The reaction of tert-butylacetylene (t-BA) in the presence of seven-coordinate tungsten(II) and molybdenum(II) compounds [MC;(M'Cl-3)(CO)(3)(NCR)(2)] (M = Mo, W; M' = Sn, Ge; R = Me, Et) leads to the catalytic coupling of t-BA molecules and the formation of high-molecular-weight ((M) over bar(n) > 10(5)) soluble polymers. The geometric structure of poly(tert-butylacetylene) (P-t-BA) was determined by means of H-1- and C-13-NMR spectroscopy. The monitoring of the reaction between t-BA and the metal complex by means of H-1-NMR spectroscopy provided further insight into the initiation step of this polymerization process. The reaction of a heterobimetallic compound with a stoichiometric amount of t-BA yields compounds in which CO and/or nitrile ligands have been replaced by a t-BA Ligand. The alkyne molybdenum(II) and tungsten(II) complexes formed were characterized structurally by means of IR and NMR spectroscopy. The possible mechanisms for the formation of very reactive intermediate compounds and their role in the catalytic process are discussed.