Group 2 metallocene carbene complexes

In 1998, the same group reported the synthesis of magnesium metallocene NHC complexes, in addition to metallocene NHC complexes of other group 2 elements.21 The adducts show an interesting trend in the nature of the metal-carbene bonds which increase as metal radii increase. The trends are reflected in both the solid state structures (metal-carbene bond length increases) and the NMR spectra (downfield shift of C2) of the adducts (Table 1). In addition to mono-NHC complexes, the heavier alkaline earth elements (Sr and Ba) are capable of forming stable bis-NHC adducts. 

E. O. Fischer, F. J. Gammel, J. O. Besenhard, and A. Frank, and D. Neugebauer, J. Organomet. Chem., 1980, 191, 261. Transition-metal carbyne complexes LIV. New metallocene-carbene and carbyne complexes of Group VI, preparation and electrochemical characteristics. 

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