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 A, 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.