Nickel complex trigonal planar

The reaction of [Ni(ethene)3] with a hydride donor such as trialkyl(hydrido)-aluminate results in the formation of the dinuclear anionic complex [ Ni(eth-ene)2[2l 11 [22]. The nickel(O) centers in this complex are in a trigonal planar environment of two ethene molecules and a bridging hydride ion, with the ethene carbons in the plane of coordination. The two planes of coordination within the dinuclear complex are almost perpendicular to each other, and the Ni-H-Ni unit is significantly bent, with an angle of 125° and a Ni-Ni distance of 2.6 A [22],... 

Earlier we reported the reaction of 1 with Ni(CO)4, in which two carbon monoxide molecules are displaced to yield the tetrahedral complex, (l)2Ni(CO)2 [7]. When bis(cyclooctadiene)nickel is treated with 1 or 2, both cyclooctadiene molecules are lost and the trigonal planar complexes (l) Ni and (2)3Ni are obtained [8]. The stracture of (l)3Ni is shown in Fig. 4. This trigonal structural arrangement for nickel complexes is unusual, but not unprecedented. 

Complexes of Olefins, Acetylenes, and Related Molecules. A number of compounds have been described which contain nickel atoms in trigonal-planar co-ordination, one or more of the co-ordination sites being occupied by the multiple bond of a mono-olefin or similar species. A review of such compounds, containing structural data, has appeared. ... 

In (362), the cation is distorted trigonal pyramidal with the axial positions occupied by one S atom and the central P atom of the P3 ligand. In the anion the nickel center is bonded to six S-donors of three xanthate ligands. Upon reaction with dppe an approximately square planar NiS2P2 complex results.938 A similar coordination geometry is found for the two complexes (361a,b) 939,940... 

Both nickel(III) and nickel(IV) complexes are usually found in octahedral environments. Less common geometries are the square planar and the trigonal bipyramidal ones. 

The cyclododecatrienylnickel complex [Li(tmeda)]2Ni(cdt) (65) (92) is structurally analogous to 64 each lithium atom is apparently tetrahedrally surrounded by two olefinic carbon atoms and two nitrogen atoms with a possible additional interaction with the nickel atom. The nickel atom in 65 adopts a distorted trigonal-bipyramidal geometry with the lithiums occupying the apices, whereas 64 involves a square-planar nickel atom. 

This is particularly true now with the recent discovery of trigonal prismatic structures for some 6-coordinated metal complexes (21). Surely the proper metal-ligand combination should give a system in which the difference in energy between an octahedral and a trigonal prismatic structure is small. As a result, racemization may take place by a trigonal twist mechanism. Twisting processes of the square-planar-tetrahedral type are known to occur readily in certain nickel(II) complexes (20). 

---