Diazene ligand geometry effects on the formation and properties of metal carbonyl complexes. Group vib pentacarbonyl complexes of cis- and trans-1,2-diisopropyldiazene and 1,2-diisopropylhydrazine

The Group VIB complexes M(CO)₅L have been synthesized for the cases L= cis-1,2-diisopropyldiazene (c-DIPD) with M = Cr, Mo, W, L = trans-1,2-diisopropyldiazene (t-DIPD) with M = Or, W, and L, = 1,2-diisopropylhydrazine (DIPH) with M = Cr, W. Failure to obtain any bimetallic complexes is discussed in terms of steric interactions of these and related complexes. The significance of diazene ligand geometry is demonstrated by the differences in properties of the c-DIPD and t-DIPD complexes. The available evidence indicates that cis diazenes are better ligands than their trans isomers. Complex stability decreases in the order W > Cr > Mo and c-DIPD > t-DIPD. Infrared, visible, and NMR spectra are interpreted in terms of bonding in the complexes. A 30–60 cm^?1 reduction of v(NN) of the diazenes upon coordination is attributed to metal-ligand π-bonding with c-DIPD being a better π-acceptor than t-DIPD. The NMR spectra of the c-DIPD complexes are temperature dependent and show that the M(CO)₅ moiety undergoes coordination site exchange between the two nitrogen atoms. No exchange occurs in the t-DIPD complexes. Coalescence temperatures of 10, ?48, and 60°C were recorded for the Cr, Mo, and W complexes of c-DIPD respectively, with the Gibbs free energy barriers of 15.0, 11.5 and 15.0 kcal/ mol. A comparison with exchange in other M(CO)₅(cis-diazene) complexes is made and the role of the diazene structure on the reaction rate is discussed. The M(CO)₅(DIPH) (M = Cr, W) complexes have been oxidized by H₂O₂/Cu²⁺ and by activated MnO₂ to DIPD complexes in low yield. The tungsten DIPH complex gives only W(CO)₅(t-DIPD) but the chromium system gives predominantly Cr(CO)₅(c-DIPD).