New monitor won't let me set resolution to 2560x1440 even though it should be native

i have just purchased a dell p2715q monitor and while i understand that my early 2011 macbook pro will not run it at the full resolution, it should be able to run it at 2560x1440. However the only option from the list of resolutions that works (1080i) (i.e. any other res I select makes the monitor go dark, still active as i am able to drag windows to it, but not see anything on the monitor.) I have spent literally all today trying to configure it and nothing works. I know this isnt a hardware or cable issue as i have run the screen at 2560x1440 res perfectly on my windows bootcamp from the same mac, so the issue must lie with OSx. I have the latest version of Yosemite, have tried restarting, 3rd party programmes and even rescripting from terminal commands. nothing works. Thanks.