Allowed and Forbidden d-d Transitions i n C r ( o ~ a l a t e ) ~ ~ Introductorv treatments of electronic structure and s ~ e c t r of a transition metal comnlexes usuallv cite the small molar
a good laboratory introduction to transition metal chemistry.
Experimental The synthesis of K3Cr(oxalate)3.3HzO is described in Inorganic Synthesis ( 2 ) and, with modifications, in a recent laboratory manual (3).Both preparations work well, though sometimes the reaction mixture does not spontaneously warm up and must be heated. The crude product can be recrystallized from water by reducing the volume or by adding ethanol. The recrystallization should be done quickly with minimal heating to minimize aquation of the complex. M solutions in H 2 0 using a l-cm path-length cell yields Visible spectroscopy of spectra with convenient ahsorbance for the allowed hands and a clearly visible forbidden band (see figure). Assignment of Transitions In the Cr(oxalate),'-
x
I
i
Spectrum
20
n-' .I@
':
i
I
band maximum nm
cm-'
'using
A = 17,500 c m ' .
r(M-' em-')
B
= 660 cm-'
assignments
late)?
in aqueous solution at r o o m
temperature. Interpretation Reference to the d3 Tanahe-Sugano diagram (4) rev&& that the lowest energy electronic transitions would be expected to be from the 'A2 ground state t o thedT2 and 'TI excited states (spin allowed) and to the 2E, ZT1, excited states (spin forbidden). The transitions can be assigned as given in the table by finding the value of ligand field strength far which the observed peak maxima fit the energy levels in the Tanabe-Sugano diagram.
Discussion Observation of both allowed and forbidden transitions in room temperature spectra of dissolved transition metal complexes is relatively rare. C r ( o ~ a l a t e ) ~ happens ~to be a particularly favorable case which is also convenient for study in undergraduate laboratories. Although the fit of the spectral data to theTanabe-Suganodiagram is, in general, an iterative procedure, there is a feature for d3 which lends extra confidence to the result: for the ease of a d3 ion the energy of the first allowed transition gives the ligand field strength Ao directly (5). Literature Cited (1) (a1 Cotton, F. A..and Wilkinson.G.. "Advanced InorganieChemistry," 3rd Ed., Intcracienee, New York, 1912, p 578. (b) Heslop, R. B., and Jones, K.,"lnorganie
Chemistry? Elsevier, New York, 1976, p 160. (c) Purmll, K. F., and KoU,J. C.. "Inorgsnie Chemistry," Saunders, Philadelphia, 1977, p 564. 12) Rai1ar.J.C.. Jr..and Joner,E. M..lnorg. Synfh. 1.37 (1939). (31 Nelson,J. H.,snd K e m r K.C.,"LaboratorvEx~rimentthrBmmmdLeMey.Cbemistry:TheCentralScienee~P~ntim-Hell. E n r l e w d Cliffs,NewJe~y, 1977. p 297.
(41 la) Hall. L.H.."Croup T h w r y m d Symmetry inchemistry?
Mffirsw-Hill.NewYark, 1969, p 322. (b) Cotton, F. A.."Chomieel Applications of GmupTheow?
2nd Ed., Wiley-Interscience. New Yark, 1971, p 266. (6) Ref LC, p 667.
S. S. Eaton University of Colorado at Denver Denver, CO 80202
T. D. Yager G. R. Eaton University of Denver Denver, CO 80208
Volume 56. Number lo, October 1979 / 635
