The first silyl-substituted Si(3)-allene, namely 1,1,3,3-tetrakis(di-tert-butylmethyisilyl)trisilaallene (4), was prepared as an air- and moisture-sensitive red solid by the reaction of the dilithiosilane ('Bu(2)MeSi)(2)SiLi(2) with the dichlorosilylene-NHC complex :SiCl(2)w <- NHC (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) in benzene at room temperature. Remarkably, the reaction of 4 with methanol proceeds regioselectively to form only the 3,3-dimethoxypentasilane derivative 5, in contrast to the case for a previously reported trisilaallene. In addition, 4 undergoes unprecedented thermal isomerization to tetrakis(di-tert-butylmethylsilyl)cyclotrisilene (7).