2 Sn0 + 4 NaOHI + 6 H2IO 2 Na2(SnIII(OHI)5) + 3 H20

This is an oxidation-reduction (redox) reaction:

6 HI + 6 e- 6 H0 (reduction)

2 Sn0 - 6 e- 2 SnIII (oxidation)

Change the oxidation states