Infrared Spectra of the CH₃−CrF, CH₂WHF, and CH⋮WH₂F Molecules:  Reversible Photochemical Interconversion by α-Hydrogen Transfer

Abstract

Group 6 methylmetal fluoride, methylidene, and methylidyne complexes (CH3−MF, CH2MHF, and CH⋮MH2F) are formed by reaction of the laser-ablated metal atoms and methyl fluoride during condensation in excess argon and have been identified by matrix infrared spectroscopy. The CH3−CrF molecule is much more stable and is the only product found for Cr, and all three comparable energy products are observed for Mo, but only the more stable CH2WHF and CH⋮WH2F forms are trapped for W. The last molecules are photoreversible, owing to α-hydrogen transfer between carbon and metal atoms. The methylidene complexes are formed on ultraviolet irradiation (240−380 nm) at the expense of the methylidyne complexes, and the process is reversed on visible irradiation (λ > 420 nm). Calculations show that one α-H is distorted toward the metal atom, which provides evidence of strong agostic interaction in the methylidene ground state molecules.

Observation and Characterization of CH3CH2–MH, (CH2)2–MH2, and CH3–C≡MH3 Prepared in Reactions of Ethane with Laser-Ablated Group 6 Metal Atoms

“…On the other hand, only the insertion product was observed in reactions of Cr, showing that the higher oxidation-state product was more favored in the heavier metal system. Photoisomerization between the observed Mo products is reversible, parallel to the previously investigated group 6 metal reactions with methane and methyl halides. , …”

“…The Mo and W ethylidenes are agostic (<HCM = 96.4 and 89.2°), but the Cr analogue apparently does not show the distortion. They are compared to 83.7, 83.5, and 85.0° for CH 3 CHTiH 2 , CH 3 CHZrH 2 , and CH 3 CHHfH 2 , 84.6, 85.3, 85.6, and 84.7° for CH 2 MoHF, CH 2 MoHCl, CH 2 MoHBr, and CH 2 MoHI, and 91.0, 85.1, 84.5, and 84.6° for CH 2 WH 2 , CH 2 WHF, CH 2 WHCl, and CH 2 WHBr . The earlier investigation by means of multireference calculations shows that the group 4 metal methylidene yields the largest agostic distortion and the extent of distortion decreases with moving right in the same row (<HCM angles 81.9, 85.7, and 101.8° for CH 2 ZrH 2 , CH 2 NbH 2 , and CH 2 MoH 2 ) .…”

Rotational analysis of the $3_0^1 $31 band of the $\tilde A{}^6\Sigma ^ + \leftarrow \tilde X{}^6\Sigma ^ + $Ã6Σ+←X̃6Σ+ system of CrCCH

“…Matrix isolation infrared spectra of many CH 3 MX and CH 2 MX 2 molecules are also known. [15][16][17][18][19][20] Moving to transition metal molecules containing two carbon atoms, the strength of the carbon-carbon bond dictates that the two carbons will be chemically bound to each other. The MC 2 molecules are expected to be highly ionic due to the high electron affinity of C 2 (EA = 3.269(6) eV).…”

Periodic Trends in the Agostic Interaction in Zirconium and Hafnium Methylidene Hydride Halide Complexes

“…The transition metals in Group VI form not only the methyl halide and methylidene complexes but the methylidyne ([CH≡MH 2 X]) complexes as well, and the products are also photoreversible by migration of an α hydrogen atom 9–12. The transition metals in Group V also produce the methylidene and methyl halide complexes and form anionic methylidyne complexes ([CH≡MH 2 X] − ) 13.…”