Contrastive Study on Substitution of the Bulky Phosphanide [P(SiⁱPr₃)₂]^- toward Heavier Tetrylenes

The super bulky sodium phosphanide, NaP(SiⁱPr₃)₂, was reacted with amidinatotetrylenes LECl (L = PhC(N^tBu)₂, E = Si, Ge), resulting in the formation of phosphasilene LSi(SiⁱPr₃) = PSiⁱPr₃ (1) and phosphanido germylene LGeP(SiⁱPr₃)₂ (2), respectively. Investigation on the reactivity of 1 and 2 toward elemental sulfur was carried out, where a stepwise reaction yielding the silanethione LSi(=S)SiⁱPr₃ (3) and the silicon thioester analogue LSi(=S)SSiⁱPr₃ (4) was observed in the case of 1, while the treatment of 2 with sulfur exclusively afforded the germanium thioester analogue. In addition, the reactions of 2 with Fe₂(CO)₉ and GeCl₂·1,4-dioxane led to the germylene-coordinated iron carbonyl and the asymmetric Ge-Ge-bonded complex, respectively, exhibiting the reactivity of the lone pair as well as a weak Ge-P bond. Further investigations into the stability and bonding characteristics of the Si-P and Ge-P bonds in 1 and 2, employing DFT, NBO, QTAIM, and EDA-NOCV analyses, suggest their polarized nature, with electron density accumulation on the phosphorus atoms supported by electrostatic potential maps. EDA-NOCV results indicate that the Si-P bond is primarily stabilized by covalent interactions, whereas the Ge-P bond exhibits a dative character arising from interactions between ionic fragments [(SiⁱPr₃)₂P^-(S) + LGe⁺(S)], further confirming its polarization.