Ligands are broadly classified into two classes classical and non-classical ligands, depending on their donor and acceptor ability. Classical ligands form classical complexes while non-classical ligands form non-classical complexes. Bonding mechanism in non-classical is called synergic bonding.
Synergic bonding is absent in :
a. [Mo(CO)6]
b. [Cr(CO)3(B3N3H6)]
c. [Sc(CO)6]3+
d. [Ni(CN)4]4−