New Zwitterionic Spirocyclic λ<SUP>5</SUP>Si-Silicates with an SiX<INF>4</INF>C Skeleton (X = S, O) Containing Two Ligands of the Dithiolato(2−) or Diolato(2−) Type:  Synthesis, Structure, and Bonding Situation

Reaction of trimethoxy(morpholinomethyl)silane (<BO>7</BO>) with lithium aluminum hydride yielded (morpholinomethyl)silane (<BO>8</BO>). Treatment of <BO>8</BO> with ethane-1,2-dithiol (molar ratio 1:2) in MeCN afforded bis[ethane-1,2-dithiolato(2−)](morpholiniomethyl)silicate (<BO>5</BO>; isolated as <BO>5</BO>·MeCN). Reaction of <BO>7</BO> with ethane-1,2-diol (molar ratio 1:2) in MeCN yielded the oxygen analogue bis[ethane-1,2-diolato(2−)](morpholiniomethyl)silicate (<BO>6</BO>). Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of <BO>5</BO>·MeCN and <BO>6</BO> are distorted trigonal bipyramids. Compounds <BO>5</BO>·MeCN and <BO>6</BO> were additionally characterized by solid-state VACP/MAS NMR experiments (¹³C, ²⁹Si). The structural investigations were complemented by quantum-chemical studies of the structure, bonding, and ²⁹Si NMR chemical shift tensors of <BO>5</BO> (SiS<INF>4</INF>C skeleton) and <BO>6</BO> (SiO<INF>4</INF>C skeleton).