Macrocycles incorporating sulfur and nidocarborane cages: reactivity toward nickel(II) and palladium(II). Molecular structures of Pd{7,8-.mu.-(S(CH2CH2OCH2CH2OCH2CH2OCH2CH2)S)C2B9H10}2 and Pd{P(C6H5)3}Cl{7,8-.mu.-(SCH2CH2S)C2B9H10}