Provided herein are multicyclic compounds, methods of their synthesis, pharmaceutical compositions comprising the compounds, and methods of their use. For example, disclosed herein are compounds having formula (IIa) shown below, or a pharmaceutically acceptable salt or stereoisomer thereof, wherein the variables are defined herein. The compounds provided herein are useful for the treatment, prevention, and/or management of various neurological disorders, including but not limited to, psychosis and schizophrenia.