We report a series of calix[4] arene Mo(VI) dioxo complexes M2RC4MoO2 (M) alkali metal, R = H or Bu-t) that were fully characterized by NMR, X- ray, IR, UV/ vis, and elemental analysis. Molybdocalix[4] arene structures can be controlled via lower rim deprotonation, groups at para positions of calix[4] arene, and alkali metal counterions. Mono deprotonation at the lower rim leads to calix[4] arene Mo( VI) monooxo complexes RC4MoO (R = H, Bu-t, or allyl), and full deprotonation gives rise to calix[4] arene Mo(VI) dioxo complexes. Structural studies indicate that HC4 Mo(VI) dioxo complexes easily form polymeric structures via cation-pi interaction and coordination between different calixarene units. However, ButC4 Mo(VI) dioxo complexes tend to form dimers or tetramers due to steric hindrance of the tert-butyl groups at para positions in calixarene. The structures of the reduced side products A and C were determined by X- ray diffraction studies. The mechanism of RC4MoO formation from the reaction of calixarene monoanions with MoO2Cl2 appears to include the addition of a calixarene-OH group across a Mo=O bond.