Conformation uranocenes

Also crystallizing in space group P2 ]c, uranocene has two molecules per unit cell, so that the U(CsH8)2 molecule occupies a special position of site symmetry 1. In other words, the molecule has an eclipsed conformation, and it may be assigned to special position 2(a). Similarly, the two halves of the [Re2Clg]2- dianion in K2 [Rc2C lmolecular dimensions (indicating that the symmetry of the dianion is Z>4h within experimental error) and crystal structure are shown in Fig. 9.6.5. 

Numerous substituted uranocenes are now known and could, in principle, provide useful tests. Other factors now, however, become involved and need to be evaluated. The lower symmetry of these compounds means that X and Y are no longer constrained to be equal and the eq. 3 needs to be considered in its entirety. Moreover, the substituent could have an effect on magnetic anisotropy. Finally, some substituents have more than one possible conformation which would need to be considered. 

Figure 12. Conformations of the substituent in substituted uranocenes shown in Newman projection form with the uranium atom below the plane of the ring in each figure...

Figure 14. Several conformations of 1,1 -disubstituted uranocenes about the central axis. The 1 -substituent is shown with the dotted line.

Early attempts to factor the isotropic shifts in alkyl-uranocenes using eq. 9 were not completely successful because of failure to correctly assess the conformation of the substituent in solution and overestimation of the value of the anisotropy term jj-Xj (5,6,14). ... 

The molecular structure of uranocene has been determined by single-crystal x-ray diffraction and is shown in Fig. 22.15 [130,131]. The molecule possesses rigorous Dg), symmetry with the eight-membered rings arranged in an eclipsed conformation. The mean U-C bond distance is 2.647(4) A, and the mean C-C bond distance is 1.392(13) A. TheCgHi rings are planar to within experimental error. 

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