Molecular orbital analysis of the metal-hydrazide(2-) bonding in co-ordination chemistry

The bonding in mono- and bis-hydrazido metal complexes has been studied with the help of EHMO and ab initio calculations on various models as well as on free hydrazide. The theoretical results have been analysed together with a collection of structural data obtained through a Cambridge Data Base search covering 118 compounds. Although generally described as being a hydrazide(2-) ligand, its oxidation state is often closer -1 in early transition-metal complexes, corresponding to the following occupation of its frontier orbitals: (σ_n)²(π_NN)²-(π _σ)²(π*_NN)¹. The occupied hydrazido π_NN orbital, which does not interact significantly with the metal, is largely responsible for the significant double-bond character of the N-N bond. The partial population of the π*_NN level, which tends to reduce the N-N bond order, is partly balanced by depopulation of the somewhat antibonding π_σ orbital. Assuming the traditional hydrazido(2-) formal charge, the ligand is a six-electron donor in mono-hydrazido metal species if co-ordinated linearly. If significantly bent, it is a four-electron donor. In the case of cis bis(hydrazido) species, the two formally hydrazide(2-) ligands act generally as a 10-electron donor system.