Compounds of formula I: are disclosed, as are pharmaceutical compositions containing such compounds. Methods of treating neurological or psychiatric disorders in a patient in need are also disclosed. Such disorders include depression, bipolar disorder, pain, schizophrenia, obsessive compulsive disorder, addiction, social disorder, attention deficit hyperactivity disorder, an anxiety disorder, autism, a cognitive impairment, or a neuropsychiatric symptom such as apathy, depression, anxiety, psychosis, aggression, agitation, impulse control disorders, and sleep disorders in neurological disorders such as Alzheimer's and Parkinson's diseases.