High oxidation state transition-metal complexes with a carbon-metal double bond have proven important for understanding the nature of metal coordination and for catalysts in alkene-metathesis and alkane-activation reactions. [1][2][3] Several early transition-metal alkylidenes are agostic, [1] and these compounds provide the opportunity to characterize the agostic interaction of hydrogen to a transition-metal center in a simple carbene complex (most agostic interactions involve much more complicated systems) and to help understand the important alkane CÀH bond-activation process (a key step in many transition-metal catalyzed reactions). [4][5][6][7][8] The simplest compound of this type is the methylidene complex, [H 2 C = MH 2 ], which is an ideal model system to examine substituent effects and the agostic interaction. Such Group 4 compounds have been investigated by electronicstructure calculations using small basis sets and shown to have symmetrical structures without agostic interactions. [9][10][11] Recently we have treated laser-ablated Ti, Zr, and Hf atoms with CH 3 F and prepared the fluorine substituted [H 2 C = MHF] derivatives. [12][13][14] These methylidene species result from ahydrogen migration in the initial [CH 3 MF] intermediate. Our electronic-structure calculations using basis sets with polarization functions reveal a tilted CH 2 group and provide evidence for agostic interaction in the [H 2 C=MHF] molecules.The simple [H 2 C = MH 2 ] analogues contain two intense infrared chromophores, namely the CH 2 and MH 2 groups, which are sensitive to the distortion associated with intramolecular agostic bonding. Hence the IR spectrum of [H 2 C= ZrH 2 ] can provide evidence for agostic interaction through symmetry lowering in the molecule, as can computations using large basis sets. In particular, a simple count of [CHD = ZrHD] isotopomers provides a ready method to confirm agostic interactions in this simplest possible carbene complex, as only two isotopomers (cis and trans) will be formed if there is no agostic interaction (C 2v or C s symmetry), but four isotopomers (cis and trans for H and for D in the agostic position) if there is agostic interaction (C 1 symmetry). We report herein the first experimental characterization of [H 2 C = ZrH 2 ] and demonstrate the agostic CÀH···Zr interaction from the IR spectrum in solid neon and from density functional structure and frequency calculations.The laser-ablation matrix-IR experiment has been described elsewhere. [15,16] Briefly, laser-ablated zirconium atoms (Johnson-Matthey) reacted with CH 4 (Matheson), 13 CH 4 , CD 4 , CH 2 D 2 (Cambridge Isotopic Laboratories) in excess neon (Spectra Gases) during condensation on a CsI window at 5 K. IR spectra were recorded at 0.5 cm À1 resolution on a Nicolet 750 spectrometer with HgCdTe detector. Samples were irradiated by a mercury arc lamp (175 W, globe removed), were annealed, and more spectra were recorded.Complementary density functional theory (DFT) calculations were performed using the Gaussian 98 package, [...