In the process of isolation under aerobic conditions phenylethynyl-di-(tert-butyl)-phosphine bridged dicobalt complex [(micro-PPh(2)CH(2)PPh(2))Co2(CO)4(micro,eta-PhC[triple bond]CP(t-Bu)2)] 4a underwent a partial oxidation. The identity of the oxidized product, [(micro-PPh(2)CH(2)PPh(2))Co2(CO)4(micro,eta-PhC[triple bond]C-O-P([double bond]O)(t-Bu)2)] 5, was established by spectroscopic means as well as the single-crystal X-ray diffraction method. This is the first crystallographic evidence that unambiguously supports the formation of an organometallic version of a phosphinate ester. The mechanism for the formation of 5 from 4a was proposed, and its validity was examined by DFT means. For the purpose of comparison, a similar mechanism illustrating the transformation of PhC[triple bond]CP(t-Bu)2 1O into PhC[triple bond]C-O-P([double bond]O)(t-Bu)2 5O, the organic counterpart of 5, was examined by the same method. It was found that the metal fragment is indeed capable of assisting the oxidation process by lowering the activation energy, although the effect is small. The impact of the presence of an electron-withdrawing substituent such as a fluorine atom in the alkynylphosphine was also investigated. Results demonstrated that the conversion of fluorine-substituted phosphines to the corresponding phosphinate esters can be achieved more readily. In addition, the energy barrier for the reaction of a phosphine with dioxygen yielding the phosphine oxide was calculated to be much lower than that on the way to the phosphinate ester.