MONOMERIC CYCLOPENTADIENYLNICKEL METHOXO AND AMIDO COMPLEXES - SYNTHESIS, CHARACTERIZATION, REACTIVITY, AND USE FOR EXPLORING THE RELATIONSHIP BETWEEN H-X AND M-X BOND-ENERGIES

Reactive monomeric amido and methoxo complexes, CpNi(PEt3)NHTol and C pNi(PEt3)OMe, have been synthesized and fully characterized. The form er is the first monomeric 18-electron nickel amide to be synthesized a nd the latter is the first structurally characterized monomeric nickel methoxide complex, The amido complex CpNi(PEt3)NHTol reacts with var ious Bronsted acids (HX) to produce complexes of the type CpNi(PEt3)X (X = NHAr, OR, Osilica, SR), and compounds with hydridic hydrogens to give the hydridonickel complex CpNi(PEt3)H. The polarity of Ni-N and Ni-O bonds is also demonstrated by reactions with alkali metal salts and trimethylsilyl chloride, and by the crystallographic and NMR chara cterization of phenol adducts of CpNi(PEt3)OTol. The phosphine ligand s in CpNi(PEt3)X (X = OTol, SAr) compounds exchange with PMe3 through an associative mechanism; the rate increases with the electronegativi ty of X. The thermodynamics of reactions interconverting CpNi(PEt3)X + HX' and CpNi(PEt3)X' + HX have been analyzed using solution equilib rium studies and calorimetry. Instead of being 1:1, the correlation be tween H-X and M-X bond energies shows a marked preference for nickel b inding to more electronegative ligands. This preference is not specifi c to nickel: examples of similar thermodynamic preferences occur throu ghout transition metal chemistry. These results may be attributable to a large electrostatic component in the bonding between Ni and X. This qualitative E-C model explains the reactivity, thermodynamics, and ph osphine exchange rates of this series of nickel complexes, and may be general to many metal-ligand bonds.