A new molybdenum-tungsten-sulphur cluster complex family, [{(OC)4M}xM′S4]2− (M = Mo or W, M′ = Mo or W; x = 1,2). Synthesis, structure and properties of a new trinuclear mixed-valence MoS cluster, [Et4N]2[(OC)4MoS2MoS2Mo(CO)4]