The effect of μ-CN linkage isomerism and ancillary ligand set on directional metal-metal charge-transfer in cyanide-bridged dimanganese complexes

The reaction of [Mn(CN)L'(NO)(eta(5)-C5R4Me)] with cis- or trans-[MnBrL(CO)(2)(dppm)], in the presence of T1[PF6], gives homobinuclear cyanomanganese(I) complexes cis- or trans-[(dppm)(CO)(2)LMn(mu-NC)MnL'(NO)(eta(5)-(C5RMe)-Me-4)](+), linkage isomers of which, cis- or trans-[(dppm)(CO)(2)LMn(mu-NC)MnL'(NO)(eta(5)-C5R4Me)](+), are synthesised by reacting cis- or trans-[Mn(CN)L(CO)(2)(dppm)] with [MnIL'(NO)(eta(5)-C5R4Me)] in the presence of T1[PF6]. X- Ray structural studies on the isomers trans-[(dppm)(CO)(2){(EtO)(3)P}Mn(mu-NC)Mn(CNBut)(NO)( eta(5)-C5H4Me)](+) and trans-[(dppm)(CO)(2){(EtO)(3)P}Mn(mu-CN)Mn(CNBut)(NO)(eta(5)-C5H4Me)](+) show nearly identical molecular structures whereas cis-[(dppm)(CO)(2){(PhO)(3)P}Mn(mu-NC)Mn{ P(OPh)(3)}(NO)(eta(5)-C5H4Me)](+) and cis-[(dppm)(CO)(2){(PhO)(3)P}Mn(mu-CN)Mn- P(OPh)(3)}(NO)(eta(5)-C5H4Me)](+) differ, effectively in the N- and C- coordination respectively of two different optical isomers of the pseudo-tetrahedral units (NC)Mn{P(OPh)(3)}(NO)(eta(5)-C5H4Me) and (CN)Mn{P(OPh)(3)}(NO)(eta(5)-C5H4Me) to the octahedral manganese centre. Electrochemical and spectroscopic studies on [(dppm)(CO)(2)LMn(mu-XY) MnL'(NO)(eta(5)-C5R4Me)](+) show that systematic variation of the ligands L and L', of the cyclopentadienyl ring substituents R, and of the l- CN orientation (XY = CN or NC) allows control of the order of oxidation of the two metal centres and hence the direction and energy of metal-metal charge-transfer (MMCT) through the cyanide bridge in the mixed-valence dications. Chemical one-electron oxidation of cis- or trans-[(dppm)(CO)(2)LMn(mu-NC)MnL'(NO)(eta(5)-C5R4Me)](+) with [NO][PF6] gives the mixed-valence dications trans-[(dppm)(CO)(2)LMnII(mu-NC)MnIL'(NO)(eta(5)-C5R4Me)](2+) which show solvatochromic absorptions in the electronic spectrum, assigned to optically induced Mn(I)-to-Mn(II) electron transfer via the cyanide bridge.