The corresponding coefficients of PH₃ and ${\mathrm{H}}_2{\mathrm{PO}}_2^{-}$  in the following equation after balancing are ${\mathrm{P}}_4+{\mathrm{OH}}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{PH}}_3+{\mathrm{H}}_2{\mathrm{PO}}_2^{-}$