Thermolysis of the η 2 -alkyne complex (η 5 -C 5 H 5 )Co(PPh 3 )[η 2 -(MeO 2 C)C C(CO 2 Me)] (1-DMAD) leads to formation of an ortho-metalated vinyl-phosphonium derivative (η 5 -C 5 H 5 )Co[η 2 -(MeO 2 C)HC C(CO 2 Me)PPh 2 (η 1 -o-C 6 H 4 )] (4) as well as a small amount of the tricobalt bis(carbyne) cluster [(η 5 -C 5 H 5 )Co] 3 [μ 3 ,η 3 -C(CO 2 Me)] 2 (3-CO 2 Me). Thermolysis of 1-DMAD in the presence of added triphenylphosphine gives 4 as the major product, with no evidence for formation of 3-CO 2 Me. Complexes 4 and 3-CO 2 Me were characterized by X-ray crystallographic analyses.