REDUCTION OF (ETA-C(5)PH(4)Q)RU(CO)(2)BR (Q = 2,5-BENZOQUINONYL) - EVIDENCE FOR AN 18-DELTA METALATE ANION WITH A REDUCED (SEMIQUINONE) SUBSTITUENT

Three-electron reduction of the cyclopentadienyl complex (C(5)Ph(4)q)R u(CO)(2)Br (q = 2,5-quinone) produces metalate and bromide anions. The metalate anion is best described as an 18 - delta complex with a redu ced (semiquinone) substituent, i.e., it is (C(5)Ph(4)sq)Ru(CO)(2)(2-) (sq = 2,5-semiquinone) or (C(5)Ph(4)sqH)Ru(CO)(2)(-) (sqH = protonated 2,5-semiquinone). A semiquinone substituent for the metalate anion is required by electron accounting, and delocalization between the semiq uinone and metalate centers explains the higher v(C = O) frequencies a nd lower nucleophilicity of the metalate anion compared to (C5Ph4Ar)Ru (CO)(2)(-) (Ar = 2,5-dimethoxyphenyl), which has the same primary coor dination sphere.