Which chemical is used as a refrigerant in refrigerators?

Among the following,the correct statement is

Hydrogen does not combine with

Triatomic hydrogen is called

Identify the correct statements from the following:
$I$. $B_2H_6$ is an electron deficient hydride.
$II$. $NH_3$ is an electron rich hydride.
$III$. $NaH$ is a covalent hydride.
$IV$. $YbH_{2.55}$ is an interstitial hydride.

The hydride ion $H^{-}$ is isoelectronic with which of the following?