Absorption Spectra and Electrical Resistivities of Bis(1,2-dione dioximato)nickel(II) Complexes at High Pressures

The absorption spectra of [Ni(dmg)2], [Ni(niox)2], [Ni(dpg)2], and [Ni(bqd)2] were studied at various pressures. Two characteristic absorption bands, the metal-to-ligand charge-transfer (M–L) and the 3d–4p bands, were observed in the visible region for these complexes. The interesting spectral features observed under high pressures were: (1) The excitations shift remarkably to lower energies with increasing pressure. (2) The rates of the peak shifts for the d–p and M–L bands increase in the order [Ni(niox)2] < [Ni(dmg)2] < [Ni(dpg)2]. (3) The peak shifts of the M–L bands are much smaller than those of the d–p bands. (4) The absorption intensity of the M–L band of [Ni(niox)2] decreases rapidly above 10 GPa, and the band is hardly observed at around 14 GPa. (5) A new absorption band in [Ni(dpg)2] exists above about 3 GPa. The electrical resistivity of [Ni(dmg)2], [Ni(niox)2], and [Ni(dpg)2] has been studied under high pressures. These complexes are insulators with a resistivity of 1015 Ω cm at atmospheric pressure. The resistivities at around 23 GPa were 50 Ω cm for [Ni(dmg)2], 7 × 105 Ω cm for [Ni(niox)2] and 2 × 103 Ω cm for [Ni(dpg)2].