| Abstract
| - Single ligands (H2O,NH3, OH-, F-) are calculated tobind to first series transition metal ions up to 50% more tightly thanin hexacoordinated complexes. The M2+L complexes(all dn) show classical ligand field behavior,but the dn andsdn-1M+L complexes behave differently, defying the usual trendacross the series to stronger-and-shorter bonding. Anions,attached inside the metal 4s radial maximum, display very irregularbinding patterns.
- Single-ligand complexes of first series transitionmetals with ammonia, water, hydroxide, and fluoride, manyknown in the gas phase, have been studied in calculations covering the20 mono- and divalent cations and somevery unusual binding patterns have been found. Binding energiesand binding geometries were calculated atMP2 level, using a basis with a (6d/4d) contraction in the metal dspace and 6-311+G** sets for the ligands.The results were used to distinguish the effect of steadilyincreasing nuclear charge across the series from thevarying effects of d shell occupation. Even with only one ligand,the M2+ adducts displayed the familiarligandfield effects, d shell repulsion in the expected dδ<dπ< dσ order being superimposed on aregular progressionto stronger binding and shorter bonds; that progression was disturbedonly at the d5 and d10 positions, whenthedσ orbital was occupied. Monovalent metal adductsbehaved in strikingly different fashion, with irregularchangesacross early and late series metals in both bond length and bondstrength. The irregularities are only partlyattributable to the presence of bothdn-1s anddn ground states in the series. The otherpart of the explanation isthe binding of anionic ligands inside the radial maximum of the 4sorbital. At these distances the normal bindingpreference shown by H2O and NH3 fordn oversdn-1 cations isreversed. In contrast to steeply rising bindingenergies across the divalent metal ion adducts, the trend lines for themonovalent series are flat, the incrementsin nuclear charge being insufficent to offset the extra repulsion ofelectrons added to the d shell.
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