Contrastive Study on Substitution of the Bulky Phosphanide [P(Si^<i>i</i>Pr₃)₂]⁻ toward Heavier Tetrylenes

The super bulky sodium phosphanide, NaP(Si^<i>i</i>Pr₃)₂, was reacted with amidinatotetrylenes LECl (L = PhC(N^<i>t</i>Bu)₂, E = Si, Ge), resulting in the formation of phosphasilene LSi(Si^<i>i</i>Pr₃) = PSi^<i>i</i>Pr₃ (<b>1</b>) and phosphanido germylene LGeP(Si^<i>i</i>Pr₃)₂ (<b>2</b>), respectively. Investigation on the reactivity of <b>1</b> and <b>2</b> toward elemental sulfur was carried out, where a stepwise reaction yielding the silanethione LSi(=S)Si^<i>i</i>Pr₃ (<b>3</b>) and the silicon thioester analogue LSi(=S)SSi^<i>i</i>Pr₃ (<b>4</b>) was observed in the case of <b>1</b>, while the treatment of <b>2</b> with sulfur exclusively afforded the germanium thioester analogue. In addition, the reactions of <b>2</b> 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 <b>1</b> and <b>2</b>, 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 [(Si^<i>i</i>Pr₃)₂P^-(S) + LGe⁺(S)], further confirming its polarization.