First example of cyclodimerisation of a phospha-alkyne to a 1,3-diphosphacyclobutadiene. Syntheses of complexes of the type [M(η5-C5R5){η4-(ButCP)2}] R = H or Me; M = Co, Rh, or Ir): crystal and molecular structure of η5-pentamethylcyclopentadienyl-2,4-di-t-butyl-1,3-diphosphacyclobutadienecobalt(I), [Co(η5-C5Me5){η4-(ButCP)2}]