Paramagnetic compounds with at least partially boron-centered electron spin can be constructed using either the prototypically electron-accepting boron atoms bridged by planar pi-conjugated organic systems, or by taking advantage of the three-dimensional delocalized bonding in oligonuclear borane, haloborane, or carborane clusters. The concept of mixed valency can thus be transferred from organic and transition-metal chemistry to main-group-element molecules, and density functional theory is capable of reproducing the variable spin distribution.