Electronic structures and MCD spectra of trigonal Cr(III)S6 systems

Abstract

MCD, electronic absorption, external heavy atom, and crystal field data are presented for the low energy region (2Eg, 2T1g, 4T2g) and high energy region (2T2g, 4T1g) of Cr(dtp)3, Cr(dtc)3, and Cr(exan)3. At low energy, MCD intensities of 2E(2Eg) and 2E(2Tg) are as large or larger than “4T2g”, and the MCD technique is advantageous over electronic absorption in this respect. The MCD positions of “2Eg” and “2T1g” are nearly the same for these molecules (∼ 13 kK and ∼ 13.6 kK) · 4T2g of this region appears trigonally split (∼ 500 cm−1) in the MCD of dtp but to a smaller extent than in the electronic crystal spectrum of Lebedda and Palmer (∼ 600 cm−1). MCD did not resolve such components for exan and dtc. The higher energy region includes 2T2g and 4T1g, and the combined MCD and electronic absorption data of the three compounds taken together lead us to conclude the ordering 2A1(2T2g)<2E(2T2g)<4E(4T1g). The potentially useful external heavy atom affect on the solution-observed electronic 2E and 4E bands of Cr(dtp)3 did not shed additional light on this order of E states. Finally, it is concluded that the order of 4T1g and 2T2g cannot be decided from Oh crystal field calculations because of experimental uncertainties about choosing centers of gravity. In addition, “4T1g” and “2T2g” are close together so that ordering 2E<4E does not guarantee 2T2g<4T1g. However, it can be concluded that the ratio C/B≅4 is not correct, whereas the larger 7<(C/B)<8 is consistent with the data of all three molecules because of small B parameters (∼ 0.4). Locating O→O transitions may somewhat decrease C/B and Dq.