The preferred geometric structures of the allene analogues CH2=Si=CH2 (1), NH=Si=NH (2), PH=Si=PH (3), NH= C=NH (4), and NH=Si=CH2 (5) are examined at the RHF /6-31 G(d) level of theory, and that of 2 is further examined at the MP2/6-31G(d) computational level. The prediction that at the SCF level compounds 1, 3, 4, and 5 prefer orthogonal geometries like allene, while 2 prefers a planar structure, is examined with the aid of localized molecular orbitals, correlated energies, and, in the case of 2, a detailed analysis of the potential energy surface. The latter is very flat, and at the highest level of theory this molecule is predicted to be nonplanar with a nonlinear N-Si-N angle.