Cyanide ion as a four-electron donating bridging ligand in a dimanganese compound

Abstract

Reaction of [Mn₂(CO)₅(dppm)₂](dppm = Ph₂PCH₂PPh₂) with HBF₄·Et₂O in the presence of acetonitrile gives the complex [Mn₂H(CO)₅(NCMe)(dppm)₂][BF₄] which reacts with cyanide ion to give [Mn₂H(CN)(CO)₅(dppm)₂], shown by ¹H, and ¹³C (90% enriched ¹³CN) n.m.r. to contain a terminal cyanide ligand. Decarbonylation with Me₃NO·2H₂O gives [Mn₂H(CN)(CO)₄(dppm)₂], n.m.r. spectra of which at –90 °C are consistent with an asymmetrically bridging CN, probably C-bonded at one Mn atom and η²-bonded at the other. The ¹³C n.m.r. signal with coupling to ³¹P changes from a 1 : 4 : 6 : 4 : 1 quintet (J= 10.8 Hz) at 25 °C to a 1 : 2 : 1 triplet (J= 22.1 Hz) at –90 °C because the CN group is oscillating between the two metal atoms.