A brown ring is formed in the ring test for ${\mathrm{NO}}_3^{-}$ ion. It is due to the formation of:

In diborane, the ${\mathrm{H}}_{\mathrm{t}}-\mathrm{B}-{\mathrm{H}}_{\mathrm{t}}$ and ${\mathrm{H}}_{\mathrm{b}}-\mathrm{B}-{\mathrm{H}}_{\mathrm{b}}$ angles are:

(Where, $\mathrm{t}$ denotes terminal and $\mathrm{b}$ denotes bridge)