Unique bonding and geometry in .eta.-cyclopentadienyltantalum-diene complexes. Preparation, x-ray structural analyses, and EHMO calculations

Properties of electron deficient 1,3-diene complexes of the group 4A metals

“…So far we have no evidence that this asymmetry plays any significant role in the reactivity of the compound. In all the compounds studied the diene fragments did not show any fluxional behaviour, such as cis-trans interconversion, in keeping with the rigidity observed for the diene fragment in Cp Ta(diene)Cl 2 [6].…”

“…ketones, aldehydes, esters, nitriles, and acetylenes. Although several electronically unsaturated early transition metal diene complexes are known, such as the 16 electron species (diene) 2 M(dmpe) (M = Ti, Zr, Hf) [3], (C 8 H 8 )M(diene) (M = Ti, Zr, Hf) [4], CpV(diene)(PR 3 ) [5] and (C 5 Me 5 )Ta(diene)Cl 2 [6], very little has been reported about their reactivity. Studies on the unsaturated 14 electron complexes Cp M(diene)X (M = Ti [7], Zr, Hf [7,8]; X = Cl, alkyl, aryl; Cp = η 5 -C 5 Me 5 ) indicate that electronic and steric unsaturation can cause significant changes in the reactivity and reaction selectivity of the diene ligand.…”

Preparation of Diamidochloro(cyclopentadienyl)titanium Derivatives as Pre‐Catalysts for Olefin Polymerization − X‐ray Molecular Structure of [Ti(η5‐C5H5){1,2‐C6H4(NCH2CH2CH3)2}Cl] and [Ti{η5‐C5H4(SiMe3)}{1,2‐C6H4(NCH2CH2CH3)2}Cl]

“…Most importantly in this type of complexes, the diamido ligand can adopt different coordination modes [4e,11] and has two possible conformations (supine orprone) [12] relative to the cyclopentadienyl ring. The 1 H NMR spectra (C 6 D 6 and CDCl 3 , room temperature) show the presence of only one species, ruling out a mixture of the two rotamers (supineprone automerization) observed in solutions of similar monocyclopentadienyl diene complexes.…”

Olefin Complexes of Low‐Valent Rhenium

“…[16,17] The cis-η 4 -diene complexes are known mainly with early transition metal centers, such as in Cp 2 M(diene) (M = Ti, Zr, Hf), [16] as well as with Ta. [18] Two extreme forms of butadiene cis coordination were found: the cis-η 4 -1,3-diene structure and the metallacyclopent-3-ene structure. NMR spectroscopy is able to distinguish the various coordination modes of the diene ligand on the basis of their coupling constants.…”