THe thiocyanate ion acts as a Lewis base, donating a pair of electrons to the Fe ^ 3+ ion. Both sulfur and nitrogen atoms have lone pair electrons that can potentially be donated. Therefore, ther are two different structures (linkage isomers) that can be created when Fe(H2O)6 ^+3 reacts with SCN-.
How does the Lewis structure of the two isomers look like??
Here is an article that describes both linkages, known as linkage isomers. Thiocyanates are discussed more fully but scroll down to Organic thiocyanates to see how they are bonded as isothiocyanates.
http://en.wikipedia.org/wiki/Thiocyanate
To determine the Lewis structures of the two linkage isomers formed between Fe(H2O)6^+3 and SCN-, we first need to understand the electronic configurations and bonding abilities of the atoms involved.
1) In the Fe(H2O)6^+3 complex, the iron ion (Fe^3+) has a 3+ charge, meaning it has lost three electrons and has an electronic configuration of [Ar] 3d^5. The six water molecules (H2O) act as ligands, each donating a lone pair of electrons to form coordinate bonds with the iron ion.
2) The thiocyanate ion (SCN-) has a negative charge, indicating that it has gained an electron and has a configuration of [S=C=N]^- or [S-C≡N]^-.
Now, let's consider the two possible linkage isomers:
a) In the first isomer, the sulfur atom of the SCN- ion acts as the donor, donating its lone pair electrons to the empty d-orbitals of the Fe^3+ ion. The nitrogen atom is not involved in the coordination, and the coordination number of the iron ion remains six.
The Lewis structure for this isomer can be represented as follows:
Fe(H2O)5(SCN)^2+
b) In the second isomer, the nitrogen atom of the SCN- ion acts as the donor, donating its lone pair electrons to the empty d-orbitals of the Fe^3+ ion. The sulfur atom is not involved in the coordination, and the coordination number of the iron ion remains six.
The Lewis structure for this isomer can be represented as follows:
Fe(H2O)5(NCS)^2+
In both isomers, the iron ion is surrounded by five water molecules (H2O) and one SCN- or NCS- ligand. The different orientations of the ligand result in the two distinct linkage isomers.