FEATURES OF THE CRYSTALLINE STRUCTURE OF (1,2-tBu2Cp)2Mo2(CO)6 AND THEIR POSSIBLE CAUSES

Comparative analysis of the structural parameters of the substituted cyclopentadienyl moieties (tBuC5H4 and tBu2C5H3) of two binuclear molybdenum complexes ((tBuCp)2Mo2(CO)6 (1) and (1,2tBu2Cp)2Mo2(CO)6 (3)) and two sandwich complexes of iron (the cation of 1,1-di-tert-butylferrocenium (tBuCp)2Fe+ (4) and 1,2,1,2-tetra-tert-butylferrocene (1,2-tBu2Cp)2Fe (5)) allowed us to identify the structural features of the tBu2C5H3 fragment in (3) and to establish the reasons that account for them. It is shown that the main changes in the structural parameters (valence and torsion angles, bond lengths) of the fragment 1,2-tBu2C5H3 are due to steric interactions of vicinal tBu-substituents between themselves, as well as with the corresponding nearest CH fragment of the ring. Differences in the degree of steric interaction of vicinal tBu-substituents with CH ring fragments in (3) are interpreted by the steric influence of the transcarbonyl (C10O3) group on the tBu-substituent at C4. The latter interaction is also reflected in the larger value (26) of the rotation angle of the tBu-substituent at C4, compared to the rotation angle (15) of the corresponding tBu-substituent in (5), caused only by the steric interaction of vicinal tBu-substituents among themselves. In (3), the steric influence of the trans-carbonyl group on the tBu-substituent at C4 is also accompanied by a change in the direction of rotation of the tBu-substituent at C5, as a result of which the conformations of the tBu-substituents with respect to each other and to the ring plane differ markedly from the conformations of the tBu-substituents in (5). In the binuclear complexes of molybdenum (3) and (1), the degree of steric interaction between the trans-carbonyl group and tBu-substituents does not correspond to the rotation angle of the substituted cyclopentadienyl ring ((6.15) tBuC5H4 and (2.12) 1,2-tBu2C5H3) around the Ct–Mo axis, which was interpreted by the inhibition of rotation of the 1,2-tBu2C5H3 ring due to increased repulsion forces between the tBu-substituent at C5 and the trans-C10O3 group in (3). The analysis of the lengths of shortened nonvalent C5-ring...cis-CiOi and cis-CO...cis-CiOi contacts between the two halves of molecules (1), (3) and unsubstituted complex Cp2Mo2(CO)6 (2), and the reasons causing them allowed us to conclude that the shortening of the lengths of cis-CO...cis-CiOi contacts in the series (2)(1)(3) is the most probable reason for the elongation of the Mo–Mo bond in (3).