A @L03508@ with an acceptor centre of low @P04711@. Other things being approximately equal, complexes of hard acids and @H02742@ or soft acids and bases have an added stabilization (sometimes called 'HSAB' rule). For example the hard O- (or N-) bases are preferred to their S- (or P-) analogues by hard acids. Conversely a @S05724@ possesses an acceptor centre of high @P04711@ and exhibits the reverse preference for @C01329@ of a @S05725@. These preferences are not defined in a quantitative sense.
class (a) metal ion