Variable-temperature carbon-13 nuclear magnetic resonance spectra of phosphine-substituted dodecarbonyl-triangulo-triosmium

13 C N.m.r. spectra of the compounds [Os3(CO)12–n(PEt3)n](n= 1 or 2) indicate that carbonyl exchange occurs selectively along one edge of the Os3, triangle at low temperatures. The process involves only six of the 12 –n carbonyl groups in [Os3(CO)12–n(PEt3)n](n= 1 or 2), and a mechanism involving a double carbonyl bridge is proposed.