Synthesis and structural characterisation of the cyclosiloxymagesium compound [(py) 2 Li] 2 -µ-Mg[{Ph 2 SiO} 2 O][{Ph 2 SiO} 3 O] having both six- and eight-membered magnesiasiloxane rings

The dilithium salt, [(Ph2SiOLi·THF)2O], reacts with MgCl2·2THF to give, on addition of pyridine, the unexpected lithium bridged spirocyclic magnesiasiloxane compound [(py)2Li]2-µ-Mg[{Ph2SiO}2O][{Ph2SiO}3O] which is the first example of a molecular magnesium siloxide compound to be characterised and also the first example of a spirocyclic metallasiloxane having rings with different degrees of oligomerisation.