A compound containing this element in plant walls 
A compound containing this element in plant walls binds two RG-II monomers in pectin.  Compounds containing this element react in the Suzuki Coupling.  One ore containing this element, ulexite, has natural fiber optical properties, and other ores include raxorite and rasorite. This element's hydride dimerizes to from three-center banana bonds, and closo nido and arachno are three configurations predicted by (*) Wade's rules.  This element, which is used in nuclear reactor control rods, is used to provide a green color in fireworks and its nitride is isoelectric with diamond.  For 10 points, name this element with atomic number five and symbol B.
A compound containing this element in plant walls binds two RG-II monomers in pectin.  
{Boron} [accept {B} before giveaway]