The geometric and electronic structures of [MOX5]n− (M = Pa, n
= 2; M = U, n
= 1; M = Np, n
= 0; X = F, Cl or Br) have been investigated using relativistic density functional theory. An inverse trans influence (i.e. in which the An–Xcis distances are longer than the An–Xtrans bonds) is found in all cases, and there is good agreement with the experimental structure of [UOCl5]−. The O–An–Xcis angle is in all cases found to be very close to 90°, by contrast to analogous transition metal systems in which this angle is between 96° and 102°, and which display a regular trans influence. Molecular orbital arguments are presented to account for the difference in angle at the metal between the d- and f-block systems, and hence
for the difference in their trans influence behaviour. Comparisons are drawn with previous explanations for the inverse trans influence in the title systems. Mayer bond orders are reported, and are used to understand further the metal–ligand interactions.
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