- The clustering gas-phase reactions of Fe(13CO)+ with Fe(CO)5 to form Fex(13CO)r(CO)s+ (x = 1-3; r = 0, 1; s = 4-8) are studied by Fourier transform ion cyclotron resonance mass spectrometry. It was found that the binary cluster cations formed by these reactions have a strong tendency to lose the 13CO ligand during cluster formation. Label preference ratios varied only slightly from 0.56 0.06 to 0.7 0.1 indicating a clear preference for loss of the labeled carbonyl ligand. This preferential loss is indicative of the nonfluxional nature of these systems. Nonfluxionality in the cationic iron carbonyl clusters results from a decrease in back-bonding to the 2 orbital of the carbonyl ligands. 1990, American Chemical Society. All rights reserved.