A combined theoretical and experimental study of simple terminal group 6 nitride and phosphide N MX3 and P MX3 molecules

Organometallic complexes containing terminal metal nitrides and phosphides are important synthetic reagents. Laser-ablated group 6 metal atoms react with NF3, PF3, and PCl3 to form the simple lowest energy N MF3, and P MX3 products following insertion and halogen transfer, with the exception of P CrF3, which is a higher energy species and is not observed. The E MX3 pnictide metal trihalide molecules are identified from both argon and neon matrix infrared spectra and frequencies calculated by density functional theory and multiconfigurational second-order perturbation theory (CASSCF/CASPT2). These simple terminal nitrides involve strong triple bonds, which range from 2.80 to 2.77 to 2.59 natural bond order for M = W, Mo, and Cr, respectively, as computed by CASSCF/CASPT2, and the M N stretching frequencies also follow this order. The terminal phosphides are weaker with bond orders 2.74, 2.67, and 2.18, respectively, as the more diffuse 3p orbitals are less effective for bonding to the more compact metal valence d orbitals.