IR Stretching in Hexacarbonyl Iron

[![](https://i.stack.imgur.com/cZZdK.png)](https://i.stack.imgur.com/cZZdK.png)


With reference to this graph, my professor stated the following: "Since the IR band for the bridging carbonyls is single, it implies that the bridging carbonyls are oriented linearly. Since we have two bands for the terminal bonds, they are not collinear."  

Precisely how and why?


