SPIN DELOCALIZATION AND THE GEOMETRY OF REDOX-ACTIVE CYANOMANGANESECARBONYL LIGANDS IN HETEROPOLYNUCLEAR COMPLEXES OF RHODIUM(I)

The reactions of trans-[Mn(CN)(CO)(dppm)2] (dppm=PhPCH2PPh2) and cis- or trans-[Mn(CN) (CO)2(PR3) (L-L)] [R = OEt or OPh, L-L = dppm; R = Et , L-L = dppe (Ph2PCH2CH2PPh2)] with [{Rh(mu-Cl)(CO)2}2] give the heter obinuclear complexes [trans-(dppm)2(OC)Mn(mu-CN)Rh(CO2Cl] and [(L-L)(R 3P)(OC)2Mn(mu-CN)Rh(CO)2Cl] respectively. Cyclic voltammetry shows tha t each complex undergoes oxidation at the manganese centre; chemical o xidation of [trans-(dppm)2(OC)Mn(mu-CN)Rh(CO)2Cl] gives [trans-(dppm)2 (OC)Mn(mu-CN)Rh(CO)2Cl]+ which may also be prepared from trans-[Mn(CN) (CO)(dppm)2]+ and [{Rh(mu-Cl)(CO)2}2]. The crystal structures of the r edox pair [trans-(dppm)2(OC)Mn(mu-CN)Rh(CO)2Cl]z (Z = 0 or +1) show th at substantial changes in geometry resulting from oxidation are limite d to the vicinity of the manganese atoms (e.g. mean Mn-P increases fro m 2.284 to 2.352 angstrom). These changes are similar to those observe d for the free cyanomanganese complexes trans-[Mn(CN)(CO)(dppm)2]z (Z = 0 or +1) and indicate that the singly occupied molecular orbital in the radical cation is largely composed of the Mn d(pi) orbital in the MnP4 plane. Changes in geometry in the Mn(mu-CN)Rh(CO)2Cl unit are ver y small. Treatment of [cis-(L-L)(R3P)(OC)2Mn(mu-CN)Rh(CO)2Cl] or [tran s-(L-L)(R3P)(OC)2Mn(mu-CN)Rh(CO)2Cl] with TIPF6 in the presence of [Mn (CN)(CO)2(PR3)(L-L)] gives the heterotrinuclear cations [{(L-L)(R3P)(O C)2Mn(mu-CN)}2Rh(CO)2]+; the crystal structure of one, [{trans-(dppm)[ (EtO)3P]-(OC)2Mn(mu-CN)}2Rh(CO)2], shows the two cyanomanganese ligand s cis-co-ordinated to the rhodium centre. Cyclic, differential-pulse, and square-wave voltammetry of [{trans-(dppm)(R3P)(OC)2Mn(mu-CN)}2Rh(C O)2]+ (R = OEt or OPh) show two closely spaced oxidation waves; the sm all separation (ca. 80-90' mV) suggests weak interaction between the t wo cyanomanganese ligands in the mixed-valence dication [{trans-(dppm) (R3P)(OC)2Mn(mu-CN)}2Rh(CO)2]2+ The results provide evidence for the d ependence of spin delocalisation from Mn(II) to Rh(I) on the geometry of the ancillary ligands bound to manganese.