The insertion of the alkynes ZC=CZ (2a-i) [Z = C02R: R = Me (a), Et (b), nPr (c), iPr (a), nBu ( e ) , tBu (f), neo-Pent ( g ) , cyc-Hex (h); Z = CF, (i)] into the P1-P2 bond of the diphosphamolybdacyclopropane (if-C5H5)(OC)2hiio-P1Ph2-fi2Mes (1) (Mes = 2,4,6-trimethylphenyl) results in the formation of the diphosphamolybdacyclopentenes ( T~~-C~H~) ( O C )~-hiio-P1Ph2-CZ=CZ-P2Mes (3a-i). According to an X-ray structural analysis, 3a crystallizes in the space group P2,lc with the mesityl function at P2 and the cyclopentadienyl fragment at the metal atom on the opposite side of the planar five-membered ring. The same orientation is found in solution by NMR experiments (NOE; 2D 'H, 13C HETCOR). Kinetic investigations confirm that the rate constants decrease in the sequence R = Me > Et > nPr > isoProp > Cy > nBu > neo-Pent > tBu. The second-order reaction and the strongly negative activation entropy which was determined in the case of the reaction 1 + 2e -+ 3e are consistent with a nucleophilic attack of P2 at one of the triply bonded carbon atoms of the alkyne. The reaction of 3a, h with sulfur affords the diastereomeric compounds (q5-C5Hs)(OC)2kfo-P1-Ph2-CZ=CZ-PZ(S)Mes (4a, h). An X-ray structural analysis of the heterocycle 4h, which crystallizes in the space group P i , shows the presence of the enantiomeric pair SWRS. C5H5)(OC)2bho-P1Ph2-S-P2(S)Mes-$ (6) which crystallizesin the space group Pi. The five-membered ring in 6 has an envelope conformation. Upon thermally induced cleavage of the P2S2Mes fragment from 6 the three-membered ring (q5-C5H5)(OC)2hiio-P1Ph2-$ (?) is formed as the final product.The transition metal-catalyzed or -mediated cyclooligomerization of alkynes with heteroalkynes offers a well developed method for the preparation of four-and six-membered organic heterocycle~ [~~~]. Recently, it could be demonstrated that the P-S group in thiaphosphametallacyclopropanes behaves like a heteroalkyne because of similar covalent radii and comparable electronegativities of the phosphorus and sulfur atomL4]. It is therefore suitable for a metal-mediated cyclo-cotrimerization with alkynes. Starting from the threemembered heterocycles [ M W , we obtained regiospecifically substituted thiophenesL5I, phosphole and furansL7I, respectively, which are obtained by oxidative or hydrolytic degradation of the resulting metallabicycloheptadienes. The latter were formed via the intermediately occurring thiaphosphametallacyclopentadienes which could be Between sulfur and the PR fragment exists an isolobal relation. Meanwhile we are able to make accessible heterocycles of the type [T61-PR:-PR2r9-101. Since the [R2PS]-and [R$PPR2]-systems exhibit similar structural features, we were interested in the question whether the phosphorus analog also displays alkyne-like properties. In principle, an alkyne may be inserted into the Mo-PR:, Mo-PR2 or intothe R&P-PR2 bond of the above-mentioned three-membered heterocycle. In a communication["] lately we reported on the insertion of dimethyl acetylenedicarboxylate into the P-P bond o...