Structure and reactivities of six-co-ordinate first-row transition-metal nitrosyl complexes

Abstract

A molecular-orbital study of the [ML₅(NO)] complexes [Fe(CN)₅(NO)]^2–, [Mn(CN)₅(NO)]^3–, and [V(CN)₅-(NO)]^3– as well as of the presently non-existent model compound [FeCl₅(NO)]^2– has been made using the INDO approximation. It has been shown that the highest occupied molecular orbital (h.o.m.o.) is an orbital of π symmetry localised on the ligands and the lowest unoccupied molecular orbital (l.u.m.o.) is a degenerate π*(MNO) orbital localised mainly on the nitrogen, but with varying amounts of metal and cis, but no trans, ligand character. Analysis of the composition of this l.u.m.o. orbital as a function of ligand electronegativity, number of electrons in the complex, and nature of the metal shows that nucleophilic attack or reduction at the nitrogen atom is favoured over the same reactions at the metal site for complexes to the right of the transition-metal series with a high formal charge on the metal. For four to six [metal d+π*(NO)] electrons the site of nucleophilic attack or reduction is independent of the number of electrons, and of the nature of the L ligand. As the L ligand becomes less electronegative the contribution of cis-ligand orbitals to the l.u.m.o. orbital increases, and it is shown that addition of electrons to the l.u.m.o. results in the expected bending of the MNO group and destabilisation of the cis M–L bond in the plane of the MNO bending. Hence, with an electropositive L ligand, loss of a cis ligand is favoured by reduction or nucleophilic attack. The energy of the l.u.m.o. decreases with the number of electrons, with increasing electro-negativity of the L ligands, and on going from left to right across the transition-metal series, making nucleophilic attack, at any site, more facile in the order given. It is shown that the trans influence of the NO ligand in these complexes cannot be measured by bond-distance data because the energy required for M–L bond-distance changes of up to 0.15 Å is extremely small. Overlap criteria cannot be used since they are dependent on the M–L distance assumed. Comparison of the total energy of [ML₅(NO)], cis-[ML₄(NO)], and trans-[ML₄(NO)] indicates that the trans M–L bond is stronger than the cis M–L bond in [ML₅(NO)] complexes.