Trifluoromethylisocyanide, CF3NC, can be prepared by elimination of halogene from CF3N=CX2 (X=Cl, Br) with magnesium in THF. Both conformers of CF3N=CHF, CF3N=CHCl and CF3N=CHBr are formed when the hydrogen halides are reacted with isocyanide. The equilibrium of the Z and E conformer of these compounds has been studied by variable temperature nmr measurements. The N-trifluoromethylhalogenemethanimenes dimerize slowly forming the corresponding aminomethanimines CF3N=C(H)N(CF3)(CX2H). CF3NC is a strong [pi]-accepting ligand. This has been demonstrated by the synthesis of several transition metal complexes like Ni(CNCF3)4, Ni2Cp2(CNCF3)2, CpCo(CNCF3)2 and CpCoI2(CNCF3). In competition with the carbonyl-ligand CF3NC seems to prefer the bridging position. This is demonstrated in Fe3([mu]-CNCF3)(CO)11, which contains a stable trifluoromethylisocyanide bridge, both in the solid state and in solution. Reaction of Cp(CO)2Mo[triple bond; length as m-dash]Mo(CO)2Cp with CF3NC in the molar ratio 1:1 leads to formation of Mo2Cp2(CO)4([eta]2-[mu]-CNCF3). According to the spectroscopic data the bridging CF3NC ligand functions as a four electron donor. If an excess of CF3NC is used [MoCp(CO)2([mu]-CNCF3)]2 can be isolated as an unstable product. [MoCp(CO)2([mu]-CNCF3)]2 undergoes cleavage of the metal--- metal-bond and carbon-carbon-bond formation between the two coordinated isocyanide ligands. The new F3CN=C=C=NCF3 ligand bridges two MoCp(CO)2 fragments.
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