electron-deficient compounds

Molecules or ions that contain too few electrons to allow their bonding to be described exclusively in terms of two-center, two-electron, i.e., covalent, bonds. Such molecules or certain fragments in these are often held together by the multicenter bonds.
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1936 [Terms] [Paper]