Ligand Field Analysis of the Doublet Excited States in Chromium(III) Trischelated Complexes

Abstract

Trischelated complexes with bidentate ligands containing a π-conjugated bridge may show a pronounced trigonal splitting of the t_2g orbitals. As has been pointed out by Orgel (1961) this splitting is due to the electronic interaction between the ρπ-orbitals of the ligator atoms via the conjugated bridge. An extended angular overlap model, which takes this interaction into account, has recently been developed (Ceulemans et al. 1985). If in the frontier molecular orbital (MO) on the bidentate ligand the ligator ρπ-functions are in phase, a ψ-type Orgel effect results, described by a π_┴(ψ) parameter. Conversely if in the frontier MO of the ligand the ρπ-functions are out of phase, a χ-type coupling occurs, described by a parameter π_┴(χ). In principle more than one MO interaction can be important so that both ψ- and χ-effects may be operative simultaneously. Defining the trigonal splitting of the t_2g shell, Δτ₂, as the energy difference between the e and a₁ orbitals of t_2g signature

$$\Delta\tau_2 = \varepsilon (\text e) - \varepsilon (\text a_1)$$

the Orgel effect in trischelated complexes is given by:

$$\Delta\tau_2 = 3\pi_\bot(\psi) - 3\pi_\bot(\chi)$$

Quite often ligand donor and acceptor levels display opposite phase coupling so that π_┴(ψ) and π_┴(χ) will adopt opposite signs. In this way quite pronounced trigonal splittings may be obtained.