Mixed-valence dinuclear molybdenum complexes with benzenediamido and dianilido bridges: comparison with related phenolato and dipyridyl species, and with their pentammineruthenium analogues

The monoanionic mixed-valence benzenediamido complexes ortho- and para-[{Mo(NO)[HB(dmpz) 3 ]X} 2 (Z 2 C 6 H 4 )] - A [HB(dmpz) 3 = tris(3,5-dimethylpyrazolyl) hydroborate; Z = NH; X = Cl, Br or I] have been characterised by electronic, EPR and IR spectroscopic and electrochemical techniques. They are delocalised at room temperature on the EPR time-scale but valence-trapped on the IR time-scale, whereas the meta isomer exhibits valence-trapped behaviour according to both techniques. The electrochemical properties of these species and of related dianilido complexes [{Mo(NO)[HB(dmpz) 3 ]X} 2 (ZC 6 H 4 EC 6 H 4 Z)] B [Z = NH; E = CH 2 , O, SO 2 , CO or OC 6 H 4 (SO 2 )C 6 H 4 O] are dependent on X and E. These results are compared with those obtained from related diphenolato species A (Z = O) and B (Z = O) and dipyridyl analogues [{Mo(NO)[HB(dmpz) 3 ]X} 2 {(NC 5 H 4 )E′(C 5 H 4 N)}] C (E′ = nothing, CH 2 CH 2 , CHCH or dimethyldecapentene). Comproportionation constants and related data for the redox equilibria A + A 2- ⇌ 2A - , B + B 2- ⇌; 2B - and C + C 2- ⇌ 2C - show that interaction between the molybdenum-based redox centres increases significantly as the distance between them decreases and as a function of the group connecting the bridge to the molybdenum, in the order O < NH < N (pyridine). The data are generally consistent with those obtained from related dinuclear pentammineruthenium complexes [{Ru(NH 3 ) 5 } 2 Q] n + (Q = bridging group) but the interaction between the metal centres is much greater in the molybdenum species than in the ruthenium complexes, and a simple explanation for this behaviour is given.