Amination of the C‐isopropyldimethylsilyl P‐chlorophosphaalkene (iPrMe2Si)2C=PCl (1) leads to the P‐aminophosphaalkenes (iPrMe2Si)2C=PN(R)R′ (R, R′ = Me (2), R = H, R′ = nPr (3), R = H, R′ = iPr (4), R = H, R′ = tBu (5), R = H, R′ = 1‐Ada (6), R = H, R′ = CPh3 (7), R = H, R′ = Ph (8), R = H, RR′ = 2,6‐iPr2Ph (= DIP) (10), R = H, R′ = 2,4,6‐Me3Ph (= Mes) (11), R = H, R′ = 2,4,6‐tBu3Ph (= Mes*)] (12), R = H, R′ = SiMe3 (13), and R, R′ = SiMe2Ph (14). 31P‐NMR spectra confirm that phosphaalkenes 2–7 and 10–14 are monomeric in solution; the structures of 7, 10, and 12 were determined by X‐ray crystallography. Freshly prepared (iPrMe2Si)2C=PN(H)Ph (8) is a monomer that dimerizes with (N→C) proton migration within several hours to the stable diazadiphosphetidine [(iPrMe2Si)2CHPNPh]2 (9). NMR‐scale reactions of deprotonated 5 and 13 with tBuiPrPCl provide by P–P bond formation the P‐phosphanyl iminophosphoranes [(iPrMe2Si)2C=](RN=)PPtBu(iPr) [R = tBu (15), R = Me3Si (17)]. Deprotonated 5 and Me3GeCl deliver by N–Ge bond formation the aminophosphaalkene (iPrMe2Si)2C=PN(tBu)GeMe3 (20), which with elemental selenium 5 undergoes (N→C) proton migration to form the alkyl(imino)(seleno)phosphorane [(iPrMe2Si)2CH](tBuN=)P=Se (21), which is a selenium‐bridged cyclic dimer in the solid state.