[AdiDex]

I've got something written in my textbook ..i'm not able to understand it...

" Low oxidation state are found in transition elements when a complex compound has ligands capable of π (pi) - acceptor character in addition to σ (sigma) - bonding.
For example, in Ni(CO)₄ , the oxidation state of nickel is zero "

So what is meant by "ligands capable of π (pi) - acceptor character in addition to σ (sigma) - bonding" ...??
 
I thought That CO is a very strong field ligand that why it can make a complex with a metal (Even in zero oxidation state) 

but according to C.F.T. ( Crystal Field Theory ) ,there is no Overlapping between ligand's and metal's orbital , they just do electrostatic interaction.

So how these "π electrons"  and "σ bonding" came into the scene .

[kriggy]

Ok:

So what is meant by "ligands capable of π (pi) - acceptor character in addition to σ (sigma) - bonding" ...??

It means that the ligand can accept electrons from metal. The ligand donase its electron pair to metal forming sigma bond and then accepts electrons from metal into is empty pi antibonding orbitals.

I thought That CO is a very strong field ligand that why it can make a complex with a metal (Even in zero oxidation state)

Strong/weak ligands tell you whether the metal will be in low spin or high spin state.
Every ligand can complex metal even in low oxidation state, however, the low state is unstable and if you dont have suitable ligands the metal will get oxidized. The unstability of such low oxidation number metal is from the fact that it has a lot of electrons around so that is the reason you need ligands like CO or PR₃ to stabilize the low state: because they can reduce the electron density at central atom by backbonding.

If you want to have complex in (unusualy) high ox. state, then the unstability comes from very low number of electrons around, so you need ligands with good sigma and pi donor capabilities to increase the electron density and they have to be resilient against oxidation because metals in high oxidation states are strong oxidants (Mn(VII), Cr(VI)..)

Hope I answered all your questions

[AdiDex]

Thanks you so much...it was really helpfull.....thank you very much..
 

I have another question....
Strength of backbonding will decrease in higher oxidation state (Of central metal atom)...…?

[kriggy]

I dont know but it seems logical.