Effect of the Substitution on the Protonation of Allyl Cyclopentadienyl Molybdenum(II) Compounds

Synthesis, characterization, and reactivity of new allyl cyclopentadienyl molybdenum­(II) compounds [(η³-C₃H₄R¹)­(η⁵-C₅H₃(R²)₂)­Mo­(CO)₂] (R¹ = H, COOMe; R² = COOMe, CONH^<i>t</i>Bu) are reported. Although these compounds are structural analogues of [(η³-C₃H₅)­(η⁵-Cp)­Mo­(CO)₂], their reactivity is very different. While protonation of [(η³-C₃H₅)­(η⁵-Cp)­Mo­(CO)₂] gives a cationic cyclopentadienyl complex, the presented compounds give cationic allyl complexes [(η³-C₃H₄R)­Mo­(CO)₂(NCMe)₃]­[BF₄] (R = H, COOMe) or stable cationic allyl cyclopentadienyl complexes. The theoretical calculations have shown that this behavior is a result of high affinity of the functional groups in the cyclopentadienyl ligand toward protonation