Metal Nitrosyl Complex - Bonding and Structure - Enemark-Feltham Notation

Enemark-Feltham Notation

The Enemark-Feltham notation is used to describe the number of d-type electrons present in a complex. It is deliberately ambiguous, because the true oxidation state of a metal coordinated by non-innocent ligands is often unclear. When written with this notation, the d-electron count is always consistent no matter how the nitrosyl ligands are treated (i.e. NO+, NO·, or NO−).

To illustrate its use, the {MNO} d-electron counts of the 3− anion is shown. In this example, the cyanide ligands are "innocent", i.e. they have a charge of −1 each, −5 total. To balance the fragment's overall charge, the charge on {CrNO} is thus +2 (−3 = −5 + 2). Using the neutral electron counting scheme, Cr has 6 d electrons and NO· has one electron for a total of 7. Two electrons are subtracted to take into account that fragment's overall charge of +2, to give 5. Written in the Enemark-Feltham notation, the d electron count is {CrNO}5. The results are exactly the same if the nitrosyl ligand were considered NO+ or NO−.