Gallium and indium hydrazides. Molecular and electronic structure of In[N(SiMe₃)NMe₂]₃ and related compounds

Gallium and indium hydrazides, Ga[N(SiMe₃)NMe₂]₃ (1) and In [N(SiMe₃)NMe₂]₃ (2), were synthesized from the reactions of metal chlorides and Li[N(SiMe₃)NMe₂]. Single crystal X-ray crystallographic analysis revealed that compound 2 was monomeric with trigonal planar geometries on the indium and the indium-bonded nitrogen atoms. The average In-N bond distance of 2.078(3) Å and the N-In-N-N dihedral angles did not provide clear structural evidence of In-N π-bonding. The electronic absorption spectra of the indium hydrazido complex revealed transitions at significantly lower energies compared to those observed in the tris(amido) compounds, In[N(SiMe₃)₂]₃ (3) and In[N(^tBu)(SiMe₃)]₃ (4). The absorptions of the indium and gallium compounds were attributed to ligand-metal charge transfer transitions. Trends in the electronic transitions for compounds 2 and 3 calculated at the time-dependent density functional and configuration interaction including single excitations levels, both using a minimal basis set, were consistent with the experimental data, and Mulliken charge analyses support the assignment to ligand-to-metal charge transfer transitions. These calculations also demonstrated the presence of π-bonding between the In and N p-orbitals, and an analogy is drawn to the frontier molecular orbitals of trimethylenemethane. The low-lying spectroscopic transition in 2, and thus its yellow color, results from mixing of the lone pair electrons on the β-nitrogens of the hydrazido ligands with the HOMO of the InN₃ core.