Electronic Properties of 4,4‘,5,5‘-Tetramethyl-2,2‘-biphosphinine (tmbp) in the Redox Series <i>fac</i>-[Mn(Br)(CO)₃(tmbp)], [Mn(CO)₃(tmbp)]₂, and [Mn(CO)₃(tmbp)]^-: Crystallographic, Spectroelectrochemical, and DFT Computational Study

Stepwise electrochemical reduction of the complex fac-[Mn(Br)(CO)3(tmbp)] (tmbp = 4,4‘,5,5‘-tetramethyl-2,2‘-biphosphinine) produces the dimer [Mn(CO)3(tmbp)]2 and the five-coordinate anion [Mn(CO)3(tmbp)]-. All three members of the redox series have been characterized by single-crystal X-ray diffraction. The crystallographic data provide valuable insight into the localization of the added electrons on the (carbonyl)manganese and tmbp centers. In particular, the formulation of the two-electron-reduced anion as [Mn0(CO)3(tmbp-)]- also agrees with the analysis of its IR ν(CO) wavenumbers and with the results of density functional theoretical (DFT) MO calculations on this compound. The strongly delocalized π-bonding in the anion stabilizes its five-coordinate geometry and results in the appearance of several mixed Mn-to-tmbp charge-transfer/IL(tmbp) transitions in the near-UV−vis spectral region. A thorough voltammetric and UV−vis/IR spectroelectrochemical study of the reduction path provided evidence for a direct formation of [Mn(CO)3(tmbp)]- via a two-electron ECE mechanism involving the [Mn(CO)3(tmbp)]• radical transient. At ambient temperature [Mn(CO)3(tmbp)]- reacts rapidly with nonreduced fac-[Mn(Br)(CO)3(tmbp)] to produce [Mn(CO)3(tmbp)]2. Comparison with the analogous 2,2‘-bipyridine complexes has revealed striking similarity in the bonding properties and reactivity, despite the stronger π-acceptor character of the tmbp ligand.