Crystal structure of tetraethylammonium bis(tetracarbonylmolybdenum)-tetrathiotungstate, [Et4N]2[(OC)4MoS2WS2Mo(CO)4] and synthesis, infrared spectra, 95 Mo NMR spectra and cyclic voltammetry of new linear trinuclear Mo(W)–S carbonyl clusters containing mixed-valence metal atoms [(OC)4MS2M′SM(CO)4]2