Isoelectronic Organometallic Molecules

Abstract

This chapter discusses isoelectronic organometallic molecules. There are a number of ways in which the term isoelectronic is used to intercompare compounds. There are a number of ways of working the isoelectronic game, but an effective way is to simply realize that isoelectronic compounds differ only in the location of one or more protons. The interactions between main group species and transition-metal fragments range from coordinate covalent bonds, where the main group species acts primarily as an electron donor and the transition-metal fragment as an electron acceptor, to cluster systems where the concept of the main group fragment as a simple ligand is difficult to apply. With the photoelectron spectroscopic technique, information is gained that relates only to the occupied molecular orbitals (MOs); hence, what follows is restricted to the lowest ionization potentials corresponding to the highest occupied MOs. Analysis of the photoelectron spectrum below the 11-eV range reveals two bands that can be assigned to metal–main group fragment interactions. The oxo-bridged dimer [Cp*Re(CO 2 ) 2 ],( μ-O) has a distorted square pyramidal configuration at rhenium. Transition-metal chemistry with ethyne and substituted ethynes is well developed. Formal reductive cleavage of the two rings of nickelocene (4H), followed by the removal of four protons from the nickel atom would yield a bis(μ 5 -pentadienyl)chromium “open chromocene” complex. With homonuclear clusters, we come to systems where an essential aspect of the bonding is delocalized.