Spin-frustrated monomer-type Mott insulator C60•− solids are discussed in this review article. For the C60•− solids, the interfullerene center-to-center distance (r) is the key parameter that controls the competition between covalent bond-formation, itinerancy, and spin frustration. Eight C60•− salts with various compositions and dimensionalities are reviewed. In all of these C60•− salts except one, neither bond-formation nor long-range magnetic ordering was observed down to low temperatures. A plot of Weiss temperature (|ΘCW|) against r shows that |ΘCW| grows rapidly below r = 10.0 Å.