Valence orbital energetics as a tool for organometallic synthesis: On the scarcity of two-legged piano-stool complexes

The stability of 18-electron two-legged piano-stool complexes of the type (ηn-CnHn,)ML2(n = 4, 5 or 6; L = CO or NO; M = first-row transition metal) is correlated to the energetic separation of the HOMO from both the LUMO and the SHOMO (second-highest occupied molecular orbital) of the complexes calculated by using the Fenske-Hall molecular orbital method. Complex stability is observed to increase with increasing HOMO/LUMO and decreasing HOMO/SHOMO separations. This correlation is reflected in an examination of known stable complexes and unsuccessful synthetic attempts at others. On the basis of these findings, predictions are made as to the stability of a number of unknown 18-electron two-legged piano-stool complexes for which syntheses have not yet been attempted.