Absorption bands of ions (teaching fits)

The absorption bands behind the site’s computed colours of first-row and lanthanide ions: positions are literature maxima, and ε values are approximate fits chosen to reproduce the observed colours.


$\ce{[Sc(H2O)6]^3+}$	3	no visible absorption	—	colourless
$\ce{[Ti(H2O)6]^3+}$	3	d–d	493 nm (ε 4); 575 nm (ε 2.4)	violet
$\ce{[V(H2O)6]^2+}$	2	d–d	810 nm (ε 3.2); 541 nm (ε 4.4); 358 nm (ε 7)	violet (lilac)
$\ce{V^3+}$	3	d–d	588 nm (ε 5); 408 nm (ε 8)	green
$\ce{VO^2+}$	4	d–d	763 nm (ε 17); 625 nm (ε 9)	blue
$\ce{VO2+}$	5	charge transfer	320 nm (ε 2500)	yellow
$\ce{[Cr(H2O)6]^2+}$	2	d–d	709 nm (ε 5)	sky blue
$\ce{[Cr(H2O)6]^3+}$	3	d–d	575 nm (ε 13); 407 nm (ε 15)	blue-violet (green when Cl⁻ binds)
$\ce{CrO4^2-}$	6	charge transfer	372 nm (ε 4830)	yellow
$\ce{Cr2O7^2-}$	6	charge transfer	350 nm (ε 3150); 440 nm (ε 370)	orange
$\ce{[Mn(H2O)6]^2+}$	2	d–d	529 nm (ε 0.02); 433 nm (ε 0.02); 401 nm (ε 0.04); 357 nm (ε 0.03)	very pale pink
$\ce{MnO4^2-}$	6	charge transfer	606 nm (ε 1710); 439 nm (ε 1380); 347 nm (ε 1830)	green
$\ce{MnO4-}$	7	charge transfer	526 nm (ε 2400)	purple
$\ce{[Fe(H2O)6]^2+}$	2	d–d	962 nm (ε 2.2); 1205 nm (ε 1.4)	pale green
$\ce{Fe^3+}$	3	charge transfer	335 nm (ε 300)	yellow-brown (chloride solution)
$\ce{FeO4^2-}$	6	charge transfer	505 nm (ε 1150)	red-purple
$\ce{[Co(H2O)6]^2+}$	2	d–d	1235 nm (ε 2); 516 nm (ε 4.8); 464 nm (ε 2)	pink
$\ce{[Co(NH3)6]^3+}$	3	d–d	476 nm (ε 56); 339 nm (ε 46)	orange-yellow
$\ce{[Ni(H2O)6]^2+}$	2	d–d	1177 nm (ε 2); 725 nm (ε 2.1); 649 nm (ε 1.5); 395 nm (ε 5.2)	green
$\ce{Cu+}$	1	no visible absorption	—	colourless
$\ce{[Cu(H2O)6]^2+}$	2	d–d	800 nm (ε 12)	light blue
$\ce{[Zn(H2O)6]^2+}$	2	no visible absorption	—	colourless
$\ce{La^3+}$	3	no visible absorption	—	colourless
$\ce{Ce^3+}$	3	no visible absorption	—	colourless
$\ce{Pr^3+}$	3	f–f	444 nm (ε 10.4); 469 nm (ε 4.4); 482 nm (ε 4.4); 588 nm (ε 1.9)	green
$\ce{Nd^3+}$	3	f–f	354 nm (ε 2.3); 512 nm (ε 1.5); 521 nm (ε 4.2); 575 nm (ε 7.2); 740 nm (ε 7); 794 nm (ε 11.5)	lilac
$\ce{Sm^3+}$	3	f–f	362 nm (ε 1.8); 375 nm (ε 1.5); 402 nm (ε 3.4)	pale yellow
$\ce{Eu^3+}$	3	f–f	394 nm (ε 2.9)	very pale pink
$\ce{Gd^3+}$	3	no visible absorption	—	colourless
$\ce{Tb^3+}$	3	no visible absorption	—	very pale pink
$\ce{Dy^3+}$	3	f–f	350 nm (ε 2.5); 365 nm (ε 2); 388 nm (ε 1)	pale yellow
$\ce{Ho^3+}$	3	f–f	361 nm (ε 2.4); 416 nm (ε 2.1); 451 nm (ε 4.4); 537 nm (ε 3.3); 641 nm (ε 1.8)	yellow
$\ce{Er^3+}$	3	f–f	379 nm (ε 8.5); 487 nm (ε 1.5); 523 nm (ε 3.4); 653 nm (ε 2.1)	rose-pink
$\ce{Tm^3+}$	3	f–f	360 nm (ε 1.5); 684 nm (ε 2.5); 785 nm (ε 1.3)	pale green
$\ce{Yb^3+}$	3	no visible absorption	—	colourless (absorbs in the near infrared)
$\ce{Lu^3+}$	3	no visible absorption	—	colourless

These are teaching fits, not measured spectra: bands are drawn as Gaussians whose widths and ε were adjusted to reproduce the documented colours. d–d and f–f transitions are weak (ε about 1–100); charge transfer is strong (ε up to thousands).

Sources

A. B. P. Lever, Inorganic Electronic Spectroscopy, 2nd ed., Elsevier (1984) (d–d band positions).
C. J. Ballhausen, H. B. Gray, Inorg. Chem. 1, 111 (1962) (vanadyl ion).
W. T. Carnall, P. R. Fields, K. Rajnak, J. Chem. Phys. 49, 4412 (1968) (Ln³⁺ aqua ions).
N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed. (1997) (observed colours).

Sources

Used in these lessons