Infrared Spectroscopic Characterization of Early 4d Transition Metal Carbene Cations, ZrCH 2 + and NbCH 2 .

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 -C 3 H 6 ) 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, WCH 2 + and TaCH 2 + . A molecular orbital analysis traces the reasons for the agostic deformation from a classic C 2v symmetric carbene.