The bridging methylidyne clusters [Co3 (CO) 9 {η1-μ3-[CP (O)(OR) 2])] 1 (R= Et) and 3 (R= SiMe3) with α-phosphoryl substituents have been synthesized by reaction of Cl3C-P (O)(OR) 2 with Co2 (CO) 8, followed by protonation. Cluster 3 was also obtained by the reaction of 1 with Me3SiBr. The structure of 1 has been determined by X-ray crystallography. The donor properties of the P= O function toward Lewis acids such as [Cp2MCl]+(M= Ti, Zr ...