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Coordination Compounds 749

TABLE IV Energy Gaps in Analogous Species Containing Cobalt, Rhodium, and Iridium
Compound Configuration Transition Energy (kK) a Color
5
6
Co(NH 3 ) 3+ (3d) 6 (t 2g ) → (t 2g ) (e g ) 1 21.3 Orange-yellow
6
5
6
Rh(NH 3 ) 3+ (4d) 6 (t 2g ) → (t 2g ) (e g ) 1 33.3 White
6
5
6
Ir(NH 3 ) 3+ (5d) 6 (t 2g ) → (t 2g ) (e g ) 1 40.0 White
6
5
6
trans-[Co(py) 4 Cl 2 ] + b (3d) 6 (t 2g ) → (t 2g ) (e g ) 1 16.0 Green
6
5
trans-[Rh(py) 4 Cl 2 ] + b (4d) 6 (t 2g ) → (t 2g ) (e g ) 1 24.5 Yellow
trans-[Ir(py) 4 Cl 2 ] + b (5d) 6 — — White
a −1
1 kK (kilokayser) = 1000 cm .
b
All are the trans-dichloro isomers; py = pyridine, C 5 H 5 N.
3 2 1 brought about not by d–d excitation but by other light ab-
from (t 2 g ) → (t 2 g ) (e g ) , the result of increasing bond
strength (i.e., stability) is to increase the ligand field split- sorptions, such as charge transfer. For example, on the
ting
E (Table V). irradiation of trisoxalatoferrate(III) ion with UV light, the
The order of increasing splitting of the d levels by process shown in Eq. (45) takes place through the photo-
the ligands is the spectrochemical series: F < OH 2 < chemical excitation shown in Eq. (46).
−
−
NH 3 < NO < CN . The last entry in Table V shows an
−
2 3− 2− −
exampleofthebaricenter(centerofgravity)rule.Ifthefre- [Fe(C 2 O 4 ) 3 ] → [Fe(C 2 O 4 ) 2 ] + CO + CO 2 (45)
quency of a particular transition in an environment (MA 6 ) +
Fe(III) ← (C 2 O 4 ) 2− + h ν → Fe(II) − (C 2 O 4 ) −
is ν 1 and that for MB 6 is ν 2 , then for [MA n B 6−n ] it is the
weighted average [Eq. (44)]. Such mixed coordination (46)
(n ν 2 + (6 − n)ν 2 )/6 (44)
spheres, where there is more than one type of molecule F. Solvent Effects
acting as ligand, are very common. The effects of sub- Nearly all the basic notions of the chemistry of complex
6
stituting ammonia by chloride for cobalt(III) [3d ] and ions are derived from aqueous systems, and striking depar-
3
chromium(III) [(3d) ] are shown in Table VI. tures often occur when other solvents are used. For exam-
ple,thecoordinationofiron(II)bydiimineligandschanges
its complexion as follows. In water β 110 > β 120 β 130 .
E. Photochemistry
2+
The tris species, for example, [Fe(phen) 3 ] , in water or
The photochemistry (chemical reactions of molecules in isolated from water as salts with chloride and so on is dia-
6
excited electronic states, made by irradiating with light of magnetic (d spin-paired). In water, where anions such
the appropriate energy to promote them from their ground as chloride are well solvated, paramagnetic solids such as
states) of complex compounds is not as useful as might [Fe(phen) 2 Cl 2 ] dismute, as in Eq. (47).
be expected. In general, the most effective reactions are
6H 2 O + 3[Fe(phen) 2 Cl 2 ] → 2[Fe(phen) 3 ] 2+
3
TABLE V Colors of d Coordination Complexes + Fe(OH 2 ) 2+ + 6Cl − (47)
6
Complex λ a n (=E /h) b Color In acetone and similar nonprotic solvents, the bis species is
/
perfectly stable, and it is now the tris species that is unsta-
[CrF 6 ] 3− 671 14.9 Green
ble [i.e., β 120 β 130 , the opposite of water; see Eq. (48)].
[Cr(OH 2 ) 6 ] 3+ 575 17.4 Violet
To speak of the spin pairing of the tris species as ligand
[Cr(NH 3 ) 6 ] 3+ 464 21.55 Orange-yellow
field stabilization is incorrect, since that stabilization is
[Cr(CN) 6 ] 3− 376 26.6 Pale yellow
effective only in water. The remarkable, and often quoted,
[Cr(NC 5 H 5 ) 3 Cl] c 629 15.9 Green
3
[Cr(OH 2 ) 4 Cl 2 ] +d 635 15.75 Green reversal of the stability constants for adding the second
and third chelating ligands to iron(II) ions must actually
a
Wavelength of absorption maximum in nanometers. stem rather from changes in solvation energies of the an-
b
Frequency of absorption maximum in kilokaysers (1 kK = ions present.
1000 cm −1 ).
c
Mer isomer; NC 5 H 5 is pyridine.
d 2+ −
Trans isomer. [Fe(phen) 3 ] + 2Cl → [Fe(phen) 2 Cl 2 ] + phen (48)