The product pattern of the reaction of the alkynes ZCECZ (4-11) [Z = C02R: R = Me (4), Et (5), nPr (6), iPr (7), nBu (8), nPent (91, neo-Pent (lo), cyc-Hex (ll)] with the diphosphamanganacyclopropanes ( O C ) 4 M F P w 2 R 2 (1 -3) depends on the polarity of the employed solvent. With the exception of phosphaalkynes these reactions proceed via metallacyclopropanes and metallacyclopentadienes as intermediates. Depending on the kind of the alkyne, the products are metallacycloheptatrienes or metallabicycioheptadienes[15-181, Reductive elimination results in the formation of highly substituted benzene and pyridine derivatives, respectively. It is surprising that analogous cyclocotrimerizations with electron-poor alkynes are possible by introduction of the )=S group which stems from depro-

The phospha-arsa-mangana-cyclopropane (OC)4Mn–PPh2–AsAr (4) [Ar = 2,4,6-(t-Bu)3C6H2] is obtained in a one-pot reaction proceeding in the sequence Mn2(CO)10 → HMn(CO)5 (1) → HMn(CO)4PPh2H (2) → Li2[(OC)4Mn—⃛PPh2] (3) → 4. According to an X-ray structural analysis 4 has a short P–As bond (229 pm) and a long Mn–As distance (262 pm).