Bridging methylene and π-allylic bonding modes in tetranuclear osmium clusters; the x-ray crystal structures of [Os₄H₂(CO) ₁₂CHCH₂Ph] and [Os₄H₂(CO) ₁₁MeCCPhCHPh]

[Os₄H₃(CO)₁₁CH:CHR] reacts with CO under pressure to give edge bridging methylenic compounds [OS₄H ₂(CO)₁₂CHCH₂R] and, when R=H, with acetylenes, R′C₂R″, to give π-allylic derivatives of the form [Os₄H₂-(CO)₁₁MeCCR′CHR″]; the structures of these two compounds have been confirmed by X-ray diffraction analyses.