Theoretical Study of Transition Metal Compounds with Molybdenum- and Tungsten-Phosphorus Triple Bonds

Quantum mechanical calculations at the HF, MP2, DFT (B3LYP), and CCSD(T) levels of theory using quasirelativistic effective core potentials for the metal and valence basis sets of DZP quality are reported for the transition metal complexes [M(P)(NH₂)₃] (1, 2), [M(PS)(NH₂)₃] (3, 4), [M(P)(NH₂)₃(NH₃)] (5, 6), [M(P)(N₃N)] (7, 8; N₃N = [(HNCH₂CH₂)₃N]^3-), and [(M(PS)(NH₂)₃(NH₃)] (9, 10) with M = Mo, W. The B3LYP-optimized geometries of 1-10 are in good agreement with experiment. Bond dissociation energies for the L_nMP-S bonds calculated at B3LYP are 8-10 kcal/mol higher than the CCSD(T) values. The L_nMP-S and M-NH₃ bonds of 9 and 10 are predicted to be stronger than the respective bonds of 3-6. ³¹P NMR chemical shifts and the anisotropic components have been calculated using the IGLO and GIAO approaches. The results are in accord with experimental data. The bonding situation of the complexes has been analyzed with the help of the NBO partitioning scheme. The phosphide complexes L_nMP have metal≡P triple bonds, while the phosphorus-sulfide complexes have L_nM=P=S double bonds. This formally reduces the number of coordination sites at the metal, which explains the significantly shorter and stronger bond with an amine trans to the M=P=S moiety.