No Electrons in Sigma Bonding or Antibonding Molecular Orbitals for Fe=O in That Spin State of Ferryl Heme: "A 'Single' Bond But No Single Sigma Bond"

Incidentally, I was going to emphasize that there's no sigma bond with iron(IV) in that spin state of ferryl heme. A lot of researchers have compared the Fe=O bond to the triple bond ("formal bond order") that Fe can form with nitrogen atoms of some ligands. It's like a triple bond, with two sets of pi interactions, without the sigma bond, from what I can tell. That's one way of thinking about the more complex reality, in any case, especially given that the Fe=O bond length in ferryl heme is 1.65 angstroms (that's significantly shorter than a usual Fe-O bond). In that article by Shaik et al. (2005), cited in the last posting, even the sulfur of the cysteine residue, as one of the axial ligands, forms a pi bond, designated "pi(sub)S." For a pentacoordinated species of ferric heme, however, those authors do show an unpaired electron in a sigma antibonding molecular orbital. Anyway, part of the reason I think it's helpful to get a sense of the bookkeeping side is that it's almost impossible to tell what the species being referred to, in some articles, actually are, given the use of all of these different types of nonconventional notation and shorthand and so on. But it's interesting to note that there isn't a sigma bond between the oxygen of a water molecular and iron(II), for example (Shaik et al., 2005). There's a "single bond," but it's a pi*(yz or xz, depending on which coordinates one assigns to which "direction") antibonding orbital and not a sigma molecular orbital. I know that no one on the planet cares, but anyway...I guess I'll have to save some more of these "goodies" for tomorrow.