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(tBu2pz)4-xClx (tBu2pz = 3,5-di-tert-butylpyrazolato; x = 1, 2) and
M(tBu2pz)5-xClx (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(tBu2pz)2(Me2C2N3)2 shows η2-coordination of the 1,2,4-triazolato
ligands, while in Ti(tBu2pz)3(C2H2N3) and Nb(tBu2pz)3(Me2C2N3)2 the analogous groups are joined in a η1-fashion in the solid-state structure. Solution NMR studies at different temperatures suggest transition states involving
η2-1,2,4-triazolato ligands for the complexes containing η1-1,2,4-triazolato ligands in the solid state. X-ray crystal
structures of analogous tetrazolato complexes Ti(tBu2pz)3(PhCN4) and Nb(tBu2pz)3(PhCN4)2 show η1-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 η2-coordination mode of 1,2,4-triazolato and tetrazolato ligands
is predicted to be more stable than the η1-coordination mode by 13.8−5.2 kcal/mol.