What is the Difference Between Closo Nido and Arachno Boranes?

The key difference between closo nido and arachno boranes lies in the number of skeletal bonding electron pairs in their structures:

Closo boranes have n+1 skeletal bonding electron pairs.
Nido boranes have n+2 skeletal bonding electron pairs.
Arachno boranes have n+3 skeletal bonding electron pairs.

These structures are named according to the Wade rules, which correlate the skeletal structures of boranes, carboranes, hetero-boranes, and their anions (closo, nido, arachno, hypho). The structures can be categorized as closo, nido, or arachno depending on the arrangement of skeletal atoms (boron atoms).

Closo boranes are closed deltahedra without B-H-B 3c,2e-bonds and are thermally stable and moderately reactive. An example of a closo borane is (\ce{[B5H5]^{2-}}), which forms a trigonal, bipyramidal polyhedron.
Nido boranes are closo boranes with one corner less and the addition of two hydrogen atoms instead, with B-H-B-bonds and B-B-bonds being possible. Their thermally stability lies between closo and arachno boranes. An example of a nido borane is (\ce{B{n} H{n + 4}}).
Arachno boranes resemble spiderwebs and are named according to Wade's rules. An example of an arachno borane is (\ce{B{n} H{n + 6}}).

In summary, the main difference between closo nido and arachno boranes is the number of skeletal bonding electron pairs in their structures, which determines their classification and properties.