(i) The exchange of free solvent molecules such as water, NH₃, alcohol etc. in the crystal lattice with a ligand in the co-ordination entity will give different isomers. These type of isomers are called solvate isomers.
(ii) If the solvent molecule is water, then these isomers are called hydrate isomers. For example, the complex with chemical formula CrCl₃ .6H₂O has three hydrate isomers as shown below.
Complex Colour No. of chloride ions in solutions [Cr(H₂O)₄Cl₂]Cl.2H₂O dark green colour 1 [Cr(H₂O)₅Cl]Cl₂.H₂O pale green colour 2 [Cr(H₂O)₆]Cl₃ violet colour 3

(i) Oxidation state of the central metal ion : +2 (i.e) Mn²⁺
(ii) Co-ordination number : 6
(iii) Nature of ligand: CN^- (Negative ligand)
(iv) Magnetic property: Paramagnetic nature
(v) Electronic configuration: 

(vi) Oxidation state: +2 (or) Mn²⁺ of central metal ion

The CFSE is defined as the energy of the electronic configuration in the ligand field minus the energy of the electronic configuration in the isotropic field.
CFSE (\(\Delta\)E_o) = {E_Lf}-{E_iso}
={[n_t2g(-0.4) + n_eg(0.6)]\(\Delta\)_o + n_pP} - {n'_pP}
Here, n_t2g is the number of electrons in t_2g orbitals;
n_eg is number of electrons in e_g orbitals;
n_p is number of electron pairs in the ligand field; &
n'_p is the number of electron pairs in the isotropic field (barycenter).
P - pairing energy

In \(\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]^{3-}\) Cobalt is in +3 oxidation state
\(\mathrm{Co}=[\mathrm{Ar}] 3 \mathrm{~d}^{7} 4 \mathrm{~s}^{2}  \)
\(\mathrm{Co}^{3+}=3 \mathrm{~d}^{6} 4 \mathrm{~s}^{\circ}\)

It is diamagnetic; n = 0
d²sp³ hybridisation; μ_s = 0

Bonding in metal carbonyls:
(i) In metal carbonyls, the bond between metal atom and the carbonyl ligand consists of two components.
(ii) The first component is an electron pair donation from the carbon atom of carbonyl ligand into a vacant d-orbital of central metal atom.
(iii) This electron pair donation forms \(M\overset { \sigma \ bond }{ \longleftarrow } \text {CO sigma bond}\)
(iv) This sigma bond formation increases the electron density in metal d orbitals and makes the metal electron rich.
(v) In order to compensate for this increased electron density, a filled metal d-orbital interacts with the empty \({ \pi }^{ \bigstar }\) orbital on the carbonyl ligand and transfers the added electron density back to the ligand.
(vi) This second component is called \(\pi\)-back bonding.
(vii) Thus in metal carbonyls, electron density moves from ligand to metal through sigma bonding and from metal to ligand through pi bonding, this synergic effect accounts for strong M⇽CO bond in metal carbonyls.
(viii) This phenomenon is shown diagrammatically as follows.