Syntheses and isolations of the tris(amino)stibine and tris(amino)bismuthine E[N(H)(C(6)H(2)(t)Bu(3))](3) (E = Sb, Bi) from ECl(3) and LiN(H)(C(6)H(2)(t)Bu(3)) are described, together with spectroscopic and structural characterization [crystal data for C54H90N3Sb, M = 903.04, space group , a = 11.491(5) Angstrom, b = 24.652(7) Angstrom, c = 10.002(5) Angstrom, alpha = 98.38(3)degrees, beta = 96.44(5)degrees, gamma = 77.25(3)degrees, V = 2724(2) Angstrom(3), D-c 1.101 Mg/m(3), Z = 2, R = 0.0547; crystal data for C54H90BiN3, M = 990.27, space group , a = 11.511(5) Angstrom, b = 24.785(15) Angstrom, c = 9.981(5) Angstrom, alpha = 98.06(5)degrees, beta = 96.50(4)degrees, gamma = 77.40(5)degrees, V = 2742(2) Angstrom(3), D-c = 1.200 Mg/m(3), Z = 2, R = 0.0619]. The compounds bear the "bulky" 2,4,6-tri-tert-butylphenyl substituent (known as supermesityl or Mes*), and their formation is considered in the context of the same reactions for PCl3 and AsCl3, which have been previously shown to produce the aminoiminopnictine structures [N(H)(C(6)H(2)(t)Bu(3))]P = N(C(6)H(2)(t)Bu(3)) and [N(H)(C(6)H(2)(t)Bu(3))]As = N(C(6)H(2)(t)Bu(3)). The observations establish the limits of the steric control by the supermesityl substituent and provide qualitative support for the thermodynamic significance of substituent steric strain.