  Supramolecular chemistry has allowed the production, by self-assembly, of
inorganic complexes with a [N x N] square matrix-like configuration of N^2
metal centers. Interest in these systems is driven by the potential
applications in information technology suggested by such a "two-dimensional"
(2D), addressable arrangement of metal ions. From the magnetic perspective, [N
x N] grids constitute molecular model systems for magnets with extended
interactions on a square lattice, which have gained enormous attention in the
context of hightemperature superconductors. Numerous [2 x 2] grids as well as a
few [3 x 3] grids with magnetic metal ions such as Cu(II), Ni(II), Co(II),
Fe(II), and Mn(II) have been created. Magnetic studies unraveled a remarkable
variety in their magnetic properties, which will be reviewed in this work with
emphasis on the underlying physical concepts. An intriguing issue is the
connection of [2 x 2] and [3 x 3] grids with "one-dimensional" (1D) rings, as
experimentally realized in the molecular wheels. For a [2 x 2] square of spin
centers the distinction between a 2D grid and a 1D ring is semantic, but also a
[3 x 3] grid retains 1D character: it is best viewed as an octanuclear ring
with an additional ion "doped" into its center. Challenging familiar concepts
from conventional magnets, the current picture of elementary excitations in
antiferromagnetic rings will be discussed, as a prerequisite to understand the
complex [3 x 3] grids.
