Soft cobalt(III) centres: electronic levelling in [CoX3(AH3)2] (X = Cl, Br or I; A = P, As or Sb)*

Abstract

The geometric and electronic structures of the trigonal-bipyramidal cobalt(III) complexes of general formula [CoX₃(AH₃)₂] (X = Cl, Br or I; A = P, As or Sb) have been computed using density functional theory (DFT). The performance of the local density approximation (LDA) and three different gradient-corrected functionals has been evaluated by comparing the computed structure of [CoI₃(PH₃)₂] with that observed for the PPh₃ analogue. This molecule, plus [CoI₃(AsH₃)₂] and [CoCl₃(SbH₃)₂], the last two being compared with the observed PMe₃ systems, were used to test the performance of different basis sets. The LDA gives too short bond lengths. However, computed structures using gradient-corrected DFT are in good agreement with experiment. The Co–P and Co–Sb bond lengths are systematically underestimated but full calculations for the trimethyl derivatives indicate this is due to the use of a model AH₃ moiety. The calculations show all the complexes to be paramagnetic spin triplets and reveal a remarkable electronic levelling such that the frontier molecular orbital energies are virtually independent of X or A. Estimates of the Co–A bond energies are also very similar for all nine molecules spanning the narrow range 191–217 kJ mol^–1. These data indicate comparable stabilities across the entire series. The unique features of these molecules result in a ‘soft’ metal centre despite its formal +3 charge. The balance between five- and six-co-ordination is discussed using calculations on the fac and mer isomers of the hypothetical [CoI₃(PH₃)₃] complex.