Catalogs are not now and never have been backwards compatible. There is no way to save a LR6 catalog to make it readable to Lightroom 5.
That's the most ridiculous thing ever. How can I revert back to LR 5? I edit for many clients who use LR5 and I need them to be able to view my catalogs once I'm done. If I use 6 and they haven't upgraded, then they can't get my catalogs. Help?!
If you want to send them a catalog file the only way is to use Lightroom 5 unfortunately. Not much you can do about this except hope your clients upgrade. In your case I would simply have both Lightroom 5 and 6 installed and agree in advance on what version Lightroom they want their edits in.