Structural Distortions in M[E(SiMe3)2]3 Complexes (M = Group 15, f‑Element; E = N, CH): Is Three a Crowd?

The tris­(bistrimethylsilylamido) species P­[N­(SiMe3)2]3 (1) and As­[N­(SiMe3)2]3 (2) have been prepared through halide metathesis in high yield. Their single crystal X-ray structures, along with that of Sb­[N­(SiMe3)2]3 (3), complete the series of structurally authenticated group 15 M­[N­(SiMe3)2]3 complexes (the bismuth analogue (4) has been previously reported). All four complexes possess the expected pyramidal geometries, with progressively longer M–N bond distances from P to Bi but closely similar N–M–N angles (107–104°). The structures of 1–4 also display distortions that are similar to those in f-element M­[N­(SiMe3)2]3 and M­[CH­(SiMe3)2]3 complexes, in which M···(β-Si–C) interactions have been identified. Such structural features include distorted M–(N,CH)–Si and (N,CH)–Si–C angles and close M···C and M···Si contacts. DFT calculations confirm that there are no M···(β-Si–C) interactions in 1–4; the bond distortions appear to result from the particular steric crowding that arises in pyramidal M­[(N,CH)­(SiMe3)2]3 complexes. This is likely the source of the most of the distortions in the structures of the f-element analogues as well, even though the latter possess attractive M···Si–C interactions.