Magnesium Cyanide or Isocyanide?

Preference for the binding mode of the CN(−) ligand to Mg (Mg−CN vs. Mg−NC) is investigated. A monomeric Mg complex with a terminal CN ligand was prepared using the dipyrromethene ligand (Mes)DPM which successfully blocks dimerization. While reaction of ((Mes)DPM)MgN(SiMe(3))(2) with Me(3)SiCN gave the coordination complex ((Mes)DPM)MgN(SiMe(3))(2)⋅NCSiMe(3), reaction with ((Mes)DPM)Mg(nBu) led to ((Mes)DPM)MgNC⋅(THF)(2). A Mg−NC/Mg−CN ratio of ≈95:5 was established by crystal‐structure determination and DFT calculations. IR studies show absorbances for CN stretching at 2085 cm(−1) (Mg−NC) and 2162 cm(−1) (Mg−CN) as confirmed by (13)C labeling. In solution and in the solid state, the CN ligand rotates within the pocket. The calculated isomerization barrier is only 12.0 kcal mol(−1) and the (13)C NMR signal for CN decoalesces at −85 °C (Mg−NC: 175.9 ppm, Mg−CN: 144.3 ppm). Experiment and theory both indicate that Mg complexes with the CN(−) ligand should not be named cyanides but are more properly defined as isocyanides.