Geometrically Compelled Disilene with λ4-Coordinate SiII Atoms.

The first hypercoordinated disilene with the longest Si═Si distance (2.623(1) Å) reported to date, the 5,6-bis(amidinato silylenyl)acenaphtene (amidinato = PhC(NtBu)2) (1), is presented. It results from a spatially compelled double dative interaction between the two singlet silylene moieties, due to their close proximity. The dynamic behavior of the Si═Si bond contraction and elongation in 1 predicted by quantum chemical calculations could be confirmed experimentally by variable temperature NMR spectroscopy. Compound 1 exhibits an ambivalent reactivity, reacting as disilene with ethylene, diphenylacetylene and benzophenone to give [2+2] cycloaddition products, but as bis(silylene) toward Ni(cod)2 (cod = cycloocta-1,4-diene) to form the corresponding bis(silylene)Ni0(η4-cod) complex.