Alkylidyne-alkyne coupling on triruthenium clusters. Isomerization of (.mu.-H)Ru3(.mu.3-.eta.3-EtSCCRCR)(CO)9 (R = Me or Ph) to Ru3(.mu.-SEt)(CO)9(.mu.3-.eta.3-CCRCHR), an intermediate in alkylidyne chain growth. The crystal structure of Ru3(.mu.-SEt)(.mu.3-.eta.3-CCPhCHPh)(CO)9