Sn(II) binds to kaempferol (HKaem, 3,4 ,5,7-tetrahydroxy-2-(4-hydroxyphenyl)-4H-1benzopyran-4-one) at the 3,4-site forming [Sn(II)(Kaem) 2 ] complex in ethanol. DPPH • scavenging efficiency of HKaem is dramatically decreased by SnCl 2 coordination due to formation of acid inhibiting deprotonation of HKaem as ligands and thus reduces the radical scavenging activity of the complex via a sequential proton-loss electron transfer (SPLET) mechanism. Moderate decreases in the radical scavenging of HKaem are observed by Sn(CH 3 COO) 2 coordination and by contact between Sn and HKaem, in agreement with the increase in the oxidation potential of the complex compared to HKaem, leading to a decrease in antioxidant efficiency for fruits and vegetables with Sn as package materials. be increased by the presence of copper (II), iron (III), and zinc (II) by other authors and our recent studies [7,8,[23][24][25][26]. In the present study, HKaem was selected as a typical flavonoid with two possible chelation sites (3,4 and 4,5) for metal ions, and complex formation by Sn(II) coordination to HKaem was investigated together with the complex as a radical scavenger. The effect on radical scavenging efficiency of HKaem by Sn(II) coordination is of relevance as Sn is used for packaging of food, and accordingly deserves more attention.Molecules 2020, 25, x FOR PEER REVIEW