Metallo-dithiolene non-innocence is explored in an oxomolybdenum-bis(dithione) complex, [Mo4+O(i-Pr2Pipdt)2Cl][PF6] (where i-Pr2Pipdt is N,N’-piperazine-2,3-dithione), that possesses a piperazine ring as an integral part of the dithiolene ligand. The title complex displays unusual spectroscopic features for a formally reduced Mo(IV) dithiolene complex, namely a low energy metal-to-ligand charge transfer band with appreciable intensity and C-C and C-S stretching frequencies that are markedly different from those of oxomolydenum complexes coordinated to dianionic dithiolene ligands. The electronic structure of the ligand has been described in valence bond terms as a resonance hybrid of dithione and dizwitterionic dithiolene contributing structures.

The syntheses of cationic nickel complexes using N,N’-dimethyl piperazine 2,3-dithione (Me2Dt0) and N,N’-diisopropyl piperazine 2,3-dithione(iPr2Dt0) ligands are reported. These ligands were used in synthesizing bis- and tris-dithione nickel(II) complexes as tetrafluroborate or hexafluorophosphate salts i.e., [Ni(iPr2Dt0)2][BF4]2 ([1a][BF4]2), [Ni(iPr2Dt0)2][PF6]2 ([1a][PF6]2), [Ni(Me2Dt0)2][BF4]2 ([1b][BF4]2), [Ni(iPr2Dt0)3][BF4]2 ([2a][BF4]2) and [Ni(iPr2Dt0)3][PF6]2 ([2a][PF6]2), respectively. Complex [2a][PF6]2 was isolated from a methanolic solution of [1a][PF6]2. Compound [1a][BF4]2, crystallizes in a trigonal crystal system (space group, P31/c) and exhibits unique packing features, whereas [2a][BF4]2 crystallizes in a monoclinic (P21/n) space group. Cyclic volt-ammograms of [1a][BF4]2, and [1b][BF4]2 are indicative of four reduction processes associated with stepwise single-electron reduction of the ligands. Spectroelectrochemical experiments on [1a][BF4]2 exhibit an inter-valence charge transfer (IVCT) transition as a spectroscopic signature of the mixed-valence [Ni(iPr2Dt0)(iPr2Dt1−)]− species. Analysis of this IVCT band suggests this ligand based mixed valence complex, [Ni(iPr2Dt0)(iPr2Dt1−)]−, behaves more like a traditional class II/III metal based mixed-valence complex. The density functional theory (DFT) and time dependent DFT calculations provide a theoretical framework for understanding the electronic structures and the nature of excited states of the target compounds that are consistent with their spectroscopic and redox properties. Vibrational spectra of [1a]2+ and [2a]2+ were investigated as discrete species in the gas phase using infrared multiple photon dissociation (IRMPD) spectroscopy.

Interligand charge transfer is examined in the novel metallo-dithiolene complex MoO(SPh)2(iPr2Dto) (where iPr2Dto is N,N′-isopropyl-piperazine-2,3-dithione). The title complex displays a remarkable 70° “envelope” type fold of the five member dithiolene ring, which is bent upward toward the terminal oxo ligand. A combination of electronic absorption and resonance Raman spectroscopies have been used to probe the basic electronic structure responsible for the large fold-angle distortion. The intense charge transfer transition observed at ~18,000 cm−1 is assigned as a thiolate → dithione ligand-to-ligand charge transfer (LL′CT) transition that also possesses Mo(IV) → dithione charge transfer character. Strong orbital mixing between occupied and virtual orbitals with Mo(x2−y2) orbital character derives from a strong pseudo Jahn-Teller effect, which drives the large fold angle distortion to yield a double well potential in the electronic ground state.

A new dithiolene ligand with 3,5-dibromo substituted phenyl groups was designed and synthesized. The protected form of the ligand was reacted with a nickel salt providing neutral Ni(S2C2(3,5-C6H3Br2)2)2 and anionic [Ni(S2C2(3,5-C6H3Br2)2)2]− isolated as a Bu4N+ salt. Both were characterized by UV-visible and IR spectroscopy and compared with the similar known molecular systems. They exhibit intense low-energy transitions that are characteristic of such systems. The electrochemical behavior of these molecules was investigated by cyclic voltammetry.

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