What is ncl5? what is pcl5? which of the two compounds does not exist and why?

The compound NCl5 is the compound that cannot exist since the atom of nitrogen can react only to at most 3 other atoms. The PCl5 can exist in nature as phosphorus may bound with this number of other atoms and is named, phosphorus pentochloride. 

Rn in the right answer

Idont know. if you want an answer you need to give us some context dude.