Transition metal tetrahydroborato complexes: An orbital interaction analysis of their structure and bonding

Detailed structural and electronic analyses based on orbital interaction models for a large number of tetrahydroborato transition-metal complexes were provided to account for the relationship between their structure and bonding. Three points were made as the result of discussion: (a) most of the tetrahydroborato transition-metal complexes conform to the 18 electron rule, especially for complexes with only one BH, ligand; (b) usually, complexes with more than 18 valence electrons are considered as ionic complexes. Our detailed analyses reveal that both ionic and electronic effects play important roles in the stereochemistry in these tetrahydroborato complexes. The ionic effect tends to maximize the coordination number while the electronic effect influences the orientation of ligands; (c) complexes with more than 18 valence electrons (which usually have two or more BH, ligands) adopt structures allowing the extra electrons to be exclusively delocalized in the BH, ligands. Although some complexes are formally 18-electron species, the number of electrons involved in the metal-ligand bonding is not necessarily 18