Unprecedented silicon(II)→calcium complexes with N-heterocyclic silylenes.

The first N-heterocyclic silylene (NHSi) complexes of any s-block element to date are reported for calcium: [(η(5)-C5Me5)2Ca←:Si(O-C6H4-2-(t)Bu){(N(t)Bu)2CPh}] (6) and [(η(5)-C5Me5)2Ca←:Si(N(t)BuCH)2] (7). Complexes 6 and 7 are isolable in a facile way upon reaction of the corresponding free N-heterocyclic silylenes (NHSis) with [(η(5)-C5Me5)2Ca] (2). Complexes 6 and 7 were fully characterised by spectroscopic means and the single crystal X-ray diffraction analysis of 6 is also reported. Analysis of the bonding situation by DFT methods including a Bader Atoms in molecules (AIM) analysis is also reported. The bonding interaction between the Si and Ca centres in complexes 6 and 7 can best be viewed as σ-donor-acceptor interactions, with a considerable ionic contribution in the bond. The reactivity towards the oxygen containing substrates THF and benzophenone is also discussed.