Multiple choice

Which of the following sets represents weak field ligands only?

  1. S2-, SCN-, NO3-

  2. NO2-, CN-, PPh3

  3. Cl-, N3-, NO2-

  4. NO3-, NO2-, I-

  5. PPh3, S2-, SCN-
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A Correct answer
Explanation

On the basis of ligand field theory, the spectrochemical series is an empirically-derived list of ligands ordered by the size of the splitting CFSE that they produce. It can be seen that the low-field ligands are all pi-donors, the high field ligands are pi-acceptors, and ligands such as H2O and NH3, which are are in the middle. The spectrochemical series is give as: I− < Br− < S2− < SCN− < Cl− < NO3− < N3− < F− < OH− < C2O42− < H2O < NCS− < CH3CN < py (pyridine) < NH3 < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO2− < PPh3 < CN− < CO. Hence, S2-, SCN- and NO3- all are weak field ligands.

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