Characterization of the diradical *NSNSC-CNSSN* and [NSNSC-CNSSN][MF6]n (n=1, 2). The first observation of an excited triplet state in dimers of 7pi -CNSSN* radicals.

Preparation and full characterization of the main-group diradical *NSNSC-CNSSN*, 8, the MF6- salt (As, Sb) of radical cation +NSNSC-CNSSN*, 8*+, and the AsF6- salt of the dication +NSNSC-CNSSN+, 82+, are presented. 8, a=6.717 (4), b=11.701(2), c=8.269(3) A, alpha=gamma=90, beta=106.69(3) degrees, monoclinic, space group P21/n, Z=4, T=203 K; 8SbF6, a=6.523(2), b=7.780(2), c=12.012(4) A, alpha=91.994(4), beta=96.716(4), gamma=09.177(4) degrees, triclinic, space group P, Z=2, T=198 K; 8[AsF6]2, a=12.7919(14), b=9.5760(11), c=18.532(2) A, alpha=gamma=90, beta=104.034(2) degrees, monoclinic, space group Pn, Z=6, T=198 K. Preparation of 8MF6 was carried out via a reduction of [CNSNS]2[MF6]2 (M=As, Sb) with either ferrocene or a SbPh3-NBu4Cl mixture. In the solid state, diamagnetic 8SbF6 contains centrosymmetric dimers [8*+]2 linked via two-electron four-centered pi*-pi* interactions with a thermally excited triplet state as detected by electron paramagnetic resonance (EPR). This is the first observation of a triplet excited state for a 7pi 1,2,3,5-dithiadiazolyl radical dimer. The singlet-triplet gap of the [-CNSSN*]2 radical pair was -1800+/-100 cm(-1) (-22+/-1 kJ/mol) with the ZFS components |D|=0.0267(6) cm(-1) and |E|=0.0012(1) cm(-1), corresponding to an in situ dimerization energy of ca. -11 kJ/mol. Cyclic voltammetry measurements of 8[AsF6]2 showed two reversible waves associated with a stepwise reduction of the two isomeric rings [E1/2 (+2/+1)=1.03 V; E1/2 (+1/0)=0.47 V, respectively]. 8MF6 (M=As, Sb) was further reduced to afford the mixed main-group diradical 8, containing two isomeric radical rings. In solution, 8 is thermodynamically unstable with respect to *NSSNC-CNSSN*, but is isolable in the solid state because of its low solubility in SO2. Likewise, 8SbF6, 8 is dimeric, with pi*-pi* interactions between different isomeric rings, and consequently diamagnetic; however, a slight increase in paramagnetism was observed upon grinding [from C=6.5(3)x10(-4) emu.K/mol and temperature-independent paramagnetism (TIP)=1.3(1)x10(-4) emu/mol to C=3.2(1)x10(-3) emu.K/mol and TIP=9.0(1)x10(-4) emu/mol], accompanied by an increase in the lattice-defect S=1/2 sites [from 0.087(1) to 0.43(1)%]. Computational analysis using the multiconfigurational approach [CASSCF(6,6)/6-31G*] indicated that the two-electron multicentered pi*-pi* bonds in [8*+]2 and [8]2 have substantial diradical characters, implying that their ground states are diradicaloid in nature. Our results suggest that the electronic structure of organic-radical ion pairs, for example, [TTF*+]2, [TCNE*-]2, [TCNQ*-]2, [DDQ*-]2, and related pi dimers, can be described in a similar way.