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Using a combination of high resolution and dipolar solid state 15N NMR we have determined H/D isotope effects on the nitrogen-hydron (L = H, D) distances and 15N chemical shielding tensors of strongly hydrogen bonded bisisocyanide salts of the type [(CO)5Cr−C⋮N···L···N⋮C−Cr(CO)5]- X+, where X+ = AsPh4 + (2) and X+ = NPr4 + (3). These compounds have been modeled theoretically by the linear system [C⋮N···L···N⋮C]-Li+ (1). The crystal field acting on the anion was generated by a variety of fixed C···Li distances. For the calculation of dynamical corrections of geometries and NMR chemical shifts, an iterative procedure based on the crude adiabatic approximation was employed, consisting of (i) ab initio calculation of the energy hypersurface at the MP2/6-31+G(d,p) level, (ii) solution of the Schrödinger equation for the anharmonic collinear hydron motion, and (iii) NMR chemical shift calculations using the IGLO-method. The two hydrogen bond distances r 1 ≡ N···L and r 2 ≡ L···N are found to change in a correlated way when H is replaced by D, as a function of X+, i.e., of the electric field at the hydrogen bond site. The correlation r 1 = f(r 2) established here experimentally and theoretically for very strong NHN-hydrogen bonds shows a good agreement with a correlation established previously (Steiner, Th. J. Chem. Soc., Chem. Commun. 1995 , 1331) based on the neutron diffraction structures of a number of weakly hydrogen bonded solids. A plot of the sum q 2 = r 1 + r 2corresponding in a linear hydrogen bond to the heavy atom separationas a function of the proton dislocation from the hydrogen bond center q 1 = 1/2(r 1−r 2) exhibits a minimum value at about 2.54 Å for the symmetric low-barrier hydrogen bond at q 1 = 0. This situation is realized experimentally for 2. When q 1 ≠ 0 anharmonic single well hydrogen bonds are obtained, typical for 3. The geometric H/D isotope effects can be split into a primary effect referring to the hydron position q 1 = 1/2(r 1−r 2) and a secondary effect referring to the heavy atom position q 2. Secondary effects have been reported previously by Ubbelohde. Both isotope effects are shown to be related in a simple empirical way to the hydrogen bond geometries and to the isotopic fractionation factors. Finally, it is shown that the chemical shielding of the nuclei in the hydrogen bridge is a qualitative probe for the primary and secondary geometric isotope effects.

The reaction of 2-azidophenyl isocyanide (7) with [M(CO)(5)(thf)] (M=Cr, W) yields the isocyanide complexes [M(CO)(5)(7)] (M=Cr 8, M=W 9). Complexes 8 and 9 react with tertiary phosphines such as triphenylphosphane at the azido function of the isocyanide ligand to give the 2-triphenylphosphiniminophenyl isocyanide complexes 10 (M=Cr) and 11 (M=W). The polar triphenylphosphiniminophenyl function in complexes 10 and 11 can be hydrolyzed with H(2)O/HBr to afford triphenylphosphane oxide and the complexes containing the unstable 2-aminophenyl isocyanide ligand. This ligand spontaneously cyclizes by intramolecular nucleophilic attack of the primary amine at the isocyanide carbon atom to yield the 2,3-dihydro-1H-benzimidazol-2-ylidene complexes 12 (M=Cr) and 13 (M=W). Double deprotonation of the cyclic NH,NH-carbene ligands in 12 and 13 with KOtBu and reaction with two equivalents of allyl bromide yields the N,N'-dialkylated benzannulated N-heterocyclic carbene complexes 14 (M=Cr) and 15 (M=W). The molecular structures of complexes 9 and 11-15 were confirmed by X-ray diffraction studies.

A free tricarbene is most likely a long‐lived intermediate in the formation of the first hexacarbene iron complex 1 (R = Me) from trimethylated tris(1‐imidazolyl)borate, n‐butyllithium, iron chloride, and tetraphenylborate. This tripodal C,C′,C″‐chelating ligand, for which the authors predict a brilliant future, is an isomer of Trofimenko's tris(1‐pyra‐zolyl)borate and represents a novel class of negatively charged six‐electron donors with the greatest possible similarity to the cyclopentadienyl anion.

With Sharpless' and Meldal's discovery of the immensely supportive effect that metal catalysis has on Huisgen's classical 1, 3‐dipolar cycloaddition, azides (RN3) – long underappreciated in organic synthesis – suddenly got in the focus of attention as most crucial players in sensational ‘click chemistry'. Less noisy though with the same commitment and even a much broader scope of scientific topics and objectives, the inorganic azide chemistry has made just as great strides in the last few decades. This review (Part I) gives an introductory survey of the most important results, and informs about modern developments and general trends. Particular emphasis is placed on the recent successful approaches to highly unstable homoleptic azido metal complexes of the main group and early transition elements, as well as on the enormous structural versatility caused by the ‘flexidentate' N3– ligand with its unsurpassed bridging capacities. The presentation in this paper of selected compounds and reactions is meant, in a way, as a prelude to the [3+2]‐cycloadditions of metal azides and related species which will be covered in‐depths in Part II. A large part of the comments finally deals with applications in fields such as catalysis, high explosive performance or magnetism of metal compounds containing azide, today certainly one of the most attractive research areas world‐wide.