Structure and bonding in low-spin octahedral manganese(II) carbonyls: ligand-set control of spin delocalisation

Abstract

X-Ray structural studies on the redox pair trans-[Mn(CN)(CO)(dppm)₂](dppm = Ph₂PCH₂PPh₂) and trans-[Mn(CN)(CO)(dppm)₂][PF₆]·CH₂Cl₂ showed that one-electron oxidation results in changes consistent with depopulation of an orbital involved in Mn–P π-back bonding. The ESR spectra of trans-[Mn(CN)(CO)(dppm)₂]⁺, [Mn(CO)(CNCH₂CHCH₂)(dppm)₂]²⁺, trans-[Mn(CN)(CO)₂(PEt₃)(dppe)]⁺(dppe = Ph₂CH₂CH₂PPh₂) and trans-[MnBr(CO)₂(PEt₃)(dppe)]⁺ in frozen dichloromethane–dichloroethane (1:1) solutions at 90 K, and extended-Hückel molecular-orbital calculations on the model compounds [Mn(CN)(CO)(H₂PCH₂PH₂)₂]⁺, [Mn(CO)(CNMe)(H₂PCH₂PH₂)₂]²⁺, cis- and trans-[Mn(CN)(CO)₂(PH₃)₃]⁺ and trans-[MnBr(CO)₂(PH₃)₃]⁺, showed that the semi-occupied molecular orbital of these low-spin octahedral cyanomanganese(II) carbonyls is always primarily manganese d_π in character and in the plane of the phosphorus ligands, either aligned along the Mn(CN) axis as in trans-[Mn(CO)₂(PEt₃)(dppe)]⁺ or perpendicular to this axis as in trans-[Mn(CN)(CO)(dppm)₂]. The relative arrangement of the cyanide and carbonyl ligands is shown to control the extent of spin delocalisation onto the cyanide ligand.