Question

1. Describe back bonding by ligands and how it affects the t2g level in octahedral coordination

2. Describe what a pi-donor ligand is and its effects on the t2g level

3. Discuss how the spectrochemical series relates to the pi-donor or acceptor role of the ligand.

4. Describe the 18-electron rule and the situations in which the 16 electron rule apply.

Answer 1

1. If the metal present in a metal complex has high electron density (low valent / zero valent) and the ligand has suitable vaccant orbital, then metal can donate electron density to the ligand. This is known as back bonding.

Pi back bonding from the ligand decreases the energy of t2g level.

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2. Pi donor ligand is a ligand with filled non-bondling orbital which can overlap with metal orbitals. Example : F-

It increases the energy of the t2g orbital

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3. Pi acceptor ligands are the strongest ligand in the spectrochemical series. As, in this case, t2g is stabilize and energy of eg remains same, the net energy difference between t2g and eg increases. As a result ligands like CO, PPh3 are strongest ligand and lies in the extreme right side of the spectrochemical series.

On the other hand, in complex containing Pi donor ligand energy gap between t2g and eg decreses , thus these ligands are weak ligand althoough they are negatively charged. These ligands can be found at the extreme left side of the series.

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This rule states that When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period and as a result the complex becomes stable.

d^8 square planar or tetrahedral complexes obey 16 electron rule.