This is because Cu²⁺ oxidizes I^- to I₂(2Cu²⁺ + 4I^{- \(\rightarrow \)} Cu₂I₂(s)+I₂) or I^- ions reduce Cu²⁺ to Cu⁺.

tetracyanonickelate (II) ion.

(i) mercuric tetrathiocyanatocobaltate (II)
(ii) potassium dicarbonylcyanonitrosocobaltate (II)

(i) pentaamminechloridocobalt (III) chloride
(ii) chloridobis (ethane-1, 2-diamine) nitritocobalt (III) ion.

In both the complexes, Fe is in +3 oxidation state with the configuration 3d⁵. CN- is a strong ligand. In its presence, 3d electrons pair up leaving only one unpaired electron. The hybridization is d 2sp³forming inner orbital complex. H₂0 is a weak ligand. In its presence, 3d electrons do not pair up. The hybridization is sp³ d²  forming an outer orbital complex containing five unpaired electrons. Hence, it is strongly paramagnetic.

Stability of coordination compounds in aqueous solution is determined with the help of satbility constant. Higher the value of stability constant, greater will be stability. Smaller the cation, higher the charge on the cation, more stable will be the complex. Chelating agents form more stable complex than monodentate ligand. Stronger the ligand, more stable will be the complex.

(I) Dichlorido-bis (ethane 1,2-diamine) cobalt(ill) chloride.

(il) Diamminechloridonitrito-N-platinum (II)​​
 

-1 ∵ + 1 + x + 0 = 0  ⇒ x = -1
[oxidation number of 'K' is +1, CO is neutral ligand].

Let us assume that the six ligands are positioned symmetrically along the cartesian axes, with metal atom at the origin. As the ligands approach, first there is an increase in energy of d-orbitals relative to that of the free ion just as would be the case in a spherical field  The orbitals lying along the axes (\({ d }_{ { z }^{ 2 } }\)nd \({ d }_{ { x }^{ 2 }-{ y }^{ 2 } }\))get repelled more strongly than dx¹ d and d_yz and d_zx orbitals which have lobes directed between the axes. The \({ d }_{ { z }^{ 2 } }\)and  orbitals get raised in energy and d_xy d_yz d _xz orbitals are lowered in energy relative to the average energy in the spherical crystal field. Thus, the degenerate set of d-orbitals get split into two sets : the lower energy orbitals set and the higher energy orbitals eg set. The energy is separated by t_2g and the higher energy orbitals e_g set. The energy is separated by \({ \Delta }_{ 0 }\)

(a) Ligand is an atom or group of atoms or ions which can donate a pair of electrons to vacant d-orbitals of metal atom or ion. Ethane -1, 2-diamine H₂NCH₂CH₂NH₂ or oxalate ion is an example of bidentate ligand.
(b) Ni(28) : [Ar]4s²3d⁸,
     Ni²⁺        : [Ar] 4s⁰3d⁸
 CN^- is strong field ligand will cause pairing of electrons.
 
It has dsp² hybridisation, square planar shape and diamagnetic in nature.
Ni (28) : [Ar] 4s²3d⁸
Ni(28) : [Ar]4s⁰3d¹⁰
 

(i) +3 (III)  
(ii) dsp, octahedral
(iii) paramagnetic
(iv) Two geometrical isomers
(v) Yes, there may be optical isomer also due to presence of polydentate ligand.
(vi) Dichlorido bis-(ethane 1, 2-diamine) Iron (III)

\({ \triangle }_{ t }=\left( \frac { 4 }{ 9 } \right) { \triangle }_{ 0 }\).  Thus,  \({ \triangle }_{ t }\) is smaller than \({ \triangle }_{ 0 }\).  Hence, less energy (higher wavelength) is absorbed by tetrahedral complexes than by octahedral complexes of the same metal and ligands.  Therefore, the observed colour are different.

When white light falls on the complex, some part of it is absorbed.  Greater the CFSE, greater is the energy absorbed or shorter is the wavelength absorbed \((E={hc\over\lambda })\).  The observed colour is the complementary colour of the colour absorbed.