The present application provides, among other things, novel compounds for metathesis reactions, and methods for preparing and using provided compounds. In some embodiments, the present invention provides compounds having the structure of formula I or II. In some embodiments, the present invention provides methods for preparing a compound of formula I or II. In some embodiments, the present invention provides methods for using a provided compound. In some embodiments, a provided compound is useful for stereoselective ring-opening metathesis polymerization. In some embodiments, a provided metathesis method provides cis and/or isotactic polymers.