I am missing the nut on the back end of the a/c compressor. Does anyone know the metric size? It looks like a M10 thread.

 

I just had the same problem. I pulled the bolt off a part yard car along with a tensioner bolt that I needed.

you could probably make it work by going to home depot and finding the carriage bolt/nut that fits with your 1/2" socket. I would assume it's really close to the same as the one that was on your car and you just need it to hold fit in the mounting holes.