The Diagnostic Vibrational Signature of Pentacoordination in Heme Carbonyls

[Image: see text] Heme-carbonyl complexes are widely exploited for the insight they provide into the structural basis of function in heme-based proteins, by revealing the nature of their bonded and nonbonded interactions with the protein. This report presents two novel results which clearly establish a FeCO vibrational signature for crystallographically verified pentacoordination. First, anisotropy in the NRVS density of states for ν(Fe–C) and δ(FeCO) in oriented single crystals of [Fe(OEP)(CO)] clearly reveals that the Fe–C stretch occurs at higher frequency than the FeCO bend and considerably higher than any previously reported heme carbonyl. Second, DFT calculations on a series of heme carbonyls reveal that the frequency crossover occurs near the weak trans O atom donor, furan. As ν(Fe–C) occurs at lower frequencies than δ(FeCO) in all heme protein carbonyls reported to date, the results reported herein suggest that they are all hexacoordinate.