Formation and Crystal Structure of Stable Five‐coordinate Diorgano Cobalt(III) Complexes

The mer‐octahedral complexes(2‐carbonyl)(4‐Me)(6‐tBu)phenolatoC,O]hydridotris(trimethylphosphine)cobalt(III) ( 1 ) or ‐(1‐carbonyl)(2‐oxo)(1,2‐diphenylethene)C,O]hydridotris(trimethylphosphine)cobalt(III) ( 2 ) via formal insertion of propynoic acid ethyl ester into Co‐H functions afford pentacoordinate vinylcobalt(III) 3 and 4 , respectively, that are diamagnetic and attain a square pyramidal structure as exemplified by an X‐ray diffraction analysis of 3 .