Eight-coordinate molecules

The eight-coordinate vanadium complex V(S2CMe)4 contains both dodecahedral and square prismatic eight-coordinate molecules in the same crystal.322 Of particular interest is the chain-like, mixed valence platinum complex [Pt2(S2CMe)4( i-I)]A, which displays unusual electrical properties metallic conduction between 300 and 340 K and semiconducting properties below 300 K,323 whereas the analogous nickel complex, [Ni2(S2CMe)4(p-I)]x is a semiconductor.324... 

Crystal structures of V(S2CMe)4 (containing both dodecahedral and square prismatic eight-coordinate molecules in the same crystal),337 Ni(S2CPh)2,338 Ni(S2CCH2Ph)2,339... 

In a similar manner, the potentially sexadentate macrocycle (21) yields Pb(n) and Cd(n) complexes which have unusual geometries (Drew etal., 1979). With Pb(n), the stereochemistry is hexagonal pyramidal with one axial site occupied by a water molecule and the other filled by a sterically-active lone pair of electrons on the metal ion. The Cd(n) complex is eight-coordinate (with a water molecule and a perchlorate group occupying axial positions) however the cadmium ion is not held centrally in the macrocyclic hole but is displaced to one side, presumably reflecting the... 

Apromising new class of stable Gd3+ complexes based onTREN-Me- 3,2 -HOPO shows relatively fast water exchange rates. The water soluble tris(2-hydroxymethyl)-TREN-Me-3,2-HOPO complex is eight-coordinate with two water molecules in the first coordination sphere (234). The water exchange rate is more than one order of magnitude faster than on DTPA and DOTA complexes and the mechanism is proposed to be a-activated. 

Complexes of PyzO with lanthanide perchlorates (2 79) and hexafluorophosphates 180) are eight coordinate. However, La(III) perchlorate gives the complex La(Pyz0)7(C104)3 2 H20 in which both the water molecules are coordinated to La(III). In the case of complexes of PyzO with lanthanide chlorides 180), the number of coordinated ligands increases as the ionic radius of the lanthanide ion decreases. These complexes probably contain bridging ligands. 

The compound Gd(H20)6 Cl3 is eight coordinate with six water molecules and two chloride ions coordinating to the metal ion 331). The third chloride remains ionic. The complex units are held together by hydrogen bonds through both the ionic and coordinated chloride ions. Each noncoordinated Cl" ion forms six Cl... H-0 bonds and each coordinated Cl- ion forms three Cl... H-0 bonds in addition to the Cl—Gd bond. The coordination geometry around Gd(III) has been described as distorted dodecahedron. In [Eu(H20)6 C12]C1, the coordination geometry has been described as distorted square antiprism 332). Similar compounds of Nd(III), Sm(III), Dy(III), Ho(III), and Er(III) are isostructural. 

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