Question

The complex that has the highest crystal splitting energy \[(\Delta )\], is:
A.\[{K_3}[Co{(CN)_6}]\]
B.\[\left[ {Co{{\left( {N{H_3}} \right)}_2}\left( {{H_2}O} \right)} \right]C{l_3}\]
C.\[\left[ {Co{{\left( {N{H_3}} \right)}_3}\left( {{H_2}O} \right)} \right]C{l_3}\]
D.\[\left[ {Co{{\left( {N{H_3}} \right)}_5}Cl} \right]C{l_2}\]

Answer 1

Hint: To answer this question, you should recall the concept of crystal field theory. Crystal field theory helps to describe the breaking of energies of electron orbital states, due to another electric field produced by a surrounding charge distribution known as a ligand. This can describe many properties such as spectroscopies of transition metal coordination complexes, magnetic properties, colours, hydration enthalpies, and spinel structures of transition metal complexes.

Complete step by step answer:
We know that word ligand means to tie or bond. The role of a ligand is that it donates a pair of electrons to the central metal atom or ion to form a coordination complex. Ligands can be divided into a strong field and weak field ligands. Strong field ligands result in large splitting in crystal field energy and weak field ligands produce a small splitting.
According to Crystal Field Theory, metal and ligand are considered as point charges. This theory deals with the electrostatic force of attraction which means it talks about the ionic character of \[M - L\] the bond. The complex with the greater number of unpaired electrons is known as the high spin complex, the low spin complex contains the lesser number of unpaired electrons. High spin complexes result in cases with weak field ligands as expected where the crystal field splitting energy is small. The opposite applies to the low spin complexes in which strong field ligands cause maximum pairing of electrons in the set of three atomic orbitals due to large.
As complex \[{K_3}[Co{\left( {CN} \right)_6}]\;\] have \[C{N^ - }\;\] ligand which is strong field ligand amongst the given ligand in other complexes.

Hence, the correct option is option A.

Note:
Make sure that you remember the spectrochemical series of ligands. Here they are ordered by the size of the splitting crystal field energy:\[{I^ - }\; < \;B{r^ - }\; < \;{S^{2 - }}\; < \;SC{N^ - }\;\left( {S-bonded} \right){\text{ }} < \;C{l^ - }\; < \;N{O_3}^ - \; < \;{N_3}^ - \; < \;{F^ - }\; < \;O{H^ - }\; < \;{C_2}{O_4}^{2 - }\; < \;{H_2}O\; < \;NC{S^ - }\;\left( {N-bonded} \right){\text{ }}\]
\[ < \;C{H_3}CN\; < \;py\; < \;N{H_3}\; < \;en\; < \;2,2' - bipyridine\; < \;phen\; < \;N{O_2}^ - \; < \;PP{h_3}\; < \;C{N^ - }\; < \;CO.\]