Alkaline Earth Metal Superatom of W@Si₁₆: Characterization of Group 6 Metal Encapsulating Si₁₆ Cage on Organic Substrates

Transition metal atom (M)-encapsulating silicon cage nanoclusters (M@Si₁₆) exhibit a superatomic nature, depending on the central M atom owing to the number of valence electrons and charge state on organic substrates. Since M@Si₁₆ superatom featuring group 4 and 5 transition metal atoms exhibit rare-gas-like and alkali-like characteristics, respectively, group 6 transition metal atoms are expected to show alkaline earth-like behavior. In this study, M@Si₁₆, comprising a central atom from group 6 (M^VI = Cr, Mo, and W) were deposited on C₆₀ substrates, and their electronic and chemical stabilities were investigated in terms of their charge state and chemical reactivity against oxygen exposures. In comparison to alkali-like Ta@Si₁₆, the extent of charge transfer to the C₆₀ substrate is approximately doubled, while the oxidative reactivity is subdued for M^VI@Si₁₆ on C₆₀, especially for W@Si₁₆. The results show that a divalent state of M^VI@Si₁₆²⁺ appears on the C₆₀ substrate, which is consistently calculated to be a symmetrical cage structure of W@Si₁₆²⁺ in C_3v, revealing insights into the “periodic law” of M@Si₁₆ superatoms pertaining to the characteristics of alkaline earth metals.