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 (https://doi.org/10.1351/pac199971101919)