A complex whose IUPAC name is not correctly written is :

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Complex	Name
$F e {(σ - C_{5} H_{5})}_{2}$	Bis( $η^{5}$ -cyclopentadienyl)iron(0)

Complex	Name
Cr(C₆H₆)₂	Bis( $η^{6}$ -benzene)chromium(0)

Complex	Name
[CoCl₂(H₂O)₄]Cl.2H₂O	Tetraaquadichlorocobalt (III)chloride-2-water

Complex	Name
[Zn(NCS)₄ ]^2-	Tetrathiocyanato-N-zincate (II) ion

answer is A.

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Detailed Solution

The IUPAC name of the complex Fe(σ−C₅H₅)₂ is Bis(η¹-cyclopentadienyl)iron(0) and not Bis(η⁵-cyclopentadienyl)iron(0).
this is because the ligand is attached to metal through sigma bond only. Thus option 1 is incorrectly named.
The IUPAC name of the complex Cr(C₆H₆)₂ is Bis(η⁵-benzene)chromium(0). In this complex, the ligand is attached to metal ion through pi bonds.
Thus option 2 is correctly named.
The IUPAC name of the compound [CoCl₂(H₂O)₄]Cl.2H₂O is tetraaquadichlorocobalt(III)chloride-2-water.
The prefix tetra indicates four ligands and the prefix di indicates two ligands. The ligands are named in alphabetical order.
Thus option 3 is correctly named.
The IUPAC name of the complex [Zn(NCS)₄]²⁻ is tetrathiocyanato-N-zincate(II)ion.
Here N represents the linkage through N atom of thiocyanato group.The prefix tetra indicates four ligands. For complex anion, the name of the zinc is written as zincate.
Thus option 4 is correctly named