Metallorganische Lewis–Säuren, LVI: Vom Bis(alkin)-Komplex [(OC)4Re(η2-MeCCMe)2]+BF4− zu [(OC)4Re(η4-C4Me4)]+BF4− mit rechteckigem Cyclobutadien–Liganden. Die erste beobachtbare cis-Bis(alkin)/Cyclobutadien–Umlagerung

Reaction of the 14-electron cation `[Re(CO)4]+' [as (OC)4Re(OEt2)FBF3] with 2-butyne yields the bis(alkyne) complex [(OC)4Re(η2-MeCCMe)2]+BF4− (1), the first isolated cis-bis(alkyne) transition metal complex with a d6 electron count. At ambient temperature and in the presence of O- or N-donors 1 isomerizes to the stable tetramethylcyclobutadiene complex [(OC)4Re(η4-C4Me4)]+BF4− (2), which forms a distorted trigonal bipyramid with a rectangular, cyclobutadiene ring in the equatorial position, and with remarkably different axial and equatorial Re–C–O bonds. Bonding, electronic structures and molecular geometries of 1 and 2 have been studied by means of Extended Hückel MO calculations. The isomerization reaction 1→2 is commented upon with respect to its mechanistic implications. A rhenacyclopentatriene intermediate is proposed, its electronic structure is described and its transformation to 2 is analyzed.