VARIABLE-TEMPERATURE IR SPECTROELECTROCHEMICAL INVESTIGATION OF THE STABILITY OF THE METAL-METAL-BONDED RADICAL-ANIONS [(CO)(5)MNRE(CO)(3)(L)](CENTER-DOT-) (L=2,2'-BIPYRIDINE (BPY), 2,2'-BIPYRIMIDINE (BPYM), 2,3-BIS(2-PYRIDYL)PYRAZINE (DPP)) AND [(CO)(5)MNRE(CO)(3)(L)RE(BR)(CO)(3)](CENTER-DOT-)(L=BPYM, DPP) CONTROLLED BY THE LOWEST PI-ASTERISK (ALPHA-DIIMINE) ORBITAL ENERGY

Cyclic voltammetry in combination with IR thin-layer spectroelectrochemistry was employed for detection and characterization of the radical anionic complexes [(CO)(5)MnRe(CO)(3)(L)](.-) (L = 2,2'-bipyridine (BPY), 2,2'-bipyrimidine (BPYM), and 2,3-bis(2-pyridyl)pyrazine (DPP)) and [(CO)(5)MnRe(CO)(3)(mu-L)Re(Br)(CO)(3)](.-) (mu-L = mu-BPYM, mu-DPP) as the primary products formed upon one-electron reduction of the neutral parent compounds. The stability of the radical anions is determined by the degree of polarization of the Mn-Re bond which increases in the order Mn-Re(mu-BPYM)Re < Mn-Re(mu-DPP)Re < Mn-Re(BPYM) similar to Mn-Re(DPP) < Mn-Re(BPY), i.e., with increasing energy of the pi* LUMO of the alpha-diimine ligand in the parent complexes and with increasing sigma- and pi-donor character of the alpha-diimine anion in the reduced species. In the same order, the unpaired electron in the metal-metal-bonded radical anions becomes more delocalized over the (Mn)Re-L chelate bond. This delocalization in turn also considerably destabilizes the d(z)(2) orbital of Re(L) involved in the Mn-Re bonding, making its occupation no more energetically convenient. Consequently, the Mn-Re bond will split heterolytically to give [Mn(CO)5](-) and [Re(CO)3(L)](.). The trinuclear radical anion with mu-L = mu-BPYM was found to be stable already at room temperature and could be characterized also by UV-vis and ESR spectroscopy. The related radical anion with mu-DPP could only be stabilized at properly low temperatures, just as for the compounds with nonbridging BPYM and DPP. In the case of the BPY compound, the metal-metal-bonded radical anion could not be detected at all. Descriptions of the secondary reaction pathways of the radicals [Re(CO)(3)(L)](.), formed in the course of the irreversible reduction of the above Mn-Re-bonded complexes, are given as well.