Mononuclear electron diffraction

It may be worthwhile to mention the mononuclear nature of Cr(0-r-Bu)4, the structure of which was elucidated by the allied technique of gas-phase electron diffraction (65). 

The CK" ion can act either as a monodentate or bidentate ligand. Because of the similarity of electron density at C and N it is not usually possible to decide from X-ray data whether C or N is the donor atom in monodentate complexes, but in those cases where the matter has been established by neutron diffraction C is always found to be the donor atom (as with CO). Very frequently CK acts as a bridging ligand - CN- as in AgCN, and AuCN (both of which are infinite linear chain polymers), and in Prussian-blue type compounds (p. 1094). The same tendency for a coordinated M CN group to form a further donor-aceeptor bond using the lone-pair of electrons on the N atom is illustrated by the mononuclear BF3 complexes... 

Although the metal-carbon bond in cyano compounds has been confirmed by neutron diffraction in only a limited number of substances (24—29), it is generally and reliably accepted that in the stable mononuclear complexes the metal is always bonded to carbon. Support for the M—C—N linkage is provided by electronic spectra (30) and by vibrational spectroscopic data obtained with different isotopes (31). 

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