The geometries and energies of the iminopnictoranes (H(3)M=NH; M=P, As, Sb, and Bi) and their reaction paths with formaldehyde are predicted by means of ab initio calculations. The multiplicity of M=N bonds is discussed by comparing the M=N and M-H bond lengths, the bond length ratios M=N:M-N, the bond angles of M-N-H, and the barrier to internal rotation about the M=N bond with those of the ylides, and it was concluded that the contribution of the ionic canonical form M(+)-N- is much more important than that of the M=N form. For the two reaction routes of the iminopnictoranes with formaldehyde (aza-Wittig and Corey-Chaykovsky-type reactions), all of the stationary points and transition states were fully optimized by using an analytical gradient with the LANL1DZ and the 9s5p-d/[3s2p-d] basis set (DZ-d) at the MP2 level. For M=P and As, the aza-Wittig reaction is more favorable than the Corey-Chaykovsky-type reaction from both the thermodynamic and the kinetic viewpoint. In the case of M=Sb and Bi, the Corey-Chaykovsky-type reaction takes place predominantly. The higher level calculations, such as MP4(SDTQ)/DZ-d//MP2/DZ-d and QCISD(T)/DZ-d// MP2/DZ-d, were performed to get accurate energies of these intermediates and transition States.