Early transition metal complexes containing 1,2,4-triazolato and tetrazolato ligands: synthesis, structure, and molecular orbital studies.

Several early transition metal complexes bearing 1,2,4-triazolato and tetrazolato ligands have been prepared by reaction of the pyrazolato complexes Ti(tBu(2)pz)(4-x)Cl(x) (tBu(2)pz = 3,5-di-tert-butylpyrazolato; x = 1, 2) and M(tBu(2)pz)(5-x)Cl(x) (M = Nb, Ta: x = 2, 3) with the sodium or potassium salts derived from 1,2,4-triazoles and tetrazoles. The X-ray structure analysis of Ti(tBu(2)pz)(2)(Me(2)C(2)N(3))(2) shows eta(2)-coordination of the 1,2,4-triazolato ligands, while in Ti(tBu(2)pz)(3)(C(2)H(2)N(3)) and Nb(tBu(2)pz)(3)(Me(2)C(2)N(3))(2) the analogous groups are joined in a eta(1)-fashion in the solid-state structure. Solution NMR studies at different temperatures suggest transition states involving eta(2)-1,2,4-triazolato ligands for the complexes containing eta(1)-1,2,4-triazolato ligands in the solid state. X-ray crystal structures of analogous tetrazolato complexes Ti(tBu(2)pz)(3)(PhCN(4)) and Nb(tBu(2)pz)(3)(PhCN(4))(2) show eta(1)-coordination of the 2-nitrogen atoms of the tetrazolato ligands. Molecular orbital calculations have been carried out on several model titanium complexes and provide detailed insight into the bonding between early transition metal centers and 1,2,4-triazolato and tetrazolato ligands. The eta(2)-coordination mode of 1,2,4-triazolato and tetrazolato ligands is predicted to be more stable than the eta(1)-coordination mode by 13.8-5.2 kcal/mol.