The [5 + 2] cycloaddition reactivity of the Cp*Co( η 5 -1,2,5-trimethylpentadienyl) + complex and ethyne has been demonstrated. This reactivity was predicted computationally and runs against previous synthetic observations of decreasing reactivity with increasing substitution. The pentadienyl complex and the resulting Cp*Co( η 2 , η 3 -1,2,5-trimethylcycloheptadienyl) + species have been characterized spectroscopically and crystallographically. The X-ray structure of the pentadienyl complex has a dihedral angle between the two η 5 -bound planes that falls within the range expected for reactivity (3.1(8)°), providing strong support for our computationally proposed structure/reactivity relationship.
