The Contax and Nikon RF mounts are similar, but have a different thread pitch, which means they are only compatible for lenses 35mm and wider. I have seen the Cocntax adapters sold on eBay and they do work; however, finding Nikon adapters is much harder. I have an Orion Nikon adapter from the 1960's which I use on my R-D1 all the time, and am very pleased with it. However, to replace this these days will cost over 1 grand. The Nikon-pitched Orion adapters were copied a few years ago, but never seem to pop up on eBay and would cost $3-400 if they did. Tom Abrahamsson (of Rapidwinder fame) is thinking of making another Orion adapter copy for Nikon lenses, but I think needs convincing that there is a market for them. Email him (TTAbrahams@aol.com) to support the idea.
In any case, you can buy the Contax adapter and very probably you will have no problems, given the caveats above.
Ed