The molybdenum-mercury bond. A multinuclear (1H, 13C, 31P, 95Mo and 199Hg) n.m.r. study of (η5-C5H5)(CO)2LMoHgX [L = CO, P(OMe)3 and PPh3; X = Cl, Br, I, S2COEt, S2P(OEt)2, S2CNEt2 and Mo(CO)3(η5-C5H5)] complexes and the crystal and molecular structure of (η5-C5H5)(CO)3MoHgS2CNEt2