Density Functional BP86 calculations are reported for the interconversion reactions of WH(CH)(CO)(PH 3 ) 3 and WH(CH)(CO) 2 (PH 3 ) 2 to the corresponding five-coordinated carbene complexes W(CH 2 )(CO)(PH 3 ) 3 and W(CH 2 )(CO) 2 (PH 3 ) 2 , respectively. Transition states have been calculated to be 9.9 and 0.7 kcal mol - 1 higher in energy than the carbynes. A comparison with the isoelectronic osmium system OsHCl 2 (CMe)(PH 3 ) 2 with a transition state for interconversion 27.2 kcal mol - 1 above the carbyne leads to the conclusion that additional π-acceptors competing with the carbyne or carbene ligand for back donation facilitate the interconversion reaction, whereas additional π-donors undergoing synergic push-pull interactions impede the interconversion reaction.