Question

What is the charge on the complex $[Cr{{({{C}_{2}}{{O}_{2}})}_{2}}{{({{H}_{2}}O)}_{2}}]$ formed by Cr(III)?
A.$+3$
B.$+1$
C.$+2$
D.$-1$

Answer 1

Hint: The coordination complexes consist of positive ligands, negative ligands and neutral ligands. The charge of the complex depends on the summation of the charges on the ligands. To calculate the overall charge, note the charges of the associated ligands.

Complete answer:
The given complex’s name is: Diaquadioxalatochromate(III).
To calculate the charge, you have to subtract the negative charges and add the positive charge. As the neutral ligand has no charge, the charge can be taken as zero. Thus, the total charge is the algebraic sum of all the charges of all the ligands.
In the complex- $[Cr{{({{C}_{2}}{{O}_{2}})}_{2}}{{({{H}_{2}}O)}_{2}}]$
Chromium (Cr) is the central metal atom in the complex. The oxidation number of Chromium will be the charge on the atom. Cr is with +3 charge. (Chromium has multiple oxidation numbers)
There are two ligands in the complex: Oxalato $({{C}_{2}}{{O}_{2}})$and Aqua $({{H}_{2}}O)$. Both these ligands are neutral as they have a linear structure which cancels out the charges. So their charge will be zero.
Now we have to calculate the charge on the complex,
Charge on complex $[Cr{{({{C}_{2}}{{O}_{2}})}_{2}}{{({{H}_{2}}O)}_{2}}]$= Sum of all charges on ligands
 = \[\left[ +3+\left( 0 \right)+\left( 0 \right) \right]\]
= \[+3\]
Final answer: The charge on the complex $[Cr{{({{C}_{2}}{{O}_{2}})}_{2}}{{({{H}_{2}}O)}_{2}}]$ formed by Cr(III) is \[+3\].

Note:
Please remember that for identifying the charge of the complex compound, the oxidation number of each component i.e. the ligand and the central metal ion should be known. To find the oxidation number, you can draw the Lewis structure which will give you a clear idea of the charges the ligands carry.