The trimethyl phosphite substituted pentacarbonylpropargyldicobalt cation, [(CH = CCH2) Co2(CO)5L]+ [L = P(OMe)3], is significantly more stable than its phosphine analogues (L = PPh3, PBu3), and has been converted to a series of ethers, (CH = CCH2OR)Co2(CO)5L. Reaction of the (cholesteryl propargyl ether)hexacarbonyldicobalt complex with trimethyl phosphite proceeds without stereoselectivity, and the product suffers metal-carbon rather than oxygen-carbon cleavage on treatment with acid.