightly different molecular structures and therefore different barriers to H2 rotation when crystallized with different counterions or lattice solvent molecules. The rotational tunnel splitting (17 cm-1) is larger [and the barrier is thus lower (Table 6.1)] for Mo(CO)(H2)(dppe)2-2-toluene solvate than the 

Braga and coworkers have comprehensively examined the crystal structures of H2 complexes for both intramolecular and intcrmolecular interactions and their effect on the rotational barrier.40 Intermolecular contacts are extremely rare because the H2 is well protected by ancillary ligands. The first and only well-documented example is IrHQ2(H2)(PlFr3)2, which has both intramolecular and intermolecular 

M ean be considered to be a fixed reference frame wherein the H2 is described by the usual spherical coordinates r, 6, and j . The complete description of the exchange process involves the foin dynamical coordinates r, d, f , and z, which can be simplified because presumably the H2 remains perpendicular to the z axis during the rotation and is not displaced appreciably relative to M along z. In contrast to r and l , the two coordinates z and 6 do not play an important role in the dynamics 

For three other systems studied, the 2D model also gives excellent results that compare well with experiment, e.g., /hh in [F H(H2)(dppe)2] is 0.82 A by neutron diffraction and is 0.822 A with the 2D model. However, it is only useful if a sufficient number of INS transitions are observed. While instrumental limitations confine the observation of the tunneling transition to H2 complexes with a rotational barrier of less than 3 kcal/mol, transitions to the torsional states can in principle be observed for any such compound. The 2D methodology could then be applied to any experiment in which the dynamic process involves permutation of two identic particles, e.g., the quantum exchange coupling phenomenon (see Section 6.3.4). 

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Asymmetry parameters <5(X) have been obtained for a number of substituents8. Several factors limit the reliability of such derived parameters, e.g. the validity of additivity and other assumptions, the influence of conformation, electronic and steric interactions of substituents, additional strain in polycyclic systems, crystal packing effects and size and quality of the data sample. 

Investigation of environmental effects. As has been stressed in this chapter, homoaromaticity is just a matter of a few kcal mol-1 stabilization energy in most cases, and therefore environmental effects may have a large impact on structure, stability and other properties of a homoaromatic compound. Future work in theory (as well as in experiment) has to clarify how environmental effects can influence electron delocalization, through-space interactions and bonding in homoaromatic molecules. The theoretical methods are now available to calculate solvent and counter ion effects (for homoaromatic ions in solution) or to study intermolecular and crystal packing forces in the solid state. 

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