We first discuss the **f bond in **f. We then outline the theory of the interaction of paramagnetic dipoles with nuclei and show that the theory is in excellent agreement with experiment. Indeed it is possible to separate electron paramagnetic from nuclear effects. The information provided by the electron paramagnetic effects is then discussed, and finally the nuclear effects are interpreted in terms of various motional modified models of the **f bond in **f.

Theoretical studies of the hydrogen bond generally agree that the **f bond will be linear in the absence of peculiarities of packing in the solid. Moreover, it will be asymmetric until a certain critical **f distance is reached, below which it will become symmetric. There is ample evidence from many sources that the **f bond in **f is symmetric. The **f distance in **f is 2.26 A. There is evidence, though less convincing than for **f, that the **f bond in nickel dimethylglyoxime is symmetric. Here the **f distance is 2.44 A. A number of semiempirical estimates by various workers lead to the conclusion that the **f bond becomes symmetric when the **f bond length is about 2.4 to 2.5 A, but aside from the possible example of nickel dimethylglyoxime there have been no convincing reports of symmetric **f bonds. Douglass has studied the crystal structure of **f by x-ray diffraction. He finds the structure contains an **f bond with the **f distance of **f. There is, then, the possibility that this **f bond is symmetric, although Douglass was unable to determine its symmetry from his x-ray data.