A metal ion that combines preferentially with ligands containing ligating atoms other than the lightest of their Periodic Group.
See also: class (a) metal ion, hard acid
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1097 (https://doi.org/10.1351/pac199466051077)