Disclosed are chemical entities of formula I: wherein X, Y, Z, R1, R3, R4, R5 and R6 are defined herein, as NR2B subtype selective receptor antagonists. Also disclosed are pharmaceutical compositions comprising a chemical entity of formula I, and methods of treating various diseases and disorders associated with NR2B antagonism, e.g., diseases and disorders of the CNS, such as depression, by administering a chemical entity of formula I.