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 MN bond distances from P to Bi but closely similar N-M-N angles (107-104 degrees). The structures of 14 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...(beta-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...(beta-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.