Diradical Character of Neutral Heteroleptic Bis(1,2-dithiolene) Metal Complexes: Case Study of [Pd(Me(2)timdt)(mnt)] (Me(2)timdt = 1,3-Dimethyl-2,4,5-trithioxoimidazolidine; mnt(2–) = 1,2-Dicyano-1,2-ethylenedithiolate)

[Image: see text] The reaction of the bis(1,2-dithiolene) complex [Pd(Me(2)timdt)(2)] (1; Me(2)timdt(•–) = monoreduced 1,3-dimethyl-2,4,5-trithioxoimidazolidine) with Br(2) yielded the complex [Pd(Me(2)timdt)Br(2)] (2), which was reacted with Na(2)mnt (mnt(2–) = 1,2-dicyano-1,2-ethylenedithiolate) to give the neutral mixed-ligand complex [Pd(Me(2)timdt)(mnt)] (3). Complex 3 shows an intense solvatochromic near-infrared (NIR) absorption band falling between 955 nm in DMF and 1060 nm in CHCl(3) (ε = 10700 M(–1) cm(–1) in CHCl(3)). DFT calculations were used to elucidate the electronic structure of complex 3 and to compare it with those of the corresponding homoleptic complexes 1 and [Pd(mnt)(2)] (4). An in-depth comparison of calculated and experimental structural and vis–NIR spectroscopic properties, supported by IEF-PCM TD-DFT and NBO calculations, clearly points to a description of 3 as a dithione-dithiolato complex. For the first time, a broken-symmetry (BS) procedure for the evaluation of the singlet diradical character (DC) of heteroleptic bis(1,2-dithiolene) complexes has been developed and applied to complex 3. The DC, predominant for 1 (n(D)(C) = 55.4%), provides a remarkable contribution to the electronic structures of the ground states of both 3 and 4, showing a diradicaloid nature (n(D)(C) = 24.9% and 27.5%, respectively). The computational approach developed here clearly shows that a rational design of the DC of bis(1,2-ditiolene) metal complexes, and hence their linear and nonlinear optical properties, can be achieved by a proper choice of the 1,2-dithiolene ligands based on their electronic structure.