Question

The figure below shows the magnetic susceptibility property of a metal complex, (phen) Fe(NCS)2. 4 It appears that at 180 K, there is a subtle change in its magentic property. (i) Suggest which type of spin state it has at temperature below and above 180 K. (ii) suggest a probable reason why there is a change in spin state at that temperature. (6 + 3 point) 3.5 3.0 2.5 2.0 xT, emu:K/mol OOO OOO 1.5 ZEO 1.0 0.5 0 0.0 0 50 100 150 200 250 300 350 Temperature, K structure of (phen)2Fe(NCS)2 complex

Answer 1

This is an example of spin state crossover or spin isomerism.

Fe (II) > The magnetic moment of Fe (pheniz Cres)2] was found to be approximately around 3.0 B.M. But you Fe (IT) low spion, it should be around zero and for high spin it should be around 5.0 B.M. Hence 3.0 B. M is an intermediate magnetic moment. Thus, it was suggested that there exists equilibrium between the high spin (Hos) and low spin (Los) state at 180 K. Variable temperature magnetic moment suggests that aut lower temperature ranges , the complex will be Lis and with increase in temperature, the His state will gradually be populated. But it is never possible to excite all the molecules from Lis to his state. So, below 180k, will it be Ls and above 180k, itt will be hos. forms of the the have comples formula, they can be considered as the phenomenon is called spin since the L'S & His same molecular isomers isomerism.

do (Fe Cphen), (NCS72] 구 eg S=0 s=2 tag do Los Meff « 1.73 Meff ~ 5.0 B.M Spin isomerism The complex exhibits because ho value A value and pairing energy (P) are close to other. each At lower than that of L'S field strength (lower to value), the CFSE for His is more negative & hence his state is more stable. At higher field strength Chigher 4 value) negative than that of Hos while there iu a region spin state crossones, where there exists a temperature dependent His & Lis, energy of the his is more where 4 ñ p equilibrium