The pair ${\mathrm{NH}}_{3 }\mathrm{and} {\mathrm{BH}}_3$ is isoelectronic with

${\mathrm{NH}}_{3 }\to$1(N) 3(H)

$\to$ $1\left(7\right)+3\left(1\right)=10{\mathrm{e}}^{-}$

${\mathrm{BH}}_3\to 1\left(5\right)+3\left(1\right)=8{\mathrm{e}}^{-}$

${\mathrm{NH}}_{3 }\mathrm{and} {\mathrm{BH}}_3$$\Rightarrow$10+8=18electrons

Iso electronic means both the species having same number of electrons

${\mathrm{B}}_2{\mathrm{H}}_6\Rightarrow 2\left(5\right)+6\left(1\right)=16{\mathrm{e}}^{-}$

${\mathrm{C}}_2{\mathrm{H}}_6$$\Rightarrow 2\left(6\right)+6\left(1\right)=18{\mathrm{e}}^{-}$

${\mathrm{C}}_2{\mathrm{H}}_4\Rightarrow$$2\left(6\right)+4\left(1\right)=16{\mathrm{e}}^{-}$

${\mathrm{CO}}_2\Rightarrow 1\left(6\right)+2\left(8\right)=22{\mathrm{e}}^{-}$

$\therefore \mathrm{The} \mathrm{pair} {\mathrm{NH}}_3 \& {\mathrm{BH}}_{3 }$is isoelectronic with ${\mathrm{C}}_2{\mathrm{H}}_6$