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The d 2 ‐M(NR) 2 Fragment, High Valent and Low Valent Organometallic Equivalents, and an Unusual Mode of Ethylene Complexation
Author(s) -
Radius Udo,
Hoffmann Roald
Publication year - 1996
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19961291106
Subject(s) - chemistry , trigonal bipyramidal molecular geometry , crystallography , ethylene , molecular orbital , olefin fiber , ligand (biochemistry) , valence (chemistry) , stereochemistry , extended hückel method , atomic orbital , molecule , crystal structure , electron , physics , biochemistry , polymer , receptor , organic chemistry , catalysis , quantum mechanics
We report approximate molecular orbital calculations on models of the ethylene complex W(NMes) 2 (PMe 3 ) 2 (η 2 ‐C 2 H 4 ) 1 . The ethylene unit in 1 is oriented perpendicular to the equatorial plane – an unusual orientation for trigonal bipyramidal L 4 M‐(η 2 ‐C 2 H 4 ) complexes. In order to understand the bonding in 1 , the tungsten fragment W(NR) 2 L 2 is discussed in some detail and we compare the “high valent” ethylene complex 1 and the “low valent” olefin complex Fe(CO) 4 (η 2 ‐C 2 H 4 ). 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 d 2 ‐[M(NR)L 2 ] 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 d 2 ‐ cis ‐M(E) 2 L 3 . Both C 2 v fragments d 2 ‐W(NR) 2 L 2 and d 8 ‐Fe(CO) 4 have characteristic frontier orbitals, qualitatively similar to the frontier orbitals of CH 2 . These orbitals guide the ethylene ligand into or out of the equatorial plane. The general analysis leads to the conclusion that d n ( 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.