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 tetra zolato ligands have been prepared by reaction of the pyrazolato complexes T i(tBu(2)pz)(4-x)Cl-x: (tBu(2)pz = 3,5-di-tert-butylpyrazolato; x = 1, 2) an d 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 anal ysis of Ti(tBu(2)pz)(2)(Me2C2N3)(2) shows eta (2)-coordination of the 1,2,4 -triazolato groups are joined ligands, while in Ti(tBu(2)pz)(3)(C2H2N3) and Nb(tBu(2)pz)(3)(Me2C2N3)(2) the analogous eta (1)-fashion in the solid-sta te structure, Solution NMR studies at different temperatures suggest transi tion states involving eta (2)-1,2,4-triazolato ligands for the complexes co ntaining eta (1)-1,2,4-triazolato ligands in the solid state. X-ray crystal structures of analogous tetrazolato complexes Ti(tBu(2)pz)(3)(PhCN4) and N b(tBu(2)pz)(3)(PhCN4)(2) show eta (1)-coordination of the 2-nitrogen atoms of the tetrazolato ligands. Molecular orbital calculations have been carrie d out on,several model titanium complexes and provide detailed insight into the bonding between early transition metal centers and 1,2,4-triazolato an d tetrazolato ligands. The eta (2)-coordination mode of 1,2,4-triazolato an d tetrazolato ligands is predicted to be more stable than the eta (1)-coord ination mode by 13.8-5.2 kcal/mol.