5.1 Sound with iMac (late 2012)

Hy Folks,

I'm planning on getting a 5.1 Sound System for my new iMac. I was doing a little bit of a research through the web and the only thing I found (that sounded logical) is a cable with a 3.5 mm jack on the one side and on the other a toslink connector. The jack goes into my Mac and the toslink into the 5.1 system.

My question: is it really that simple? I also red somewhere, that I had to get an external sound card to plug into my Mac, but i would prefer to buy just this cable...

Please Help (: