A five-coordinate, sixteen-electron manganese(I) complex [Mn(CO)(3)(S,S-C6H4)](-) stabilized by S,S pi-donation from chelating [S,S-C6H4](2-)

The five-coordinate, sixteen-electron manganese(I) complex [N(PPh3)(2)][Mn( CO)(3)(S,S-C6H4)] 1 was prepared from reaction of [N(PPh3)(2)][Mn(CO)(3)(NH ,S-C6H4)] and 1,2-benzenedithiol via the hexacoordinate intermediate fac-[M n(CO)(3)(S-C6H4SH)(NH2,S-C6H4)](-). Alternatively, oxidative addition of 1, 2-benzenedithiol to [Mn(CO)(5)](-), followed by a Lewis acid-base reaction, with evolution of H-2 gas (identified by gas chromatography), led to forma tion of the coordinatively-unsaturated complex 1. In contrast, reaction of bis(2-pyridyl) disulfide and [N(PPh3)(2)][Mn(CO)(5)] afforded hexacoordinat e fac-[N(PPh3)(2)][Mn(CO)(3)(S-C5H4-N)(S-C5H4N)] 2, with one anionic [S-C5H 4N](-) ligand bound to Mn-I in a monodentate (S-bonded) manner and the othe r [S-C5H4-N](-) ligand bound in a bidentate manner (S,N-bonded). Complexes 1 and 2 have been characterized in solution by infrared spectroscopy and in the solid state by X-ray crystallography. The strong pi-donating ability o f the bidentate [S,S-C6H4](2-) ligand stabilizes the unsaturated complex 1 which has short Mn-I-S bond lengths of 2.230(1) Angstrom (average) as a res ult. The existence of one pi and two sigma bonds between the [Mn(CO)(3)](+) and [S,S-C6H4](2-) fragments, based on qualitative frontier molecular orbi tal analysis, also indicates that the lone-pair electrons are delocalized a round the sulfur-manganese-sulfur system stabilizing the five-coordinate co mplex 1. The IR carbonyl stretching frequencies and the Mn-I-S bond distanc es of complexes 1 and [N(PPh3)(2)][Mn(CO)(3)(NH,S-C6H4)] suggest that the r elative pi-donating ability of the bidentate ligands is [NH,S-C6H4](2-) > [ S,S-C6H4](2-). The Mulliken atomic charges derived from Hartree-Fock calcul ations roughly quantify the charge distribution in the complex [Mn(CO)(3)(N H,S-C6H4)](-) (delta(N) = -1.14; delta(S) = -0.43; delta(Mn) = 1.14), and s upports the premise that the reactions of [Mn(CO)(3)(NH,S-C6H4)](-) with el ectrophiles (1,2-benzenedithiol, thiophene-2-thiol, 1,2-ethanedithiol) occu r at the more electron-rich amide site, yielding charge-controlled, collisi on complexes.