Five-coordinate transition metal complexes and the value of τ5: observations and caveats

The τ5 parameter, first proposed by Addison and coworkers, is the principal measure of the geometry of five-coordinate transition metal complexes, with τ5 = 0 said to describe a perfect square pyramidal geometry and τ5 = 1 a perfect trigonal pyramidal geometry. Therefore, the geometries of all five-coordinate complexes are assumed to lie on a continuum between these two extremes. Herein we show that there are a significant number of examples of transition metal complexes having τ5 > 1, leading to an equatorially distorted trigonal bipyramidal geometry with the transition metal ion lying out of the plane of the equatorial donor atoms. We also show that complexes having τ5 = 0 and displaying perfect square pyramidal geometry are very much the exception, and that the majority of complexes for which τ5 = 0 have the metal ion sitting above the mean plane of the donor atoms in the square plane, in a basally distorted square pyramidal geometry. Density functional theory computations on a number of these complexes show that the structural distortions are inherent features of the complexes, and not merely the result of intermolecular interactions.

Single-site N-N bond cleavage by Mo(IV): possible mechanisms of hydrazido(1-) to nitrido conversion

Mo(NMe(2))(4) and the tridentate, dipyrrolyl ligand H(2)dpma(mes) were found to form 5-coordinate Mo(NMe(2))(2)(dpma(mes)) (1), which exhibits spin-crossover behaviour in solution. The complex is a ground state singlet with a barrier of 1150 cm(-1) for production of the triplet in d(8)-toluene. The complex reacts with 1,1-disubstituted hydrazines or O-benzylhydroxylamine to produce nitrido MoN(NMe(2))(dpma(mes)). The mechanism of the 1,1-dimethylhydrazine reaction with 1 was examined along with the mechanism of substitution of NMe(2) with H(2)NNMe(2) in a diamagnetic zirconium analogue. The proposed mechanism involves production of a hydrazido(1-) intermediate, Mo(NMe(2))(NHNMe(2))(dpma(mes)), which undergoes an α,β-proton shift and N-N bond cleavage with metal oxidation to form the nitrido. The rate law for the reaction was found to be -d[1]/dt = k(obs)[1][hydrazine] by initial rate experiments and examination of the full reaction profile. This conversion from hydrazido(1-) to nitrido is somewhat analogous to the proposed mechanism for O-O bond cleavage in some peroxidases.

N2 functionalization at iron metallaboratranes

The reactivity of the anionic dinitrogen complex [(TPB)Fe(N(2))](-) (TPB = tris[2-(diisopropylphosphino)phenyl]borane) toward silicon electrophiles has been examined. [(TPB)Fe(N(2))](-) reacts with trimethylsilyl chloride to yield the silyldiazenido complex (TPB)Fe(NNSiMe(3)), which is reduced by Na/Hg in THF to yield the corresponding sodium-bound anion [(TPB)Fe(NNSiMe(3))]Na(THF). The use of 1,2-bis(chlorodimethylsilyl)ethane in the presence of excess Na/Hg results in the disilylation of the bound N(2) molecule to yield the disilylhydrazido(2-) complex (TPB)Fe≡NR (R = 2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopentyl). One of the phosphine arms of TPB in (TPB)Fe≡NR can be substituted by CO or (t)BuNC to yield crystalline adducts (TPB)(L)Fe≡NR (L = CO, (t)BuNC). The N-N bond in (TPB)((t)BuNC)Fe≡NR is cleaved upon standing at room temperature to yield a phosphoraniminato/disilylamido iron(II) complex. The flexibility of the Fe-B linkage is thought to play a key role in these transformations of Fe-bound dinitrogen.