A new generation of simple methylidene complexes has been prepared by reactions of excited group 4−6 transition metal atoms with methyl halides and methane in solid argon. These CH2MHX (X = F, Cl, Br, I) and CH2MH2 methylidene complexes exhibit agostic bonding effects of CH2 and MH2 distortion. The reactions proceed through the CH3MX insertion product followed by α-H transfer on an excited potential energy surface. The higher valence of group 6 metals sustains a second α-H transfer to form the CH⋮MH2X (M = Mo, W, X = H, F, Cl, Br) methylidyne complexes, and electron capture by group 5 CH2MHX (M = Nb, Ta, X = H, F, Cl, Br) methylidene complexes gives rise to the analogous CH⋮MH2X- methylidyne anion complexes. These simple organometallic complexes are identified by matrix infrared spectra through isotopic substitution and by comparison with vibrational characteristics calculated by DFT. Periodic trends in agostic interactions are illustrated for different metals and halogen substituents. Complementary investigations for group 3 and for group 7−9 transition metals and for early lanthanide and actinide metals are also discussed and compared.

Laser-ablated W atoms react with CH4 in excess argon to form the CH3-WH, CH2=WH2, and CH[triple bond]WH3 molecules with increasing yield in this order of product stability. These molecules are identified from matrix infrared spectra by isotopic substitution. Tungsten methylidene and methylidyne hydride molecules are reversibly interconverted by alpha-H transfers upon visible and ultraviolet irradiations. Matrix infrared spectra and DFT/B3LYP calculations show that CH[triple bond]WH3 is a stable molecule with C3v symmetry, but other levels of theory were required to describe agostic distortion for CH2=WH2. Analogous reactions with Cr gave only CH3-CrH, which is calculated to be by far the most stable product.

Reaction of laser-ablated Zr with CH(4) ((13)CH(4), CD(4), and CH(2)D(2)) in excess neon during condensation at 5 K forms CH(2)=ZrH(2), the simplest alkylidene hydride complex, which is identified by infrared absorptions at 1581.0, 1546.2, 757.0, and 634.5 cm(-)(1). Density functional theory electronic structure calculations using a large basis set with polarization functions predict a C(1) symmetry structure with agostic C-H- - -Zr bonding and distance of 2.300 A. Identification of the agostic CH(2)=ZrH(2) methylidene complex is confirmed by an excellent match of calculated and observed isotopic frequencies particularly for the four unique CHD=ZrHD isotopic modifications. The analogous reactions in excess argon give two persistent photoreversible matrix configurations for CH(2)=ZrH(2). Finally, methane activation by CH(2)=ZrH(2) gives the new (CH(3))(2)ZrH(2) molecule.

The photoreversible system of titanium methylidene and methyltitanium fluoride complexes has been formed by the reaction of methyl fluoride and laser-ablated Ti atoms and isolated in a solid argon matrix. There are two major groups of absorptions based on broad-band irradiation and annealing behavior. The increase of group I is accompanied by the decrease of group II on photolysis with UV light (240 nm < λ < 380 nm) and the reverse with visible light (λ > 530 nm). The methylidene CH 2 dTiHF and methyltitanium fluoride CH 3 -TiF are responsible for these absorptions, and the photochemical interconversion occurs via R-H migration between carbon and titanium atoms. The methylidene complex appears to be stabilized by an agostic hydrogen interaction. Absorptions for the higher coordinated (CH 3 ) 2 TiF 2 complex are observed to increase on UV irradiation and on annealing where CH 3 TiF spontaneously adds CH 3 F.

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