# Why is Fe3+ SCN- Dark Red?

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Hi this is a question on my chemistry test, it is a follow-up question after asking us to calculate the frequency of a 460nm photon, and was given a conversion table with colours, complementary colours, and frequency range.

I don't get how does it relate to each other...

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This is odd, I have received this exact same question from my school's chemistry test before on "Calculating frequency of a 460nm photon"

- Use Planck's Equation of c = λ ×υ by plugging the wavelength into the equation.

(v = 3.00*10^8m/s  / [460nm * 1m/1*10^9nm]) and you will find the frequency with the unit of (l/s)

And for the rest of the question, I need to read my Chemistry booklet again

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460 nm is blue light, which is the light absorbed. Other colors are transmitted, especially the complementary colour, red. Red is the apparent colour because it was least absorbed.

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4 minutes ago, kw0573 said:

460 nm is blue light, which is the light absorbed. Other colors are transmitted, especially the complementary colour, red. Red is the apparent colour because it was least absorbed.

But I thought the complementary colour of blue is orange?

Also, though the two questions are subquestions of the same question, how could I know it is related in this way?

Quote

(iii) Calculate the frequency of a 460 nm photo. The speed of light is 3.00 x 10^8 ms
(iv) State and explain what colour the Fe3+ SCN- Solution would be. A frequency-colour table is provided below:

Red: 3.8-4.8 x 10^14  Complimentary colour: green
...

Blue: 6.1-6.59 x 10^14 Complimentary colour: orange

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(Replied in private message too)

f= C/wavelength(in meters)
f= 300,000,000/(4.6 x 10^-7)
f= 6.52173913 × 10^14 Hz  (this part must be correct)

So according to the frequency graph, it is indeed blue light.  However, complementary colour is when the ion absorbs blue light, then reemits orange light due to  the following below, (which will appear in later in Chem HL).

When ligands bond to a transition metal ion, electrons in the ligands repel electrons in the five d orbitals of the metal ion, splitting them into high and low energy orbitals. When white light is passed through a complex, a specific frequency of light is absorbed to promote an electron from a lower energy orbital to an upper one. As a consequence, the other colours are transmitted (which is, complementary colour). Different complex ions absorb different frequencies of visible light, giving rise to different colours.

The complementary color is orange (though in real life, [FeSCN2+] would be blood red due to it is 447nm wavelength, that's a whole new answer).

I am indeed not talented in Chemistry.

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