This is a double blind study designed to select 1 or 2 LY2189265 doses for evaluation in Phase 3 studies (dose-finding portion) and to evaluate efficacy and safety of selected doses of LY2189265 in comparison to Sitagliptin (100 milligrams) up to 104 weeks and Placebo up to 26 weeks in participants with type 2 diabetes mellitus on Metformin (confirmatory, Phase 3 portion). The primary objective is to show non-inferiority of the higher LY2189265 dose (if 2 doses are selected) to Sitagliptin with respect to change in glycosylated hemoglobin (HbA1c) at 52 weeks. The final endpoint is 104 weeks.