(Trimethylsilyl)diazomethane (1-H) reacts with nBuLi in THE at elevated temperature to afford (previously reported) 1-Li.(3)/2THF. However, reaction in hexane/TMEDA at low temperature affords instead the N-lithiate Me3SiCNNLi-TMEDA (9), which is a novel "open" pseudo-cubic tetramer in the solid state. Variable-temperature NMR spectroscopy suggests that N-metallated 9, apparently the kinetic product of the reaction, irreversibly rearranges at high temperature in solution to give the thermodynamically preferred C-lithiated isomer. These observations, supported by DFT calculations, influence our understanding of the reactivity of lithiated di-azomethanes towards aryl isothiocyanates, suggesting as they do that previously observed product selectivity in these reactions is critically dependent on temperature control exercised during the process. (C) Wiley-VCH Verlag GmbH Co.