Crystal Field Theory and Coordination Complexes

In a weak field ligand system, the electron filling of splitting of d-orbital corresponds to Hund’s rule, followed by pairing. In strong field ligand system, the electron filling of splitting of d-orbital corresponds to pairing at initial stage.
For Fe³⁺ electronic configuration is d⁵.
For d⁵ in weak field ligand system,
CFSE = 3 (- 4) + 2 (6) = - 12 + 12 = 0 Dq (All electrons is unpaired, 3 in t_2g and 2 in e_g)
For d⁵ in strong field ligand system,
CFSE = 5 (- 4) + 0 = - 20 Dq + 2P
(two paired electrons and one electron is unpaired in t_2g and 0 electron in e_g orbital)
Hence, the CFSE for octahedral complexes of Fe³⁺ in a weak field and strong field ligand systems are 0 and -20 Dq+2P respectively.

V²⁺ has d³ electronic configuration and it is empowered in a weak field ligand.
The term symbol is calculated by the formula-
2s+1 LJ
For d³,
n = 3
s = n/2 = 3/2
2s + 1 = 4
L = 3 = F
J = (L + s) to (L - s) = 2, 1, 0
term symbols = 2s + 1 LJ = ⁴F_9/2, ⁴F_7/2, ⁴F_5/2, ⁴F_3/2
The ground state term for V²⁺ is ⁴F_3/2.

Ground state term for Mn⁴⁺ is ⁶S_5/2.
Ground state term for Fe²⁺ is ⁵D₄.
Ground state term for Co³⁺ is ⁵D₄.

Fe³⁺ and Fe²⁺ have d⁵ and d⁶ electronic configurations and both are in high spin ligand system.
The term symbol is calculated by the formula-
2s+1 LJ
For d⁵,
n = 5
s = n/2 = 5/2
2s + 1 = 6
L = 0 = S
J = (L + s) to (L - s) = 5/2
Possible term symbol for Fe³⁺ = 2s + 1 LJ = ⁶S_5/2
Similarly, possible term symbols for Fe²⁺ = ⁵D₄, ⁵D₃, ⁵D₂, ⁵D₁, ⁵D₀
The ground state term for Fe³⁺ is ⁶S_5/2.
The ground state term for Fe²⁺ is ⁵D₄.

(η-C₅H₅)₂Ti has zero number of unpaired electrons and possesses a diamagnetic nature.

The ground state terms for d¹, d², d³ and d⁴ electronic configurations are ²D_3/2, ³F₂, ⁴F_3/2 and ⁵D₀ respectively.
Hence, the representation of codes P - 2, Q - 3, R - 1, S - 4 is the correct answer.

On the basis of ligand field theory, the spectrochemical series is an empirically-derived list of ligands ordered by the size of the splitting CFSE that they produce. It can be seen that the low-field ligands are all pi-donors, the high field ligands are pi-acceptors, and ligands such as H₂O and NH₃, which are neither of the two, are in the middle.
The spectrochemical series is give as:
I− < Br− < S²⁻ < SCN− < Cl− < NO₃⁻ < N₃⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < CH₃CN < py (pyridine) < NH₃ < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO₂⁻ < PPh₃ < CN⁻ < CO
Hence, S^2-, SCN^- and NO₃^- are all weak field ligands.

[FeF₆]^3- and [Mn(H₂O)₆]³⁺ both have sp³d² hybridisation and hence, are outer orbital or high spin complexes.

A and B both have a paramagnetic nature with 2 unpaired electrons respectively.
Hence, these are identical in the number of unpaired electrons.

The coordination complexes [Mn(NH₃)₆]²⁺, [(Ph₄As)₂Mn(NO₂)₄], [Mn(EDTA)(NH₃)]^2- and [Mn(S₂C₆H₃Me)₂]^- have the coordination numbers of 6, 8, 7 and 4 respectively.
Hence, the representation of the codes P - 5, Q - 2, R - 1, S - 4 is the correct answer.

(PPh₃)₃RhCl is a Wilkinson’s catalyst and catalyses the hydrogenation of alkenes, Rh(CO)₂I₂ is a Monsanto catalyst and is an important process for the manufacture of the acetic acid. PdCl₄ is a Wacker catalyst. The Wacker process is originally referred to as the oxidation of ethylene to acetaldehyde by oxygen in water in the presence of a tetrachloropalladate catalyst. Al(C₂H₅)₃/TiCl₄ is a Ziegler-Natta catalyst. It is as a catalyst in the synthesis of polymers of alpha-alkenes.
Hence, the correct answer using the codes is A - 2, B - 1, C - 4, D - 3.