A unusual bridging mode of vinylidene ligands in nonacarbonyl(vinylidene)dichromium complexes

Abstract

Thermolysis of the pentacarbonyl(vinylidene)chromium complexes [(CO) 5 CrÄ/CÄ/C(R 1)R 2 ] (1a Á/d) (C(R 1)R 2 0/CMe 2 (a), C(CH 2) 5 (b), C(Et)Me (c), C(t Bu)Me (d)), affords binuclear, vinylidene-bridged nonacarbonyldichromium complexes, [(CO) 5 Cr(m-h 1 ,h 2-CÄ/C(R 1)R 2)Cr(CO) 4 ] (2a Á/d). The solid-state structures of 2a and 2d have been established by X-ray structural analyses. The structural data prove the m-h 1 ,h 2-C Ä/C(R 1)R 2 bonding mode. To account for the NMR spectroscopic features a dynamic process is proposed involving rapid coordination site exchange of the CÄ/C bond between Cr(1) and Cr(2) (windshield wiper mechanism) and of one CO ligand.

Computational Discovery of Stable Transition-Metal Vinylidene Complexes

“…For these pairs, a wider range of metal fragments, shown in Scheme , were considered to probe the effects of the transition-metal group (e.g., Cr / Mo , Pd / Pt , Ag / Au-1 ) and period (e.g., Mn / Ru-2-CO ) on the predicted tautomer energy preferences. Of the fragments employed here, well-defined vinylidene complexes are known for [Cr(CO) 5 ] ( Cr ) and [Mn(η 5 -C 5 H 5 )(CO) 2 ] ( Mn ), as well as complexes related to Rh and Ru-1-H , as discussed above. In contrast, Au-1 is known to support η 2 -alkynes (discussed above), and so is [PdCl 3 ] − ( Pd ) .…”

Unprecedented Allenylidene Transfer from Chromium to Tungsten

“…The alternative dissociative mechanisms involves initial and rate-determining Cr−( cis -CO) dissociation followed by reaction of the resulting allenylidene tetracarbonyl complex with [(CO) 5 W(THF)] to give a binuclear allenylidene-bridged (CO) 4 Cr−W(CO) 5 complex. Related vinylidene-bridged homobinuclear dichromium complexes have been prepared and structurally characterized . Subsequent rearrangement and reaction with THF and/or CO would afford the final products (Scheme ).…”

“…In intermediate E1 the Cr−W distance is 3.07 Å and corresponds to a Cr−W single bond. For comparison the W−W bond in, for example, [(CO) 5 W] 2 (μ-C(Ph)H) is 3.118(1) Å, and the Cr−Cr distance in the vinylidene-bridged complex [(CO) 5 Cr(μ-η 1 ,η 2 -CCMe 2 )Cr(CO) 4 ] is 3.000(1) Å . The “migrating” CO occupies a bridging position, but is closer to chromium than to the tungsten atom.…”

“…This step is exergonic by 14 kJ/mol. A related dynamic coordination site exchange of the CC and one CO ligand between the two chromium atoms (windshield wiper mechanism) in [(CO) 5 Cr(μ−η 1 ,η 2 -CCR 2 )Cr(CO) 4 ] (CR 2 = CMe 2 ; C(CH 2 ) 5 ; C(Me)Et; C(Me) t Bu) has been proposed to account for their NMR spectroscopic features . The energy barrier for dissociation of “Cr(CO) 4 ” from G1 to afford (CO) 5 WCCCMe 2 ] ( Ia ) again is very high (Figure ), rendering this pathway unlikely.…”

“…Related vinylidene-bridged homobinuclear dichromium complexes have been prepared and structurally characterized. 27 Subsequent rearrangement and reaction with THF and/or CO would afford the final products (Scheme 7).…”