ال�مركبات� في التآصر وطبيعة أنواع�التناسقية ) ( المركبات� الكيمياوية

The type and nature of bonding in chemical compounds

(coordination compounds)

أ.د. ماهر عبد الرزاق محمد

المركبات في التآصر وطبيعة أنواع)الكيمياوية التناسقية( المركبات

التساهمي Covalent)التآصرbonding )

األيوني Ionic )التآصرbonding)

التناسقي Coordination)التآصرbonding)

التناسقي Coordination)التآصرbonding )

سيكما bonding )σ( - تآصر

باي bonding )π( - تآصر

لتكوي�ن الليكاندات أوربتاالت م�ن أنواع ثالث�ة هنال�كأوربتاالت( πباي (أواصر وهي : dمع للفلز

أوربتالp سيكما آصرة محور على (( . σعمودي

أوربتالd. الفلز ذرة على يشتمل الذي المستوي في يقعآوربتال *π. الفلز ذرة على يشتمل الذي المستوي في يقع

Pi-bonding and MO Theory

أوربتاالت π تآصر وأوربتاالت dبين للفلزالليكاند

M LM (t2g) L(p)

M LM (t2g) L(d)

C O

M LM (t2g) L()

M CN

L M (bond)

M L

L M (bond)

Remember that pi-bonding is much weaker than sigma bonding, so this interaction simply perturbs the sigma MO diagram.

Note that the )t2g(represents triple degeneracy, )eg( double degeneracy and )a( single degeneracy.

Example: [CoF6]3-

In the case of F-, the t2g orbitals are full with ligand electrons and they are lower than the metals because 2p orbitals are lower in energy than 3d and the electrons make them more electronegative.

As the ligand pi-orbitals overlap with the sigma orbitals of the metal, the F- orbitals raise the Co orbitals but the eg orbitals are un-affected

This means the bonding orbitals are mostly F- in character and the anti-bonding orbitals are mostly Co in character.This effectively decreases the 10Dqand thus F- is a weak ligand.

Now look at pi-interactions of phosphine as a ligand )PR3(.

In phosphorous the 3d orbitals are empty so they are not very electronegative and thus are of higher energy compared to the metal sigma orbitals.

The same change in the orbital energies occurs but this time the bonding t2g orbitals are mostly metal in character and they decrease in energy .

This effectively increases the 10Dq and thus the phosphine ligands are considered strong field ligands.

Practical application and observations of MO Theory. The pi-bonding hypothesis can be corroborated by the IR stretch of CO.

Remember, IR indicates the strength of the CO bond.

The higher the wave number, the stronger bond.

For example, take the following complexes.

[Mn1+)CO(6]1+ > [CrO)CO(6]

0 > [V1-)CO(6]1-

As the metal increases in electron density )i.e. reduced(, then more electron density will be donated to the antibonding pi orbital of CO and the stretching frequency of CO will go down )i.e. weaker bond(.

Trans-ligand pi competition

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