Google do not provide APIs to do this. In fact, they HATE people crawling their index for keyword positions and remove access to all APIs for anyone they discover doing so when tying it into another API-suite (for instance, RavenTools.)
If you want a keyword tool, I'm afraid you'll need to build a scraper from scratch or use one of the off-the-shelf options (which are all known by Google and in my experience consistently receive gamed results.)