The neutral pentacoordinate silicon(IV) complexes 8 and 9 with an SiO2N3 skeleton and the neutral hexacoordinate silicon(IV) complex 10.1/2 CH3CN with an SiO4N2 skeleton were synthesized, starting from tetra(cyanato-N)silane or tetra(thiocyanato-N)silane. Compounds 8 and 9 contain one tridentate dianionic ligand derived from 4-[(2-hydroxyphenyl)amino]pent-3-en-2-one and two monodentate singly charged cyanato-N or thiocyanato-N ligands bound to the silicon(IV) coordination center, whereas the silicon(IV) center of 10 is coordinated by two of these tridentate dianionic ligands. All compounds were characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of the compounds studied, the experimental investigations were complemented by computational studies.

Reaction of trimethoxy(morpholinomethyl)silane (7) with lithium aluminum hydride yielded (morpholinomethyl)silane (8). Treatment of 8 with ethane-1,2-dithiol (molar ratio 1:2) in MeCN afforded bis[ethane-1,2-dithiolato(2−)](morpholiniomethyl)silicate (5; isolated as 5·MeCN). Reaction of 7 with ethane-1,2-diol (molar ratio 1:2) in MeCN yielded the oxygen analogue bis[ethane-1,2-diolato(2−)](morpholiniomethyl)silicate (6). Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 5·MeCN and 6 are distorted trigonal bipyramids. Compounds 5·MeCN and 6 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 5 (SiS4C skeleton) and 6 (SiO4C skeleton).

The hexacoordinate silicon(IV) complexes [NEt4]2[Si(NCS)6] (2), [K(18‐crown‐6)]2[Si(NCS)6]·2CH3CN (3·2CH3CN), and cis‐[(acac)2Si(NCS)2] (cis‐4; acac = acetylacetonato‐O, O) were synthesized, starting from Si(NCS)4 (1). Compounds 1, 2, 3·2CH3CN, and cis‐4 were structurally characterized in the solid state (13C, 15N (cis‐4 only), and 29Si MAS NMR; crystal structure analyses) and in solution (1H, 13C, and 29Si NMR; cis‐4 only). The experimental investigations were complemented by computational studies (RI‐MP2 geometry optimizations of 1, [Si(NCS)6]2—, cis‐4, and trans‐4; calculations of the 29Si NMR chemical shifts using the optimized structures).

A series of novel hexacoordinate silicon(IV) complexes with an SiO2N4 skeleton (compounds (OC-6-12)-3, (OC-6-12)-4, (OC-6-12)-5, (OC-6-12)-6, and (OC-6-2'2)-7) were synthesized, starting from Si(NCO)4 or Si(NCS)4. These compounds contain (i) two bidentate O,N-chelate ligands (or one tetradentate O,N,N,O-chelate ligand) derived from 4-aminopent-3-en-2-ones of the formula type Me-C(NRH)=CH-C(O)-Me (R = organyl) and (ii) two monodentate cyanato-N or thiocyanato-N ligands. Formally, the bidentate singly negatively charged O,N-chelate ligands (tetradentate 2-fold negatively charged O,N,N,O-chelate ligand) behave as ligands of the imino-enolato type. In addition, the adduct trans-8 was synthesized by reaction of Si(NCS)4 with 2 molar equiv of Me-C(Ni-PrH)=CH-C(O)-Me. This hexacoordinate silicon(IV) complex contains (i) four monodentate thiocyanato-N ligands and (ii) two neutral monodentate ligands of the iminio-enolato type. All compounds synthesized were structurally characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of these compounds, the experimental investigations were complemented by computational studies.