STRUCTURE AND BONDING IN LOW-SPIN OCTAHEDRAL MANGANESE(II) CARBONYLS - LIGAND-SET CONTROL OF SPIN DELOCALIZATION

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 t hat one-electron oxidation results in changes consistent with depopula tion of an orbital involved in Mn-P pi-back bonding. The ESR spectra o f trans-[Mn(CN)(CO)(dppm)2]+, [Mn(CO)(CNCH2CH=CH2)(dppm)2]2+, trans-[M n(CN)(CO)2(PEt3)(dppe)]+ (dppe = PhCH2CH2PPh2) and trans-[MnBr(CO)2(PE t3)(dppe)]+ in frozen dichloromethane-dichloroethane (1:1) solutions a t 90 K, and extended-Huckel molecular-orbital calculations on the mode l compounds [Mn(CN)(CO)(H2PCH2PH2)2]+, [Mn(CO)(CNMe)(H2PCH2PH2)2]2+, c is- and trans-[Mn(CN)(CO)2(PH3)3]+ and trans-[MnBr(CO)2(PH3)3]+, showe d that the semi-occupied molecular orbital of these low-spin octahedra l cyanomanganese(II) carbonyls is always primarily manganese d(pi) in character and in the plane of the phosphorus ligands, either aligned a long the Mn(CN) axis as in trans-[Mn(CO)2(PEt3)(dppe)]+ or perpendicul ar to this axis as in trans-[Mn(CN)(CO)(dppm)2]. The relative arrangem ent of the cyanide and carbonyl ligands is shown to control the extent of spin delocalisation onto the cyanide ligand.