Novel primary phosphanes with bulky 2,6-bis(benzhydryl)phenyl substituents could be prepared and fully characterized.

Reaction of the 6π-electron aromatic fourmembered heterocycle (IPr) 2 C 2 P 2 (1) (IPr = 1,3-bis(2,6diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) with [Fe 2 CO 9 ] gives the neutral iron tricarbonyl complex [Fe(CO) 3 -η 3 -{(IPr) 2 C 2 P 2 }] (2). Oxidation with two equivalents of the ferrocenium salt, [Fe(Cp) 2 ](BArF 24 ), affords the dicationic tricarbonyl complex [Fe(CO) 3 -η 4 -{(IPr) 2 C 2 P 2 }](BArF 24 ) 2 (4). The one-electron oxidation proceeds under concomitant loss of one CO ligand to give the paramagnetic dicarbonyl radical cation complex [Fe(CO) 2 -η 4 -{(IPr) 2 C 2 P 2 }](BArF 24 ) (5). Reduction of 5 allows the preparation of the neutral dicarbonyl complex [Fe(CO) 2 -η 4 -{(IPr) 2 C 2 P 2 }] ( 6). An analysis by various spectroscopic techniques ( 57 Fe Mössbauer, EPR) combined with DFT calculations gives insight into differences of the electronic structure within the members of this unique series of iron carbonyl complexes, which can be either described as electron precise or Wade-Mingos clusters.

In contrast to cyclic π‐conjugated hydrocarbons, the coordination chemistry of inorganic heterocycles is less developed. Dicarbondiphosphides stabilized by N‐heterocyclic carbenes (NHCs) NHC→C2P2←NHC (1 a,b) (NHC=IPr or SIPr) contain a four‐membered C2P2 ring with an aromatic 6π‐electron configuration. These heterocycles coordinate to a variety of complex fragments with metals from groups 6, 9, and 10, namely [M0(CO)3] (M=Cr, Mo), [CoI(CO)2]+, or [NiIIBr2], through an η4‐coordination mode, leading to complexes 2 a,b, 3 a,b, 5 a,b, and 6 a,b, respectively. These complexes were characterized by X‐ray diffraction methods using single crystals, IR spectroscopy, and DFT calculations. In combination these methods indicate that 1 a,b behave as exceptionally strong 6π‐electron donors.

Iron tetracarbonyl complexes of the biradicaloids [P(μ-N-Ter)]2 (Ter: 2,6-dimesitylphenyl) and [P(μ-C-IPr)]2 (IPr: 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) have been isolated. These two biradicaloids act here as σdonating, monodentate ligands. Experimental and theoretical data suggest that they can best be described in their zwitterionic resonance structures in these complexes. Single-crystal XRD and IR spectroscopy reveal that the donor strength of both is comparable to N-heterocyclic carbenes (NHCs). [P(μ-C-IPr)]2 is a slightly stronger donor than [P(μ-N-Ter)]2. Furthermore, an improved, scalable, and facile synthesis of both biradicaloids is presented.