ONE-EIGHTH STAGE 2026
Aburi Girl’s SHS 38 points
St. Joseph’s Seminary SHS 20 points
Fafraha Community SHS 12 points
St. John's Grammar School: 40 points
Mpraeso SHS: 23 points
Tema Secondary School: 13 points
St. Augustine’s College
Anlo SHS
Kumasi High School
QUESTION
PREAMBLE
For the given complex ion, determine the oxidation state of the central metal and its coordination number.
1. $\text{Cu}(\text{NH}_3)_4^{2+}$
ANSWER: Oxidation State = $+2$, Coordination Number = $4$
SOLUTION:
$\text{O.S.} = \text{Overall Charge} - \text{Total Ligand Charge}$
Where:
$\text{Overall Charge} = +2$
$\text{Ligand Charges} = 4 \times 0$
$\text{Ligand Charges} = 0$
(neutral ammine ligands)
$\text{O.S.} = (+2) - (0)$
$\text{O.S.} = +2$
$\text{C.N.} = 4\text{ monodentate ligands}$
2. $\text{Fe}(\text{CN})_6^{3-}$
ANSWER: Oxidation State = $+3$, Coordination Number = $6$
SOLUTION:
$\text{O.S.} = \text{Overall Charge} - \text{Total Ligand Charge}$
Where:
$\text{Overall Charge} = -3$
$\text{Ligand Charges} = 6 \times (-1)$
$\text{Ligand Charges} = -6$
(anionic cyano ligands)
$\text{O.S.} = (-3) - (-6)$
$\text{O.S.} = +3$
$\text{C.N.} = 6\text{ monodentate ligands}$
3. $\text{Cr}(\text{H}_2\text{O})_4\text{Cl}_2^{+}$
ANSWER: Oxidation State = $+3$, Coordination Number = $6$
SOLUTION:
$\text{O.S.} = \text{Overall Charge} - \text{Total Ligand Charge}$
Where:
$\text{Overall Charge} = +1$
$\text{Ligand Charges} = 4(0) + 2(-1)$
$\text{Ligand Charges} = -2$
(neutral aqua and anionic chloro ligands)
$\text{O.S.} = (+1) - (-2)$
$\text{O.S.} = +3$
$\text{C.N.} = 4 + 2$
$\text{C.N.} = 6\text{ monodentate ligands}$
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PRACTICE QUESTIONS
1. QUESTION: For the complex ion $[Co(NH_3)_5Cl]^{2+}$, determine the oxidation state of the central metal and its coordination number.
ANSWER: Oxidation state = +3, coordination number = 6
SOLUTION:
$\text{O.S.} = \text{overall charge} - \text{total ligand charge}$
$\text{overall charge} = +2$
$\text{ligand charge} = 5(0)+1(-1)$
$\text{ligand charge} = -1$
$\text{O.S.} = (+2) - (-1)$
$\text{O.S.} = +3$
$\text{C.N.} = 5+1$
$\text{C.N.} = 6$ (number of ligand atoms bonded to the metal)
2. QUESTION: For the complex ion $[Ni(CN)_4]^{2-}$, determine the oxidation state of the central metal and its coordination number.
ANSWER: Oxidation state = +2, coordination number = 4
SOLUTION:
$\text{O.S.} = \text{overall charge} - \text{total ligand charge}$
$\text{overall charge} = -2$
$\text{ligand charge} = 4\times(-1)$
$\text{ligand charge} = -4$
$\text{O.S.} = (-2) - (-4)$
$\text{O.S.} = +2$
$\text{C.N.} = 4$ (number of ligand atoms bonded to the metal)
3. QUESTION: For the complex ion $[Ag(NH_3)_2]^{+}$, determine the oxidation state of the central metal and its coordination number.
ANSWER: Oxidation state = +1, coordination number = 2
SOLUTION:
$\text{O.S.} = \text{overall charge} - \text{total ligand charge}$
$\text{overall charge} = +1$
$\text{ligand charge} = 2\times0$
$\text{ligand charge} = 0$
$\text{O.S.} = (+1) - (0)$
$\text{O.S.} = +1$
$\text{C.N.} = 2$ (number of ligand atoms bonded to the metal)
4. QUESTION: For the complex ion $[Fe(CN)_6]^{4-}$, determine the oxidation state of the central metal and its coordination number.
ANSWER: Oxidation state = +2, coordination number = 6
SOLUTION:
$\text{O.S.} = \text{overall charge} - \text{total ligand charge}$
$\text{overall charge} = -4$
$\text{ligand charge} = 6\times(-1)$
$\text{ligand charge} = -6$
$\text{O.S.} = (-4) - (-6)$
$\text{O.S.} = +2$
$\text{C.N.} = 6$ (number of ligand atoms bonded to the metal)
5. QUESTION: For the complex ion $[Cr(NH_3)_4Cl_2]^{+}$, determine the oxidation state of the central metal and its coordination number.
ANSWER: Oxidation state = +3, coordination number = 6
SOLUTION:
$\text{O.S.} = \text{overall charge} - \text{total ligand charge}$
$\text{overall charge} = +1$
$\text{ligand charge} = 4(0)+2(-1)$
$\text{ligand charge} = -2$
$\text{O.S.} = (+1) - (-2)$
$\text{O.S.} = +3$
$\text{C.N.} = 4+2$
$\text{C.N.} = 6$ (number of ligand atoms bonded to the metal)
6. QUESTION: For the complex ion $[Zn(NH_3)_4]^{2+}$, determine the oxidation state of the central metal and its coordination number.
ANSWER: Oxidation state = +2, coordination number = 4
SOLUTION:
$\text{O.S.} = \text{overall charge} - \text{total ligand charge}$
$\text{overall charge} = +2$
$\text{ligand charge} = 4\times0$
$\text{ligand charge} = 0$
$\text{O.S.} = (+2) - (0)$
$\text{O.S.} = +2$
$\text{C.N.} = 4$ (number of ligand atoms bonded to the metal)