Mössbauer and UV−vis spectroscopic measurements of the two diazene-bridged systems [{Fe‘NHS4'}2(N2H2)] (‘NHS4'2- = 2,2‘-bis(2-mercaptophenylthio)diethylamine(2−)) and [{Fe‘S4'(PPr3)}2(N2H2)] (‘S4'2- = 1,2-bis(2-mercaptophenylthio)ethane(2−)) have been combined with self-consistent-field Xα scattered-wave (SCF-Xα -SW) calculations in order to describe the electronic properties of diazene coordinated to Fe(II)−sulfur centers. For both model systems, the experiments and calculations indicate a Fe(II)−N2H2−Fe(II) structure with Fe(II) low-spin centers. The stabilization of diazene is due to σ-donation from its highest occupied MO and π -backbonding into its lowest unoccupied MO (LUMO). The intense absorption band at approximately 600 nm is ascribed to a charge-transfer transition from metal-d orbitals into the diazene LUMO. Importantly, the diazene LUMO also is the LUMO of the complexes and is located between the fully occupied and empty orbitals of the iron centers. It is concluded that the Fe(II) high-spin configuration present in a sulfur environment of lower ligand-field strength leads to reduction of diazene. Therefore diazene is likely to be involved in a reduction pathway of dinitrogen bound symmetrically (i.e., with both nitrogen atoms) to the iron−molybdenum cofactor (FeMoCo) of the enzyme nitrogenase.

The blue compound [FeH(dppe) 2 ]BPh 4 (1) (dppe ϭ 1,2bis(diphenylphosphino)ethane) is investigated by optical absorption, vibrational, NMR and Mössbauer spectroscopy as well as magnetic susceptibility measurements. In the solid state, 1 is found to be paramagnetic (Sϭ1) whereas in tetrahydrofuran (thf) solution it is diamagnetic. Importantly, 1 reacts with dinitrogen in thf solution and in the solid state to give the yellow and green dinitrogen complexes 2a and 2b, respectively, both of which have the formula [FeH(N 2 )(dppe) 2 ]BPh 4 . Apart from their colour and optical spectroscopic properties, the dinitrogen compounds 2a and 2b also dif-Prof. Dr. F. Tuczek Inst. Für Anorgan. 1247 fer in their magnetic, Mössbauer and vibrational properties. The N-N stretching frequency of 2a is found at 2120 cm Ϫ1 whereas 2b exhibits an N-N stretching vibration of 2145 cm Ϫ1 . Moreover, 2a is diamagnetic whereas 2b is paramagnetic. DFT is employed to determine the relative energies and structure of 1 and its dinitrogen adducts in the singlet and triplet state.

The bonding of N(2) to the five-coordinate complexes [FeX(depe)(2)](+), X = Cl (1a) and Br (1b), has been investigated with the help of X-ray crystallography, spectroscopy, and quantum-chemical calculations. Complexes 1a and 1b are found to have an XP(4) coordination that is intermediate between square-pyramidal and trigonal-bipyramidal. Mössbauer and optical absorption spectroscopy coupled with angular overlap model (AOM) calculations reveal that 1a and 1b have (3)B(1) ground states deriving from a (xz)(1)(z(2))(1) configuration. The zero-field splitting for this state is found to be 30-35 cm(-1). In contrast, the analogous dinitrogen complexes [FeX(N(2))(depe)(2)](+), X = Cl (2a) and Br (2b), characterized earlier are low-spin (S = 0; Wiesler, B. E.; Lehnert, N.; Tuczek, F.; Neuhausen, J.; Tremel, W. Angew. Chem, Int. Ed. 1998, 37, 815-817). N(2) bonding and release in these systems are thus spin-forbidden. It is shown by density functional theory (DFT) calculations of the chloro complex that the crossing from the singlet state (ground state of 2a) to the triplet state (ground state of 1a) along the Fe-N coordinate occurs at r(C) = 2.4 A. Importantly, this intersystem crossing lowers the enthalpy calculated for N(2) release by 10-18 kcal/mol. The free reaction enthalpy Delta G degrees for this process is calculated to be 4.7 kcal/mol, which explains the thermal instability of N(2) complex 2a with respect to the loss of N(2). The differences in reactivity of analogous trans hydrido systems are discussed.

The vibrational properties of the two octahedral Fe II dinitrogen complexes [FeH(N 2 )(depe) 2 ] ϩ (1) and [FeCl(N 2 )(depe) 2 ] ϩ (2, depe ϭ 1,2-bis(diethylphosphino)ethane) are investigated with the help of infrared and Raman spectroscopies.

Not a dimer but a monomer was found in the X-ray structure analysis of the complex "trans-[{FeCl(depe) } (µ-N )](BPh ) " (depe=Et PCH CH PEt ). The complexes [FeXN (depe) ]BPh (X=Cl, Br; structure of the cation for X=Cl shown on the right) are much less stable than the analogous hydride compounds and undergo N exchange at room temperature even in the solid state.