Highly stable five-membered metallacycloallenes were synthesized under mild conditions. Calculations revealed that the incorporation of transition-metal moieties relieves considerable strain and indicates a trend toward ring enlargement in the five-membered metallacycloallenes. Conversion into six-membered metallacycloallenes was confirmed experimentally.

The magic of Os: An unprecedented formal [3+3] cycloaddition reaction of 1 with alkynols affords stable iso-osmabenzenes at room temperature (see scheme). The phosphonium substituent at the Cβ position and the 18e− nature of the compound play key roles in the origin of the high thermal stability of the products. Isomerization of iso-osmabenzenes into η5-cyclopentadienyl complexes through metalated cyclopentadiene intermediates is also described.