New ligand substitution modes of M2(μ-SR)2(CO)8 (M=Mo, W; R=alkyl, aryl): synthesis, structure and properties of new di-nuclear metal(I) carbonyl thiolates, [M2(μ-SPh)2(CO)xLy]n− (M=Mo, W; x=6, 8; y=0, 1; L=PhS, (Ph)2PCH2P(Ph)2 and S2CNEt2; n=1, 0) and a polymer [W2(μ-SPh)2(CO)6(Ph)2PCH2P(Ph)2]m