Cp*Ir(dab) (dab = 1,4-Bis(2,6-dimethylphenyl)-1,4-diazabutadiene): A Coordinatively Unsaturated Six-π-Electron Metallaheteroaromatic Compound?

1,4-Bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene (dab)
forms the structurally characterized iridium(III)
complex
[Cp*IrCl(dab)](PF6): 
C28H35ClF6IrN2P,
orthorhombic, space group Pnma, a =
16.187(2) Å, b = 15.823(2) Å,
c
= 11.677(1) Å, V = 2990.8(6) Å3,
Z = 4, and R = 0.0588. On reaction with
NaBH3CN this compound does not
form an iridium(III) hydride but the coordinatively unsaturated
reduced product Cp*Ir(dab): 
C28H35IrN2,
monoclinic,
space group P21/n, a =
8.484(2) Å, b = 14.535(3) Å, c =
20.956(4) Å, β = 98.88(3)°, V =
2553.2(9) Å3, Z =
4, and R = 0.0586. The inverted relation
dCC (=1.334(15) Å) <
dCN (=1.379(13) and 1.366(14) Å) in
the dab
ligand of Cp*Ir(dab) suggests that the reduction has occurred
primarily at that ligand to form an ene-1,2-diamido/iridium(III) moiety or, alternatively, a six-π-electron
metallaheteroaromatic system. Ab initio
pseudopotential
calculations of model complexes
[CpIr(HNCHCHNH)]0/2+ support this description of
the bonding.