How Does d-d Transition Occur with No Free Higher Energy d-Orbitals Available?

[leojun]

how does d-d transition take place when there are no free higher energy level d-orbital's left(as the only two higher energy d orbitals form coordinate bond with CN^-?

 

[DrDu]

The d-orbitals form a covalent 2-electron bond with the sp hybrid orbitals on C. So in each bond there are 2 orbitals involved which can take up up to 4 electrons.

 

[leojun]

The d-orbitals form a covalent 2-electron bond with the sp hybrid orbitals on C. So in each bond there are 2 orbitals involved which can take up up to 4 electrons.

nope,they form a special covalent bond called coordinate covalent bond with cn^- and d-d transition happens within a transition metal or ion only.d-d transition doesn't happen between two atoms especially not with c.

 

[DrDu]

The dative bond looks like $\mathrm{Fe \quad C \uparrow\downarrow \longleftrightarrow Fe \uparrow \quad C\downarrow \longleftrightarrow Fe \downarrow \quad C \uparrow }$.
If you excite a d electron into that bond, you get a weaker 3 electron bond $\mathrm{Fe\uparrow \quad C \uparrow\downarrow \longleftrightarrow Fe \uparrow \downarrow \quad C\uparrow }$.

 

[leojun]

The dative bond looks like $\mathrm{Fe \quad C \uparrow\downarrow \longleftrightarrow Fe \uparrow \quad C\downarrow \longleftrightarrow Fe \downarrow \quad C \uparrow }$.
If you excite a d electron into that bond, you get a weaker 3 electron bond $\mathrm{Fe\uparrow \quad C \uparrow\downarrow \longleftrightarrow Fe \uparrow \downarrow \quad C\uparrow }$.

but there can only be two electrons in an hybridized orbital

 

[DrDu]

Yes, if you look, neither the d orbital un Fe nor the sp hybrid orbital on C contains more than 2 electrons in any of the resonance structures.

 

[leojun]

sorry for the disturbance brother,Dr Du . I wasn't satisfied,so i went to a near by book store and searched every single book on this topic.guess what i found...I FOUND THAT THERE WAS NO RELATION BETWEEN VALENCE BOND THEORY AND CRYSTAL FIELD THEORY(they are two different theories used to explain different properties of complex compound's.for example-VALENCE BOND THEORY is used to explain the orientation of the ligands around the central metal atom or ion where as the CRYSTAL FIELD THEORY is used to explain the coloration of complex compound ).

 

[DrDu]

It is true that crystal field theory is usually used to explain absorption spectra of complexes. However, that doesn't mean that an explanation in terms of VB theory isn't possible and as a trained theoretical chemist I would be rather worried if there were some qualitative properties of a molecule which I could explain using one theory but not, without good reason, some other.
The diagram you were showing in your first post alludes clearly to valence bond theory, as it introduces d2sp3 hybrids and first shows up in Pauling's book "The nature of the chemical bond", who uses mostly VB theory. Hence I tried to give you an explanation in terms of VB theory. If your question was in reality about crystal field theory, then I wonder why you didn't say so in the first post.

 

[leojun]

To be true,i didn't know that there was a VB theory way to explain it.i thought VB theory and CF theory were interrelated.
what i thought-
step 1 - coordination bond forms between ligand and central metal atom or ion forming complex ion
step 2 - after the formation of complex ion,CF theory comes into play and gives color to the complex ion(through d-d transition)
i wanted to know how d-d transition took place when there were no free higher energy level d-orbital's left(as the only two higher energy d orbitals formed coordinate bond with CN-. this confusion was due to my comprehensive textbook(gives info about all aspects but in a confusing way). thank you for your time:)