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Which is the least stable trihalide of nitrogen and why?
I have two conflicting theories, one is that fluorine-fluorine electron repulsion will make $\ce{NF3}$ least stable and another is steric reasons of iodine which give $\ce{NI3}$ as the answer.

Which is correct?

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Nitrogen triiodide $\ce{NI3}$ is an extremely sensitive explosive that explodes with the slightest touch when dry.

Nitrogen trifluoride $\ce{NF3}$ is so stable that it is a greenhouse gas global warming concern, with lifetime in the atmosphere of hundreds of years.

$\ce{NI3}$ is unstable due to steric strain as you are saying.

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    $\begingroup$ I agree with Dave's answer, but would add that iodoform ($\ce{CHI3}$) is stable. I suspect that $\ce{NI3}$ is unstable due to sterics and an extremely exothermic pathway leading to the formation of the very stable $\ce{N#N}$ triple bond. A similar decomposition pathway does not exist for iodoform, hence its relative stability. $\endgroup$ – ron Feb 17 '15 at 21:29
  • $\begingroup$ I could be wrong about the reason, since NCl3 is also unstable. $\endgroup$ – DavePhD Feb 17 '15 at 21:41
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    $\begingroup$ I believe something's rather different about the fluorine compounds. Even sulfur hexafluoride and carbon tetrafluoride are extremely stable. I guess it's by virtue of their high bond strengths. $\endgroup$ – Lexicon Feb 18 '15 at 9:07

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