The complex [FeF6]3– is colorless, whereas [CoF6]3– is colored; this is because :
Spin forbidden transitions in [FeF6]3– and spin allowed transitions in [CoF6]3–
Fluoride is a weak-field ligand, so both hexafluoro complexes are high spin. The colour then depends on whether a d–d transition is spin-allowed (no change in total spin, \(\Delta S=0\)) or spin-forbidden.
In [FeF6]3– the metal is Fe3+, d5 high spin (t2g3eg2), ground term \({}^{6}A_{1g}\). Every excited configuration requires pairing of an electron, and no other sextet state exists, so all d–d transitions are spin-forbidden as well as Laporte-forbidden. The absorptions are extremely weak and the complex looks almost colourless.
In [CoF6]3– the metal is Co3+, d6 high spin (t2g4eg2), ground term \({}^{5}T_{2g}\). Promotion of a t2g electron to eg gives the \({}^{5}E_{g}\) state of the same spin, so the \({}^{5}T_{2g}\rightarrow{}^{5}E_{g}\) band is spin-allowed and the complex is blue.
The fact that both are high spin is true but does not explain the colour difference, the reverse assignment of spin-allowed and spin-forbidden bands is wrong, and the iron complex is not low spin with fluoride ligands.
Hence, Spin forbidden transitions in [FeF6]3– and spin allowed transitions in [CoF6]3–.
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Important Questions from Mixed Topic (MH-SET)
.................... is a client server application that identifies each host on the internet with a unique user-friendly name.
Every OS has a ................ or .......... which primarily resides in the main memory of the computer to perform some of the basic functions of the OS.
(i) Kernel
(ii) Interface
(iii) Nucleus
(iv) Compiler
Codes:
Which of the following are referred to as low level language?
(i) Machine language
(ii) Assembly language
(iii) High level language
(iv) Natural language
Codes:
Match the following:
List I List II (a) New York Public Library (i) Shelv AR (b) Miami University Augmented Research Group (ii) Smart library (c) Oula University Library (iii) Wolf Walk (d) North Carolina State University (iv) Find the future Codes:
Which part of the QR code represents the encoded data?
