Magnetic isomers of iron(II). Bis(C-cyanotrihydroborate)bis(phenanthroline)iron - a spin triplet

The synthesis of an unusual six-coordinated, spin triplet (S = 1) ferrous complex, Fe(phen)/sub 2/(NCBH/sub 3/)/sub 2/, with cis geometry is described. This complex is characterized by its magnetic moment and infrared, visible-region, and Moessbauer spectra. On the basis of these data and the angular overlap model of metal--ligand bonding, -NCBH/sub 3/ is characterized as a ligand possessing moderate sigma- and pi-donor characteristics toward Fe(phen)/sub 2//sup 2 +/. How a proper balance of sigma- and pi-donor properties of -NCBH/sub 3/ and phen yields the spin-triplet state of the ferrous complex is discussed. Consideration of the CN stretching frequency data for the isomeric pairs of Fe(phen)/sub 2/(NCBH/sub 3/)/sub 2/ and Fe(phen)/sub 2/(CNBH/sub 3/)/sub 2/ and other bridged cyano species raises interesting questions about the effect of double coordination of cyanide on the CN sigma bonding. Finally, thermal stability studies of the Fe--CN--BH/sub 3/ and Fe--NC--BH/sub 3/ linkages eliminate the possibility of isomerization by a facile CN flip mechanism within the Fe and B cage and suggest that (phen)/sub 2/Fe(NCBH/sub 3/)/sub 2/ is the thermodynamically favored isomer.