[eta(4)-Tetrakis(arylethoxyphosphoryl)cyclobutadiene](eta(5)-Cy-clopentadienyl)cobalt(I) complexes were synthesized by reaction of bis(arylethoxyphosphoryl)acetylenes with CpCo(CO)(2) and several diastereomers were isolated. Whole molecular structure of the tetraphosphorylcyclobutadiene complex was significantly affected by relative configuration of the phosphoryl groups.