Table 1. Comparison of the HOMO-LUMO energy gaps of the chelate structures of [SCH₂N₄]²⁻ and heavy metal ions from DFT calculations and UV-vis measurements.

	HOMO-LUMO energy gaps (eV)
	DFT calculations	Experimental (λmax/nm)
SCH2N4Cd	2.01	2.17 (570 nm)
SCH2N4Co	2.09	2.12 (585 nm)
SCH2N4Zn	2.29	2.29 (541 nm)
SCH2N4Pb	2.30	2.38 (520 nm)
SCH2N4Cu	2.37	2.30 (540 nm)
SCH2N4Mn	2.42	2.52 (492 nm)
SCH2N4Hg	2.50	2.55 (485 nm)
SCH2N4Ag	2.58	2.55 (487 nm)
SCH2N4Ni	2.73	2.73 (455 nm)

Note: experimental HOMO-LUMO energy gaps = hc/λ_max, h = 6.626 × 10⁻³⁴ J.s, c = 3 × 10⁸ m/s.