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.