BINDING PATTERNS IN SINGLE-LIGAND COMPLEXES OF NH3, H2O, OH-, AND F- WITH FIRST SERIES TRANSITION-METALS

Single-ligand complexes of first series transition metals with ammonia , water, hydroxide, and fluoride, many known in the gas phase, have be en studied in calculations covering the 20 mono- and divalent cations and some very unusual binding patterns have been found. Binding energi es and binding geometries were calculated at MP2 level, using a basis with a (6d/4d) contraction in the metal d space and 6-311+G* sets for the ligands. The results were used to distinguish the effect of stead ily increasing nuclear charge across the series from the varying effec ts of d shell occupation. Even with only one ligand, the M(2+) adducts displayed the familiar ligand field effects, d shell repulsion in the expected d(delta) < d(pi) < d(sigma) order being superimposed on a re gular progression to stronger binding and shorter bonds; that progress ion was disturbed only at the d(5) and d(10) positions, when the d(?si gma) orbital was occupied. Monovalent metal adducts behaved in strikin gly different fashion, with irregular changes across early and late se ries metals in both bond length and bond strength. The irregularities are only partly attributable to the presence of both d(n-1)s and d(n) ground states in the series. The other part of the explanation is the binding of anionic ligands inside the radial maximum of the 4s orbital . At these distances the normal binding preference shown by H2O and NH 3 for d(n) over sd(n-1) cations is reversed. In contrast to steeply ri sing binding energies across the divalent metal ion adducts, the trend lines for the monovalent series are flat, the increments in nuclear c harge being insufficent to offset the extra repulsion of electrons add ed to the d shell.