The d2-M(NR)2 Fragment, High Valent and Low Valent Organometallic Equivalents, and an Unusual Mode of Ethylene Complexation

We report approximate molecular orbital calculations on models of the ethylene complex W(NMes)2(PMe3)2(η2-C2H4) 1. The ethylene unit in 1 is oriented perpendicular to the equatorial plane – an unusual orientation for trigonal bipyramidal L4M-(η2-C2H4) complexes. In order to understand the bonding in 1, the tungsten fragment W(NR)2L2 is discussed in some detail and we compare the “high valent” ethylene complex 1 and the “low valent” olefin complex Fe(CO)4(η2-C2H4). An analysis of π-bonding shows that the metal nitrogen bond in the cis,-[M(NR)2] unit is less than a triple bond, and the d2-[M(NR)L2] fragment is more like a 16 than an 18 valence electron fragment. The nature of the occupied d-orbital explains the opening of the angle E–M–E in trigonal bipyramidal complexes d2-cis-M(E)2L3. Both C2v fragments d2-W(NR)2L2 and d8-Fe(CO)4 have characteristic frontier orbitals, qualitatively similar to the frontier orbitals of CH2. These orbitals guide the ethylene ligand into or out of the equatorial plane. The general analysis leads to the conclusion that dn (n > O) “high valent” complex fragments are actually better back-bonding donors for π-acidic ligands. For 1, we detect also another mininum on the energy surface – the olefin ligand is now oriented parallel to the equatorial plane. This raises the fascinating possibilities of the existence of multiple conformational minima for some transition metal ethylene complexes.

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