[η 4 -Tetrakis(arylethoxyphosphoryl)cyclobutadiene](η 5 -cyclopentadienyl)cobalt(1) complexes were synthesized by reac tion of bis(arylethoxyphosphoryl)acetylenes with CpCo(CO) 2 and several diastereomers were isolated. Whole molecular struc ture of the tetraphosphorylcyclobutadiene complex was signifi cantly affected by relative configuration of the phosphory groups.