Substituted osmium clusters containing labile ligands. The preparation, characterisation, and some reactions of [Os3H(OR)(CO)9(MeCN)](R = H, Me, Et, or Ph) and the isomers of [Os3H(SR)(CO)9(MeCN)](R = Me, Et, or Ph); the crystal and molecular structure of [Os3H(SPh)(CO)9(PEt3)]