The reaction of trans-Mo(C 2 H 4 ) 2 (PMe 3 ) 4 with PhC≡CCOOEt affords (PhC≡CCOOEt) 3 (PMe 3 ) (1). The molecular structure of 1 has been determined by means of X-ray analysis. NMR studies have shown that 1 undergoes a dynamic process in solution due to the alkyne rotation. The related energy barrier is experimentally determined to be 15.1 kcal mol - 1 . The well established EHMO theoretical arguments, which uniquely explain the chemical bonding in these complexes, are extended to interpret the nature of the barrier and its dependence on the π-donor or π-acceptor capability of the axial ligand. The conceptual continuity between the chemical bonding of d 6 metal species coordinated by at least two alkyne molecules (e.g. M(alkyne) 2 L 4 , M(alkyne) 2 L 2 and M(alkyne) 3 L) is highlighted.