Heteroleptic N-heterocyclic dicarbene-diphosphorus species were prepared by reaction of the carbene-phosphinidene adduct (IPr)PSiMe (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with the carbene-phosphorus trichloride adduct (IMes)PCl (2, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), which furnished the dichloride [(IPr)PPCl(IMes)]Cl (3). Reduction of 3 with potassium graphite (KC) afforded [(IPr)PP(IMes)] (4). The corresponding radical cation [(IPr)PP(IMes)]˙ (5˙) is isolated as [5]PF by reaction of 4 with ferrocenium hexafluorophosphate, whereas complexes containing the corresponding dication [(IPr)PP(IMes)] (6) can be isolated as the gallate and borate salts [6](GaCl) and [6](BAr) by chloride abstraction from 3 with GaCl or sodium tetrakis[bis(3,5-trifluoromethyl)phenyl]borate (NaBAr), respectively. The asymmetric set of N-heterocyclic carbene ligands allows to establish J coupling constants of 249 Hz for 4 and 543 Hz for [6](GaCl). Based on X-ray diffraction analyses, the molecular structures of 4, 5˙ and 6 reveal a consecutive shortening of the P-P bond lengths, in agreement with the presence of a phosphorus-phosphorus single bond in 4 and a double bond in 6, which is best described as a dicationic diphosphene according to density functional theory (DFT) calculations.

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In this work, two iron(II) coordination compounds with a N 2 O 2 coordinating Schiff base-like ligand bearing a redox active tetrathiafulvalene (TTF) unit and pyridine or trans-1,2-bis(4-pyridylethylene) as an axial ligand are synthesized. Crystals suitable for single X-ray structure analysis were obtained for the new ligand. The complexes were characterized by magnetic susceptibility measurements, T-dependent UV−vis spectroscopy, and cyclic voltammetry. Both complexes display spin transition behavior below room temperature with T 1/2 values of 146 and 156 K. The mononuclear iron(II) complex [Fe TTF L(py) 2 ] is relatively stable up to 400 K compared to similar complexes, showing no loss of axial ligands upon heating. Temperature dependent Mossbauer spectroscopy was conducted for the coordination polymer {[Fe TTF L(bpee)]} n to get more information regarding the origin of the stepwise spin crossover (SCO) behavior observed in the magnetic measurements. The change of the spin state is accompanied by a change of the optical properties, which can be monitored by VT−UV−vis spectroscopy for the mononuclear complex and has been analyzed in theoretical studies. The redox behavior of the iron(II) complexes reveals three reversible redox steps which are located at the iron center and at the TTF unit of the ligand. Oxidation of the TTF unit induces characteristic changes in the UV−vis spectrum that can be followed by spectroelectrochemical UV−vis spectroscopy. Addressing the potential of the ironcentered redox process results in similar changes in the UV−vis spectrum, which indicates an electronic coupling of the redox active unit with the metal center under certain circumstances.

The synthesis, structures, and magnetic properties of monomeric half-sandwich iron and cobalt imidazolin-2-iminato complexes have been comprehensively investigated. Salt metathesis reactions of [Cp'M(μ-I)] 2 (1-M, M = Fe, Co; Cp' = η 5 -1,2,4-tri-tertbutylcyclopentadienyl) with [Im Dipp NLi] 2 (Im Dipp N = 1,3-bis(2,6diisopropylphenyl)imidazolin-2-iminato) furnishes the terminal halfsandwich compounds [Cp'M(NIm Dipp )] (2-M, M = Fe, Co), which can be regarded as models for elusive half-sandwich iron and cobalt imido complexes. X-ray diffraction analysis confirmed the structure motif of a one-legged piano stool. Complex 2-Co can also be prepared by an acid−base reaction between [Cp'Co{N(SiMe 3 ) 2 }] (3-Co) and Im Dipp NH. The electronic and magnetic properties of 2-M and 3-Co were probed by 57 Fe Mossbauer spectroscopy (M = Fe), X-band EPR spectroscopy (M = Co), and solid-state magnetic susceptibility measurements. In particular, the central metal atom adopts a high-spin (S = 2) state in 2-Fe, while the cobalt complex 2-Co represents a rare example of a Co(II) species with a coordination number different from six displaying a low-spin to high-spin spin-crossover (SCO) behavior. The experimental observations are complemented by DFT calculations.

[(ItBu)Co(CH2SiMe3)2] serves as an efficient, homogeneous olefin hydrogenation catalyst.