Electron Density of Semi-Bridging Carbonyls. Metamorphosis of CO Ligands Observed <i>via</i> Experimental and Theoretical Investigations on [FeCo(CO)₈]^-

The electron density distribution in a transition-metal dimer containing a semibridging carbonyl is determined through experimental X-ray diffraction and quantum chemical computations. The changes occurring during the evolution from terminal to bridging coordinations are described by a “structure-correlation-like” approach and by a theoretical investigation along the conversion path. The smooth continuum of conformations observed in the solid state is explained in terms of the mutual interplay of direct M−M and M−CO and indirect M- - -M and M- - -C interactions, which can be characterized by interatomic delocalization indexes, within the framework of Quantum Theory of Atoms in Molecules.