Question

Aqueous solutions of Co²⁺ salts are pale pink. They contain the complex ion [Co(H₂O)₆]²⁺. Addition of concentrated HCl results in a profound colour change to dark blue, due to formation of [CoCl₄]^2–. The main absorption bands of [Co(H₂O)₆]²⁺ and [CoCl₄]^2– occur at 19400 cm^–1 (e ~ 10 M^–1 cm^–1) and 14700 cm^–1 (e ~ 1000 M^–1 cm^–1), respectively.

(i)    Explain the difference in the transition energies between the two complexes.

(ii)   Using a qualitative molecular orbital diagram, explain why the absorption band of [CoCl₄]^2– is much more intense than that of [Co(H₂O)₆]²⁺.   

Answer 1

en given [eo Cha do ghion which is pole pink and 5 [co clyga which is dark blue in rolour: Also [alt20) ] 2t y llanoo e-mth hung 1 (carely ga . 14700 cm? : () These two ions have difference in transition . energy because of following reasons If we take [co CHao), 2f it is an octahed real emplex to is in t? ois that means at metal ion soi dorbital Spliting is given as co2t - difference between tag and Rg feparation is the d- orbital splitting ... on the other. hand in case of [coolu ? it is a : tetrahedşal comprenant \ 12 11 19 tag na 1 tz Go2t (dt) e il eg the d-orbital splitting is as shown .. in case of [colteo), 27 af SE = [5x0.4 + 2x0.6) ao = (-2.0.71-23 Do : -0-8. do .., in case of [cocky 727.CASE = (^uX 6-6.+ 3x04] A4 (-24 +1-2707 E A do] 2 st Lift q -.2xy > 0 (A206] ?have more (FSE .. : 50 -5.3 Do: so, the separation between d orbital fs more .. in case of [co (H2O) J27. Thats why it hare e .. more transition energy, as the transition energy Is Scandependensadpen the separation between the doorbital

(") metal orbital and ligand group orbital having sama symmetay undergo eller lap to form molecular dobital. Orbital diagram for [to (Mae)32);- ☆ t28 I agttiut 6.H2O -- . Ziu! .CO24 m.o dia gream for [co clu]2 all in nptz. ins al aittz (024 CS Scanned with CamScanner LOW

Intensity of colour of transition metal complenes governed by two selection rule such as Spin Selection rule . Laporte selection sule .. Spin selection sile states that during fransition As = 0 . and laporte selection siles states that during transition Al=11 The given complenes, both are spin allowed But [co (H2O) get complen is laporte forbidden as during transition al zo i But in case of [co.cly J² it is laparte partly allowed because there is mining of symmetry between :p-d orbital as we can see from molecular orbital diagicam , thius. Hocly 32 is intense than that of [108120)6324 Csscanned with CamScanner V it