How to Make a Major Shift in a Redox Potential: Ligand Control of the Oxidation State of Dimolybdenum Units

A compound reported earlier (Polyhedron 1989, 8, 2339) as (Bun4N)2H2{Mo2[Mo(CO)4(PhPO2)2]2} has been
reexamined. We find that the hydrogen atoms in this formula are not present. Therefore, the complex must be
considered as having a central triply bonded Mo26+ unit, instead of a quadruply bonded Mo24+ unit. Our conclusion
is based on a variety of experimental evidence, including X-ray crystal structures of four crystal forms, as well as
the neutron crystal structure of one. This explains the relatively long Mo−Mo bond lengths found in the range
2.1874(7)−2.2225(7) Å and the absence of a δ → δ* transition in the visible spectrum. From electrochemistry we
also find that the diphosphonate ligand has such an exceptional ability to stabilize higher oxidation states that even
common solvents such as CH2Cl2 and C2H5OH readily oxidize the Mo24+ unit that is introduced from the Mo2(O2CCH3)4 or [Mo2(O2CCH3)2(NCCH3)6](BF4)2 employed in the preparation. The only chemically reversible wave at
E1/2 = −1.54 V vs Ag/AgCl corresponds to the reduction process Mo26+ → Mo25+.