Perturbation theory and spin coupling constants in substituted compounds. III. Octahedral fluorides

Abstract

The Pople-Santry (P-S) theory of reduced spin coupling constants 1 K(E-F) for unsubstituted fluorides EF m (E is a transition metal M or main group element atom A) has been modified in a perturbation manner to treat the Fermi-contact term in 1 K(E-F) in monosubstituted octahedral fluorides EF 5L′. The differences in diagonal matrix elements, δα′ s = 〈 s L′| H| s L′〉 − 〈 s F| H| s F〉 and δα′ p = 〈 σ L′| H| σ L′ − 〈 σ F| H| σ F〉, were taken as perturbations and first-order corrections to the P-S terms, linear in δα′ s and δα′ p , have been obtained in explicit form. It was shown that for main group compounds AF 5L′ the regularities of 1 K(A-F cis ) and 1 K(A-F tr ) may be different depending on relative energies of the s A and p F orbitals. Namely, for more electronegative atoms A the typical relationship will be 1K( A-F) < 1K( A-F cis) < 1K( A-F tr) < 0 , but for more electropositive atoms A, typically, 1K( A-F cis) < 1K( A-f tr) < 0 , where changes in 1 K(A-F) in AF 6 will increase as the donor ability of L′ increases. It was shown that the strongest donors L′ (H, CH 3, Ph, etc.) having no lone ns 2 pair, “one-pronged” donors, should cause rather small changes in both 1 K(A-F cis ) and 1 K(A-F tr ) for electropositive but not for electronegative atoms A. It was found that for transition metal compounds MF 5L′ the typical relationship will be 1K( M-F) ≅ 1K( M-K cis)> 1K( M-F tr) > 0 , where changes in 1 K(M-F) will increase as the donor ability of L′ increases but one-pronged substituents such as H or CH 3 may again occupy unusual positions in the L′ series. Some modifications of the above regularities depending on fine details of the structures of the EF 5L′ compounds were discussed. The results agree with experiment and permit a number of predictions to be made.