Structural effects of group VI metal tricarbonyl binding to benzenoid rings: Interruption of conjugation or enhanced aromaticity?

Abstract

The effect of tricarbonyl (group VI metal) complexation on the geometric aromatic character of benzenoid rings is studied as a function of bond-length alternation (localization) in the parent arene. Good agreement between theory and experiment is established for (n6-benzene) tricarbonylchromium, -molybdenum. and tungsten. It is found that, whereas the electrons of benzene become slightly more localized upon tricarbonyl metal complexation, those of 'cyclohexatriene' mimics, like in-starphenylene, become more delocalized. A combination of ab initio quantum-mechanical and high-accuracy X-ray methods leads to a linear structure-structure correlation between the free and metal-bound arene bond-alternation geometry. In all cases, the average bond length in the arene increases upon complexation. The computational observation that the average bond length increases more in benzene complexes than in in-starphenylene implies stronger back bonding in the benzene complexes and coincides with the experimental observation that more-delocalized arenes form thermodynamically favored complexes. The rotational barriers about the tricarbonylmetal-to-arene axis were computed for 1-Cr, 1-Mo, and 1-W as well as for 5-Cr, 5-Mo, and 5-W Barriers for the former group are characteristically low, almost negligible (0.05 kcal/mol for 1-Cr; 0.01 kcal/mol for 1-Mo; 0.27 kcal/mol for 1.W), whereas for the latter group they are substantial (11.2 kcal/mol for 5-Cr; 15.2 kcal/mol for 5-Mo; 13.6 kcal/mol for 5-W). The higher barriers found in 5-M compounds are consistent with previous findings.