A series of phenyl-and benzylphosphonates of the lanthanide elements has been prepared. In the lanthanum system single crystals have been obtained and the crystal structures determined. Lanthanum phenylphosphonate, La(03PC6H5)(H03PC6H5), is triclinic, space group Pi, a = 8.410 (3), b = 15.696 (7), c = 5.636 (1) A3, « = 90.24 (4)°, ß = 108.99 (1)°, y = 85.59 (4)°, Z = 2, R(F) = 0.056 for 1591 observations [/ > 3 (/)] and 172 variables. The structure is a layered one in which the lanthanum atoms are eightcoordinate. Both phosphonate groups chelate the metal atom. One oxygen of each chelate ring then bridges to an adjacent La atom, while the third oxygen bridges to lanthanum at right angles to the bridging of the chelating oxygens. The phenyl rings protrude into the interlamellar space but are disordered. This disorder arises because the true unit cell has a doubled c axis. Thus, the phenyl rings in any one row are parallel to each other in the -axis direction but are inclined to each other in adjacent rows in the c-axis direction at an angle of 58°. A short hydrogen bond (2.41 A 0-0 distance) is present within the layers. La(03PCH2C6H5)(H03PCH2C6H5)-2H20 is orthorhombic, space group Pbcn, a = 10.801 (2), b = 10.301(2), c = 33.246 (8) A, Z = 8, V = 3699 (1) A3, R{F0) = 0.033 for 1207 observations with I > 3 (/) and 277 variables. This structure is also a layered one with 8-fold coordination about the lanthanum atom. There are six phosphonate and two water oxygens in the La coordination sphere in a distorted dodecahedral arrangement. One phosphonate group chelates the lanthanum, while the remaining oxygen bridges to an adjacent La atom. One of the chelating oxygens also bonds to an adjacent lanthanum atom. The second phosphonate group bridges across two La atoms with two of its oxygens, while the third bonds to hydrogen. This acid proton and the water molecules form an extensive system of intralayer hydrogen bonds. A second type of rare-earth phosphonate of composition Ln

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