On the inverse trans influence. Density functional studies of [MOX(5)](n-) (M = Pa, n=2; M = U, n=1; M = Np, n=0; X = F, Cl or Br)

The geometric and electronic structures of [MOX(5)](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-X(cis) distances are longer than the An-X(trans) bonds) is found in all cases, and there is good agreement with the experimental structure of [UOCl(5)](-). The O-An-X(cis) angle is in all cases found to be very close to 90degrees, by contrast to analogous transition metal systems in which this angle is between 96degrees and 102degrees, 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.