Question

Which of the following complexes is an outer orbital complex?

• A. \([Co(NH_3)_6]^{3+}\)
• B. \([Fe(CN)_6]^{4-}\)
• C. \([Ni(NH_3)_6]^{2+}\)
• D. \([Mn(CN)_6]^{4-}\)

Hint:

Weak ligand \(\Rightarrow\) outer orbital complex.

Answer 1

The Correct Option is C

To determine which of the given complexes is an outer orbital complex, it is essential to understand the concept of crystal field theory and the nature of coordination complexes.

An outer orbital complex (also called a high-spin complex) occurs when the central metal ion in a complex uses its outer 4d, 5d, or 6d orbitals to form hybrid orbitals for bonding with the ligands. This usually results in weaker field ligands, which do not cause significant pairing of electrons in the d orbitals.

Let's examine each option:

1. \([Co(NH_3)_6]^{3+}\): Cobalt in the +3 oxidation state generally forms an inner orbital complex because \(NH_3\) is a strong field ligand causing pairing of electrons.
2. \([Fe(CN)_6]^{4-}\): Iron in the +2 oxidation state with cyanide, a strong field ligand, also forms an inner orbital complex due to electron pairing.
3. \([Ni(NH_3)_6]^{2+}\): Nickel in the +2 oxidation state with ammonia, which is a weak field ligand, tends to form an outer orbital complex. This occurs because \(NH_3\) does not strongly pair the electrons, and the 4s and 4p orbitals are used along with the 3d orbitals for bonding.
4. \([Mn(CN)_6]^{4-}\): Manganese in the +2 oxidation state with cyanide, a strong field ligand, forms an inner orbital complex due to electron pairing.

Hence, among the given options, \([Ni(NH_3)_6]^{2+}\) is the outer orbital complex because it involves the use of outer d orbitals for bonding, assisted by the weak ligand ammonia.