The reaction of Li[4-CH 3 C 6 H 4 C(NSiMe 3 ) 2 ] with PhECl (E = S, Se) in a 1:1 molar ratio produces the monosubstituted derivatives 4-CH 3 C 6 H 4 C(NEPh)[N(SiMe 3 ) 2 ] (3b, E = Se; 3d, E = S). The thermolysis of 3b at 130 °C produces PhSeN(SiMe 3 ) 2 and 4-CH 3 C 6 H 4 CN. Crystals of 3b slowly acquire a purple stain upon prolonged standing at 23 °C owing to the formation of 4-CH 3 C 6 H 4 C(NH 2 )(NSePh) (4c, λ max = 551 nm). The formamidine derivative HC(NH 2 )(NSePh) (4a, λ max = 472 nm) was isolated from the reaction of HCN 2 (SiMe 3 ) 3 with PhSeCl (1:3 molar ratio) in dichloromethane. The structure of 4a was shown by X-ray crystallography to involve a syn configuration at the [Formula: see text] bond and hydrogen bonding between amino nitrogen atoms. The crystals of 4a are monoclinic, space group P2 1 (No. 4), with a = 7.514(1), b = 5.575(1), c = 8.925(1) Å, β = 100.71(1)°, V = 367.40(6) Å 3 , and Z = 2. The final R and R w values were 0.055 and 0.060, respectively. The 1 H and 77 Se NMR spectra indicate that both 4a and 4c exist as a mixture of tautomers in THF solution. DFT calculations for the model tautomers HC(NH 2 )(NER) and HC(NH)(NHER) (E = S, Se; R = H, Ph) show that the former is more stable in all cases, but the difference in total bonding energy is smaller for selenium than for sulfur (17.5 vs. 29 kJ mol −1 ). The visible absorption band for 4a is assigned to the HOMO–LUMO transition (ΔE = 2.90 eV, λ max = 427 nm) of the minor tautomer. Keywords: formamidine, tautomer, selenium, purple chromophore.