The thermolysis of [Fe(L)2(CO)2(η2-CS2)] complexes (1) in refluxing benzene gives the blue tetrathiolene derivatives [Fe2(L)4(CO)2(C2S4)] (2) when L = P(OPh)3, P(OEt)3, P(OPri)3, PPh(OEt)2, PPh2(OEt). 2 are formed only when the phosphorus(III) ligands L readily dissociate from 1 and have cone angles between 109° and 133°. The two most stable are 2a {L = P(OPh)3, cone angle 128°} and 2c {L = P(OPri)3, cone angle 130°}. 2 have been characterised by IR, electronic and NMR spectroscopy and elemental analyses. Their UV-Vis spectra are dominated by an intense absorption band (ε = 13000–18000 dm3 mol−1 cm−1) with λmax = 640–690 nm. The labile triphenylphosphite ligands in 2a can be replaced in part by CO. Spectroscopic data confirms that the unstable product, 3a, contains a ligand set similar to that of 2a. Electrochemical studies show that 2a undergoes a one-electron reversible reduction to [2a]˙− and a one-electron, irreversible oxidation. The UV-Vis spectrum of [2a]˙− shows a very low energy absorption band at 2556 nm (3912 cm−1) with ε = ca. 3600 dm3 mol−1 cm−1 which is attributed to a charge transfer transition from the SOMO to an orbital with μ-C2S4 character. The ESR spectrum of [2a]˙− is consistent with the SOMO being an orbital of primarily dx2 − y2 character which is localised on one Fe atom. Under CO, one-electron reversible transfers are observed in the electrochemistry due to the redox series [3a]−/0/+. 2a is oxidised by halogens to [L2(OC)(X)Fe(S2C2S2)Fe(X)(CO)L2] derivatives.
