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We know that the Ligands which cause large degree of crystal filed splitting are termed as strong field ligands. and Ligands that cause only a small degree of crystal filed splitting are termed as weak field ligands. The common ligands can be arranged in ascending order of crystal field splitting energy. The order remains practically constant for different metals and this series is called the spectrochemical series.
The spectrochemical series is an experimentally determined series. It is difficult to explain the order as it incorporates both the effect of sigma and Pi-bonding. The halides are in the order expected from electrostatic effects. In other cases, we must consider covalent bonding to explain the order. A pattern of increasing sigma- donation is as follows:
               C-donors > P-donors > N-donors > O-donors >S-donors > X-donors
The crystal field stabilization produced by the strong CO and CN- is almost double that of halide ions. This is attributing pi- bonding in which the metal donates electrons from a filled t2g orbital into a vacant orbital on the ligand. In a similar way, many unsaturated N donors and C donors may also act as pi- acceptors.
Important Note:
Strong field ligands affects electronic configuration of central metal my making unpaired electron to pair up, while weak field ligands does not affect electronic configuration of central metal atom i.e. they does not make unpaired electrons to pair up.
Above Statement is valid for first transition series however for second and third transition series unpaired electrons, pair up irrespective of nature of ligands provide pairing of electrons is allowed.

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