Study of Electronic and Vibrational Structures of Reduced, Neutral, and Oxidized Ni₃(dpa)₄X₂ Using Density Functional Theory and Raman Spectroscopy

The electronic and vibrational structures of trinickel metal string complexes [Ni₃(dpa)₄X₂]^1–,0,1+ (X = Cl, NCS) were investigated using both theoretical calculations and spectroscopic methods. We used the density functional theory (DFT) method B3LYP*-D3, including less exact exchange energy and the van der Waals interaction of metal ions, to obtain the geometries and vibrational structures, which were found to be in excellent agreement with the experimental data. The ground state of Ni₃(dpa)₄X₂ is an antiferromagnetic (AF) singlet state, and the next state is a quintet state, which was detected using temperature-dependent Raman spectroscopy under a magnetic field. The vibrational structure of the quintet state is nearly identical to that of the AF state, according to the measured Raman spectra, except that the stretching of Ni–Cl is blue-shifted from 282.5 cm^–1 in the AF state to 283.8 cm^–1 in the quintet state. Two oxidized Ni₃ complexes were found to have [Ni₃]⁷⁺ cores, the doublet [Ni₃(dpa)₄]³⁺ without axial ligands and the quartet [Ni₃(dpa)₄X₂]⁺. Complex [Ni₃(dpa)₄X₂]⁻, which was produced from a reduction reaction by gold nanoparticles at room temperature, consists of a quartet state as the ground state and a doublet state lying nearby.