Infrared spectra and electronic structures of agostic uranium methylidene molecules.

Through reactions of laser-ablated uranium atoms with methylene halides CH2XY (XY = F2, FCl, and Cl2), a series of new actinide methylidene molecules CH2UF2, CH2UFCl, and CH2UCl2 are formed as the major products. The identification of these complexes has been accomplished via matrix infrared spectra, isotopic substitution, and relativistic density functional calculations of the vibrational frequencies and infrared intensities. Density functional calculations using the generalized gradient approach (PW91) show that these CH2UXY methylidene complexes prefer highly distorted agostic structures rather than the ethylene-like symmetric structures. The calculated agostic angles ([angle]H-C-U) are around 89 degrees for all the three uranium complexes, and the predicted vibrational modes and isotopic shifts agree well with experimental values. Electronic structure calculations reveal that these U(IV) molecules all have strong C=U double bonds in the triplet ground states with 5f (2) configurations. The calculated bond lengths and bond energies indicate that the C=U double bonds are slightly weaker in the fluoride species than in the chloride species because of the radial contraction of the U (6d) orbitals by the inductive effect of the fluorine substituent. The agostic uranium methylidene complexes are compared with analogous transition metal and thorium complexes, which reveal interesting differences in their chemistries.