RELATIVE BONDING CHARACTERISTICS OF CATIONIC ISOCYANIDE, CARBONYL, AND RELATED COMPLEXES OF IRON AS DETERMINED BY MOSSBAUER AND INFRARED SPECTROSCOPY

Mossbauer-effect isomer shifts in cationic cyclopentadienyliron complexes identify ligand (σ + π) bonding capacities to increase in the order PPh3< CNMe ≫ C(NHMe)2< CO. An analysis of substitution effects on isomer shifts in varying ligand environments illustrates the compensating effect of both carbonyl and isocyanide groups on metal electron density. In contrast, infrared force constants for carbonyl stretching modes decrease uniformly for a given substitution in each metal environment. Consideration of both isomer shifts and stretching force constants for isocyanides and diaminocarbenes distinguishes enhanced σ-donor capacities for the carbene that are paralleled by increased π acceptance on the part of the isocyanide. Both σ and π bonding are reduced in indenyl analogues of the cyclopentadienyl complexes since isomer shifts increase despite negligible differences in carbonyl stretching frequencies. © 1979, American Chemical Society. All rights reserved.