The given reaction is a reversible reaction. The sodium metaborate formed gets hydrolysed to regenerate boron trihydroxide and NaOH. B(OH)3+NaOH ⇌ NaBO2+Na[B(OH)4]+H2O In the presence of polyhydroxy compounds such as cis-1, 2-diol or glycerol, boron trihydroxide forms chelated complex (ring-structure) of higher stability. So on addition of cis-1,2- diol, due to the formation of this stable complex, the reaction proceeds in the forward direction.