Infrared Spectroscopic Characterization of Early 4d Transition Metal Carbene Cations, ZrCH₂⁺ and NbCH₂⁺

IR multiple-photon dissociation (IRMPD) action spectroscopy is combined with quantum chemical calculations to examine the [M,C,2H]⁺ species for the early 4d metals, M = Zr and Nb. These ions were formed by reacting laser ablated M⁺ ions with cyclopropane (c-C₃H₆) in a molecular beam apparatus. Both IRMPD spectra exhibit one major band near 700 cm^-1 and a second weaker band at about twice that wavenumber, more evident when irradiated in focus. The [Nb,C,2H]⁺ species also has a sharp band at 800 cm^-1. Comparison with B3LYP calculations allow assignment of the [M,C,2H]⁺ structures to agostic carbenes, which is similar to the structures found for the 5d analogues, WCH₂⁺ and TaCH₂⁺. A molecular orbital analysis traces the reasons for the agostic deformation from a classic C_2v symmetric carbene.