Mononuclear neutron diffraction

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... 

The main difference between mononuclear complexes containing either a M—H—C or a M—H—Si three-center bond is that most tj2-CH complexes correspond to an earlier stage of the addition reaction than do the 7j2-SiH complexes 7(CMH) coupling constants are usually closer to the values for /(OH), while /(SiMH) values are closer to 2/(SiMH), and the relative lengthening of the C—H distance on 172 coordination is usually smaller than that of coordinated Si—H bonds. For example, in the representative iron complex 21 [the structure of which was determined by neutron diffraction analysis (74)], the coordinated C—H bond... 

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). 

The first X-ray characterization of a mononuclear iron carbonyl dihydride has been reported. The compound studied, H2Fe(CO)2(P(OPh)3 2, was shown to have the r-dihyride, r-carbonyl structure 47. Neutron diffraction data have also been reported. " ... 

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