Polyhedral skeletal electron pair theory

The polyhedral skeletal electron pair theory or Wade's rules in chemistry are electron counting rules that apply to electron-poor cages such as boron hydrides which consist of many boron atoms linked together.

For clusters of transition metals the rules are

• closo or closed structures have 12n + 2(n +1) electrons
• nido or nest structures have 12(n-1) + 2(n+1) electrons
• arachno or spider web like structures have 12(n-2) + 2(n+1) electrons

While for main group clusters such as boranes and carboranes the rules are

• closo or closed structures have 2n + 2(n +1) electrons
• nido or nest structures have 2(n-1) + 2(n+1) electrons
• arachno or spider web like structures have 2(n-2) + 2(n+1) electrons

The number of electrons per cluster are worked out using the same rules as are used for the 18 VE rule.