New modes of attachment of C5 rings in organometallic compounds: X-ray crystal structures of the clusters [Os3H(Ph3PC5H3)(CO)x](x= 9 or 10) derived from cyclopentadienylidenetriphenylphosphorane

Abstract

The cluster [Os₃(CO)₁₀(MeCN)₂] reacts rapidly at room temperature with cyclopentadienylidenetriphenylphosphorane (Ph₃PC₅H₄) to give the oxidative-addition compound [Os₃(µ-H)(µ-Ph₃PC₅H₃)(CO)₁₀]1 which was shown by X-ray structure determination to contain the C₅ ring bonded through a single carbon atom to two osmium atoms. We consider the compound to be zwitterionic with a phosphonium substituent at the five-membered organic ring which is co-ordinated as a µ-alkylidene to the metal atoms which carry a formal negative charge. In solution the compound exists as two slowly interconverting isomers, 1a and 1b, which were separated by TLC and which differ only in the substitution position of the phosphonium group. Decarbonylation gives the compound [Os₃(µ-H)(µ₃-Ph₃PC₅H₃)(CO)₉]2, which also exists as isomers. The X-ray structure of the major isomer shows it to contain the C₅ ring co-ordinated to the three osmium atoms through two σ Os–C bonds and an η² bond. A zwitterionic description is likewise appropriate. The bonding in these compounds suggests new types of cyclopentadienylidene (C₅H₄) co-ordination.