First photochemical envelope isomerization of a late transition metal 1,3-butadiene complex: a triple stereochemical labeling experiment

Synthesis and Characterization of Dibromo-Containing Ruthenium(IV) η3-Allyl and Ruthenium(IV) η4-Diene Complexes. Formation of [Ru(η5-C5Me5)Br3]- and [Ru(η5-C5Me5)Br3]2

“…While the bonding of dienes to early transition metals is normally more accurately represented by the σ 2 ,π-metallacylopentene structure a , the vast majority of complexes containing middle and late transition metals adopt the η 4 -s-cis-1,3-diene structure b . Noteworthily, metallacyclopentene complexes of late transition metals have been postulated as intermediates; e.g., the interconversion of the diene ligand in Co(η 5 -C 5 H 5 )(η 4 -diene) complexes can be explained only by an “envelope-flip” process involving a metallacyclopentene intermediate …”

“…The ruthenium−carbon bonds to the terminal carbon atoms, on average, are about 2.181 Å while the one to the internal carbon atoms are 2.416 Å. For comparison, in the ruthenacyclopentane complex Ru(η 6 -C 6 Me 6 )(PPhMe 2 )(C 4 H 8 ) the ruthenium−carbon σ bond distance is 2.152(5) Å …”

Coordination Modes and Catalytic Carbonylative [4 + 1] Cycloaddition of Vinylallenes

“…In contrast, exo - 6 failed to undergo isomerization, indicating that exo - 6 is thermodynamically favored, presumably due to steric reasons. A ring-flipping (flip-envelope) mechanism for this thermal endo − exo isomerization is shown in Scheme 1. Decomplexation of the double bond of the bent metallacyclic form of endo - 6 and a subsequent flip through the planar σ 2 -bonded metallacycle 7 generates the η 4 -complex with an exo orientation of the trimethylsilyl group.…”

“…Stereochemical fluxionality of s- cis -diene−transition metal complexes has been often observed and can be explained by a ring-flipping mechanism . Although implicated as a transitory species in the ring flipping, examples of thermodynamically stable planar σ 2 -bonded complexes remain rare 3-5 and are mostly limited to perfluoro-1,3-butadiene 4 and vinylketene complexes …”

Synthesis, Characterization, and Structural Studies of Transition Metal Open Fulvalene Complexes

“…Thus, for the (open fulvalene)dicobalt complex, the isomerization process would require the η 3 intermediate to have the cobalt center σ bound to the terminal dienyl carbon atom, so that the exchanging endo/exo substituents would each be hydrogen atoms. A related process has already been observed for (diene)metal complexes, through which both endo/exo substituent pairs undergo exchange . Furthermore, recent observations have demonstrated that the two isomers of Fe(2,3-dimethylpentadienyl) 2 undergo interconversion, presumably also through an η 3 intermediate such as that of Scheme , with observable changes in composition occurring within hours at room temperature…”