Discussion of Planarity of Molecular Structures Using Novel Pentanuclear Cu/Ni Complexes as an Example

Two novel heterometallic complexes, [Cu2Ni3(NCS)4(Me2Ea)6] (1)
and [Cu2Ni3Cl4(Me2Ea)6] (2) (HMe2Ea = 2-dimethylaminoethanol),
have been prepared using a one-pot reaction of copper powder with
nickel thiocyanate (1) or chloride (2) in
an acetonitrile solution of HMe2Ea in air. The magnetic
investigations disclose dominant ferromagnetic exchange interactions
with S = 4 and 2 ground states for 1 and 2, respectively. A planar arrangement of five metal
centers in both compounds inspired us to study the question of planarity
of polynuclear coordination compounds having MaXb cores (where M = metal and X = bridging atom). The distribution,
obtained by the searches via the Cambridge Structural Database, of
the structures possessing a planar disposition of the metal centers,
depending on their nuclearity, is discussed, and typical planar molecular
structure types (MST) are identified. It is demonstrated that the
relative amounts of planar structures obey exponential decay in the
3–8 nuclearity region, with the exception of heptanuclear structures.
The factors that influence the disposition (planar or nonplanar) of
metal centers in the MaXb molecular structure
types are selected. Finally, we offer a model for general description
of MSTs by the graph theory toward the design of high-nuclear coordination
compounds.