Molecular geometries, electronic structures and energetics of neutral and cationic mono-ligated ammonia complexes of the d-block elements calculated by an improved modified ASED–MO model

Abstract

The potential-energy surfaces of the interaction of NH₃ molecule with the d-block metal atoms and their monopositive ions were calculated by means of an improved ASED–MO computational model. For the complexes of Cu, Zn, Sc and Ti in the first row, Ag, Cd, Y and Zr in the second row and Au, Hg, La, Hf and Ta in the third row the potential-energy surfaces revealed a global minimum corresponding to an angular conformation, either of T-shape or H-type. The angular geometry of M–NH₃ complexes of the transition metals in the right side of the rows is of H-type and those in the left of T-shape. H-type geometries of the H(1) mode are also found as local minima in the potential-energy curves of the M–NH₃ complexes of the remaining transition metal atoms of the three rows. The H-type van der Waals complexes of NH₃ ligand are the first examples of stable M–NH₃ complexes reported so far, where the NH₃ ligand acts as a hydrogen donor, the high polarizability of the transition metal atoms being responsible for their stability. For the complexes of the remaining transition metal atoms and all their monopositive ions the global minimum in the potential-energy surfaces corresponds to a linear C_3v geometry. Considering that the angular geometry, either of the T- or H-type, corresponds to the structure of the transition states in the oxidative addition reactions of NH₃ molecule to transition metal atoms, ions and their complexes, it is found that the transition metal atoms forming angular complexes are the more reactive. The N–H bond activation by these metal atoms proceeds without any activation barrier. Also very reactive are V and Nb, in spite of the fact that their ammonia complexes adopt the more stable linear geometry. However, this geometry is very close energetically to the angular one, exhibiting only a very small activation barrier. Special attention has been devoted to the analysis of the bonding mechanisms and how the charge modifies the metal–ligand bonds. The theoretical results confirmed the speculations concerning the nature of the M–N bond which for the neutral species is mostly electrostatic, owing to coulombic interactions between the nucleus of the metal and the lone electron pair of the NH₃ ligand. However, weak orbital interactions cannot be excluded, varying according to the atomic radius of the central metal atoms. On the contrary, the contribution of the covalent component in the metal–ligand bonding of the corresponding cationic complexes is comparable to that of the electrostatic one. Finally, the theoretical results obtained define and explain periodic trends and demonstrate the quality of the computed structural, energetic and spectroscopic properties when compared to those of ‘state-of-the-art’ abinitio calculations and experiment.