Reactions of some decaosmium clusters with electrophilic and nucleophilic reagents: X-ray structure analyses of [N(PPh3)2][Os10C(CO)24(μ-I)], [Os10C(CO)24(μ-I)2], [N(PPh3)2]2[Os10C(CO)22(NO)I], [Os10C(CO)23{P(OMe)3}-(μ-I)2] and of two isomers of [Os10C(CO)21{P(OMe)3}4]