Coordination numbers two and three

A very large number of stable copper(I) complexes exist in a variety of stoichiometries. In few of these complexes does the formal coordination number of the metal atom exceed four. Indeed, along with silver(I) and gold (I), it is one of the few oxidation states to exhibit regularly the low coordination numbers two and three. The simple amine and halo complexes isolated from aqueous solution fortuitously contain linear copper(I) ions. As a result, coordination number two is erroneously considered to be a common coordination number for this oxidation state. In fact, two-coordinate complexes are probably outnumbered by the trigonally coordinated complexes, whereas against the vast host of tetrahedral complexes the two- and three-coordinate complexes are numerically insignificant. 

The coordination polyhedra for coordination numbers two and three are trivial, namely linear or bent for coordination... 

A series of zinc diphosphonate complexes were synthesized in the presence of diamines of varying chain length. 1-hydroxyethylidenediphosphonate bridges the metal ions with the proto-nated diamine (ethyl-, butyl-, pentyl-, or hexylenediamine) filling in channels or residing between chains. All four structures are different with one-, two-, and three-dimensional networks formed. The coordination number (4-6) and geometry also varies.418... 

Mercury(II) arsenate Hg3(As04)2 has three independent Hg particles with coordination numbers five and seven, but each of them has two short Hg O bonds (206 < r(HgO) < 214 pm) in a nearly linear arrangement linking of these polyhedra including the As04 tetrahedra leads to a 3-D network.300... 

Metal ions having coordination number two occur in Hg(CN)2, [Ag(CN)2]- and [Au(CN)2]-, all derivatives of d ° species in each case the structure is linear. In K[Cu(CN)2] the copper atom is actually three-coordinated, the anion being an infinite chain each copper has a carbon atom of an unshared cyanide, and a carbon atom and a nitrogen atom of shared cyanides, as nearest neighbours in approximately planar coordination. Three-coordination also occurs in the [Hg(CN)3]- ion, but there are weak interactions between two of the nitrogen atoms and mercury atoms of other anions, giving rise to a loosely bonded chain structure. 

These topological requirements correspond to Euler s theorum for two and three dimensions. The number of edges joined to a given vertex is its coordination number, z. For topologically stable structures,... 

A number of hydrated sulfates exist. The nonahydrates of La and Ce have two distinct lanthanide sites, one nine-coordinate (six waters and three monodentatate sulfates) the other featuring 12-coordination (six waters and three bidentate sulfates). The other lanthanides form octahydrates with eight-coordination the larger lanthanides also form pentahydrates such as Nd2 (804)3 -5H20 in which the metal is nine-coordinate (distorted trigonal prism) by seven sulfate oxygens and two water molecules. 

Phenyllithium dissolves in hexane by addition of TMEDA. Hie phenyllithium TMEDA adduct subsequently crystallizes out of solution as the dimer (113) corresponding to general structural type (16)." With diethyl ether solvation, phenyllithium exists as a solid tetramer (114). In ether solution PhLi is known to be either dimeric or tetrameric. Monomeric phenyllithium was successfully crystallized with PMDETA as the ligand. This monomer is depicted as (115). Note the difference in the coordination number of the carbanionic center in the monomer (115), the dimer (113), and the tetramer (114), i.e. one, two and three, respectively. 

---