n-BuLi exists as a cluster both in the solid state and in a solution.  The tendency to aggregate is common for organolithium compounds.  The aggregates are held together by delocalized covalent bonds between lithium and the terminal carbon of the butyl chain.  In the case of n-BuLi, the clusters are tetrameric (in ether) or hexameric (in cyclohexane). The cluster is a distorted cubane-type cluster with Li and CHR groups at alternating vertices.  An equivalent description describes the tetramer as a Li tetrahedron interpenetrated with a tetrahedron [CHR]. Bonding within the cluster is related to that used to describe diborane, but more complex since eight atoms are involved.  Reflecting its electron-rich character, n-butyllithium is highly reactive toward Lewis acids.
Due to the large difference between the electronegativities of carbon (2.55) and lithium (0.98), the C−Li bond is highly polarized.  The charge separation has been estimated to be 55–95%.  For practical purposes, n-BuLi can often be considered to react as the butyl anion, n-Bu, and a lithium cation, Li.