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, Cp*Ni(PEt3)NHTol and Cp*Ni(PEt3)OMe, have been synthesized and fully characterized. The former is the first monomeric 18-electron nickel amide to be synthesized and the latter is the first structurally characterized monomeric nickel methoxide complex, The amido complex Cp*Ni(PEt3)NHTol reacts with various Bronsted acids (HX) to produce complexes of the type Cp*Ni(PEt3)X (X = NHAr, OR, Osilica, SR), and compounds with hydridic hydrogens to give the hydridonickel complex Cp*Ni(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 characterization of phenol adducts of Cp*Ni(PEt3)OTol. The phosphine ligands in Cp*Ni(PEt3)X (X = OTol, SAr) compounds exchange with PMe3 through an associative mechanism; the rate increases with the electronegativity of X. The thermodynamics of reactions interconverting Cp*Ni(PEt3)X + HX' and Cp*Ni(PEt3)X' + HX have been analyzed using solution equilibrium studies and calorimetry. Instead of being 1:1, the correlation between H-X and M-X bond energies shows a marked preference for nickel binding to more electronegative ligands. This preference is not specific to nickel: examples of similar thermodynamic preferences occur throughout 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 phosphine exchange rates of this series of nickel complexes, and may be general to many metal-ligand bonds.