Spectroscopic and computational studies on the trans-mu-1,2-peroxo-bridged dinickel(II) species [{Ni(tmc)}2 (O2)](OTf)2: nature of end-on peroxo-nickel(II) bonding and comparison with peroxo-copper(II) bonding.

Resonance Raman (rR) spectroscopic and density functional theory (DFT) computational studies on a trans-mu-1,2-peroxo-bridged (Ni2+)2 complex, [{(tmc)Ni2+}2 (O2)]2+ (1, tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), are presented and discussed. These studies afford a detailed description of the geometric and electronic structures of the Ni2O2 core in 1 and provide a suitable basis for a quantitative comparison between Ni-peroxo and Cu-peroxo bonding. From a normal coordinate analysis of rR data of 1, values of k(Ni-O) = 1.52 mdyn/A and k(O-O) = 2.58 mdyn/A are obtained for the Ni-O and O-O stretch force constants, respectively, which are considerably smaller than k(Cu-O) = 2.05 mdyn/A and k(O-O) = 3.09 mdyn/A reported for a representative trans-mu-1,2-peroxo-bridged (Cu2+)2 complex, [{(TMPA)Cu2+}2 (O2)]2+. These differences primarily reflect a strongly reduced covalency of the metal-peroxo bonds in 1 relative to the (Cu2+)2 dimer as a result of the lower effective nuclear charge of Ni2+ than Cu2+. DFT computations aimed at elucidating the reaction coordinate for the thermal decay of 1 reveal that the conversion of this species to a side-on peroxo (Ni2+)2 dimer is electronically feasible but energetically unfavorable by approximately 85 kcal/mol due to the steric constraints imposed by the tmc supporting ligands. These results suggest that the decay of 1 to the crystallographically characterized final product, [(tmc)Ni2+ OH]OTf, proceeds without initial end-on --> side-on peroxo (Ni2+)2 core conversion.