The key difference between closo nido and arachno boranes is that closo structure has n+1 skeletal bonding electron pairs, and nido structure has n+2 skeletal bonding electron pairs, whereas arachno structure has n+3 skeletal bonding electron pairs.
The structure of borane comes in three types as closo, nido and arachno skeletal structures, depending on the arrangement of skeletal atoms (boron atoms). These three types can be categorized according to a set of rules named “wade rules”. The wade rules are a correlation between the number of electrons, the formula and the shape of the borane structure.