Monomeric Cyclopentadienylnickel Methoxo and Amido Complexes:  Synthesis, Characterization, Reactivity, and Use for Exploring the Relationship between H−X and M−X Bond Energies

Abstract

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 Brønsted 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.

Formation, Reactivity, and Properties of Nondative Late Transition Metal−Oxygen and −Nitrogen Bonds

“…It has often been asserted that late metal-oxygen and -nitrogen bonds are weaker than their early-metal counterparts, but if we focus on specific homolytic bond dissociation energies (BDEs), rather than average bond energies, only a few studies have been carried out to test this expectation. 29,30 More systematic information is available about relative (rather than absolute) bond energies. In a widely cited study by Bryndza, Bercaw, and co-workers, a correlation was proposed between the relative bond strengths of metal-heteroatom bonds (M-X) and their corresponding acids (H-X).…”

Octahedral [TpRu(PMe3)2OR]n+ Complexes (Tp = hydridotris(pyrazolyl)borate; R = H or Ph; n = 0 or 1): Reactions at Ru(II) and Ru(III) Oxidation States with Substrates that Possess Carbon−Hydrogen Bonds

“…49 Previous experimental studies have revealed that M-OR BDEs can be greater than M-C BDEs. 40,[81][82][83][84][85][86] (PH 3 ) 2 OH is 74 kcal/mol at the same level of theory. Thus, oxidation from Ru(II) to Ru(III) is calculated to decrease the Ru-X hydroxyl BDE by only 32%, a value that is proportionately less than the impact of single-electron oxidation on Ru-alkyl BDEs.…”

Reactions of d0 alkylidene and amide complexes with silanes

“…These reactions are two of few known examples of such reactions. To our knowl- edge, the only other reported transition-metal amide reaction to give an isolated amide hydride involves Cp*Ni(PEt 3 )NHTol [12].…”