Experimental and Theoretical Investigation of Simple Terminal Group 6 Arsenide As≡MF₃Molecules

Laser-ablated group 6 metal atoms react with NF3 and PF3 to form the simple lowest energy N MF3 and P=MX3 products, and this investigation has been extended to AsF3. Mo and W atoms react with AsF3 upon excitation by laser ablation or UV irradiation to form stable trigonal As MF3 terminal arsenides. These molecules are identified by comparison of the closely related infrared spectra of the analogous phosphide species and with frequencies calculated by density functional theory and multiconfigurational second order perturbation theory (CASSCF/CASPT2). Computed CASSCF/CASPT2 triple bond lengths for the As MoF3 and As WF3 molecules are 2.240 angstrom and 2.250 angstrom, respectively. The natural bond orders calculated by CASSCF/CASPT2 decrease from 2.67 to 2.60 for P MoF3 to As MoF3 and from 2.74 to 2.70 for P WF3 to As WF3 as the arsenic valence orbitals are less effective than those of phosphorus in bonding to each metal atom and the larger metal orbital size becomes more compatible with the arsenic valence orbitals. The Cr atom reaction gives the arsinidene AsF=CrF2 product instead of the higher energy As CrF3 molecule as the Cr (VI) state is not supported by the softer pnictides.