Question

Coordination number of Ni in ${{\left[ Ni{{\left( {{C}_{2}}{{O}_{4}} \right)}_{3}} \right]}^{4-}}$
  (A) 3
  (B) 6
  (C) 4
  (D) 5

Answer 1

Hint: The given complex contains oxalate ion as a ligand. Oxalate ion is a bidentate ligand. Thus, by calculating the number of electron pairs that are donated by oxalate ions, we could find the coordination number of nickel in the given complex.


Complete step by step solution:
  - Let's start with the concept of coordination number. It can be defined as the total number of points of attachment to the central atom. Or in other words, it is the total number of ions, atoms or molecules that is bonded to a central atom.
 - we can term these ions, atoms or molecules that are surrounded to a central metal atom as ligands. The term ligancy is also used to represent the coordination number.
- In a coordination complex the ligands act as electron donors or as Lewis base and the central atom acts as electron pair acceptors or as Lewis acids.
- Hence based on the number of electron pairs that each ligand donates, we can classify them as monodentate ligand, bidentate ligand, tridentate ligand etc. We can say that a monodentate ligand has only one donor atom that can be used to bond to the central atom.
 - Similarly, bidentate ligands will have two donor atoms which will allow them to bind to a central metal atom or ion at two different points. The most common examples of bidentate ligands are the oxalate ion and ethylenediamine
 - In the given complex, the ligand is oxalate ion and the central metal ion is nickel. As we mentioned, coordination number is the number of ligand donor atoms to which the metal is directly attached. Since oxalate ion (${{C}_{2}}{{O}_{4}}^{2-}$) is a bidentate ligand ,it donate two electron pairs to nickel .Also there are three oxalate ion present in ${{\left[ Ni{{\left( {{C}_{2}}{{O}_{4}} \right)}_{3}} \right]}^{4-}}$.Hence the total number of atoms attached is
  Coordination number =2×3
                                     = 6

Therefore, the answer is option (B) 6.

Note: Do not confuse coordination number with oxidation state. The first parameter is the number of coordination bonds with which the ligands are bound to central metal ion whereas the oxidation state is a number which is assigned to an element in chemical combination and it represents the number of electrons gained or lost by an atom of that element in the given compound.