Mono‐ und Dicarbonyl‐Komplexe des Decamethylchromocens Eine CO‐induzierte η⁵ → η³ ‐ Ringverschiebung

Abstract

Decamethylchromocen, (C5(CH3)5)2 Cr, bildet mit CO den 18‐Elektronen‐Komplex (C5(CH3)5)2Cr(CO). Unter höheren CO‐Drucken entsteht der Dicarbonyl‐Komplex (C5(CH3)5)2Cr(CO)2, der auch in Abwesenheit von CO stabil ist. Eine Röntgenstrukturanalyse ergibt, daß diese Verbindung einen η3‐ und einen η5‐koordinierten Ringliganden enthält und damit ebenfalls als 18‐Elektronen‐Teilchen vorliegt.

Influence of Cyclopentadienyl Ring‐Tilt on Electron‐Transfer Reactions: Redox‐Induced Reactivity of Strained [2] and [3]Ruthenocenophanes

“…The shift from the η 5 to the η 3 ‐allyl coordination mode results in folding of the internal Cp ring, a feature consistent with other organometallic complexes of this type 39. The degree of ring folding of the Cp ring in η 3 bound organometallic complexes is defined by ω , the angle between the planes formed by the allylic and olefinic ring fragments (see Figure S3 for details),40 and is considerably greater in 9 ( ω =30.44°), than in other analogous complexes (17.4–20.0°),38a, 41 potentially reflecting the steric demands involved in formation of the cyclobutane ring.…”

“…Due to the increased strength of MCp bonds relative to those for other π‐conjugated ligands containing a C 5 ring (e.g., Ind=indenyl, Flu=Fluorenyl),42 haptotropic η 5 to η 3 shifts of the Cp ligand are extremely rare. Despite being proposed as a mechanistic step in a range of organometallic reactions,43 this hapticity transformation has only actually been detected in solution,44 or implied from X‐ray crystallographic analysis of the resulting η 3 ‐Cp bound product, on a handful of occasions 41. In contrast, examples in the literature of η 5 –η 3 haptotropic shifts in organometallic complexes containing Cp ligands fused to larger π‐conjugated ring systems (e.g., Ind, Flu) are much more common, forming the basis of the “indenyl effect”, whereby the kinetics of associative ligand substitution reactions are noticeably increased for complexes with these ligands, relative to their Cp analogues 43.…”

The Nature of the Indenyl Effect

“…A large number of ligands has been tested, as a survey of the Cambridge Structural Database (CSD) [17] shows. There are structurally characterized complexes with h 3 -coordinated Cp, in solution [18] and in the solid state, [19,20] as well as ligands with larger p systems, such as fluorenyl (C 13 H 9 À ), cyclopentha[def]phenantrenyl (C 15 H 9 À ), [21±28] and 1-hydronaphthalene (C 10 H 9 À ); [29,30] however, the most studied ligand is certainly indenyl. [21, 31±56] Since the early papers by the Hoffmann group [2,57,58] a large amount of work has been devoted to the theoretical understanding of haptotropic shifts.…”

“…The system chosen, the Group 6 bent metallocenes, such as [(h 5 -Cp')(h 5 -Cp)Mo-(CO) 2 ] 2 (Cp' Cp or Ind), presents h 5 -to-h 3 shifts of both Cp and Ind upon a two-electron reduction, and some X-ray structures of the corresponding h 3 -Cp' complexes have been determined. [19,20,32] …”

The Elusive Chemistry of Bent-Sandwich Chromocene

“…Coordination of the second CO is catalyzed by one electron oxidation of the metal center. Cp* 2 Cr (Cp* = C 5 Me 5 ) also coordinates two CO ligands at elevated CO pressures to form a stable, isolable ( 3 -Cp*)( 5 -Cp*)Cr(CO) 2 complex [22]. By contrast (C 5 Ph 4 H) 2 Cr does not coordinate CO in the presence of an atmosphere of the gas at room temperature [23].…”