A Unique Barium−Carbon Bond: Mechanism of Formation and Crystallographic Characterization

The reaction of tetrakis(tetrahydrofuran- O )barium bis[bis(trimethylsilyl)phosphanide] with diphenylbutadiyne yields dimeric (tetrahydrofuran- O )barium 2,5-diphenyl-3-(1,4-diphenylbutene-3-yne-2-ide-1-yl)-4-trimethylsilyl-1-phosphacyclopentadienide 1 . The alkenide moiety bridges two barium atoms, with Ba−C bond lengths of 2.881 and 3.071 Å, thus forming a unique three-center two-electron Ba−C−Ba bond. Furthermore, the coordination sphere of the alkaline earth metal atom contains the phosphacyclopentadienide moiety as well as one tetrahydrofuran molecule. A loose side-on coordination of the alkyne moiety completes the coordination sphere of the barium center. A similar reaction with magnesium bis[bis(trimethylsilyl)phosphanide] gives the dimeric addition product magnesium bis(trimethylsilyl)phosphanide 1,4-diphenyl-1-bis(trimethylsilyl)phosphanyl-but-1-ene-3-yne-2-ide 2 . A reaction mechanism for the formation of the barium derivative is suggested.