Trigonal-prismatic geometry complexes

Treatment of 4 with either PF3 or 13CO results in CO substitution believed to proceed via a dissociative process yielding Ti(CO)2(PF3)-(dmpe)2 (6) and Ti(13CO)3(dmpe)2. Structural characterization of 6 showed it also to be monomeric, but possessing a monocapped trigonal prismatic geometry. Complexes 4, 5, and 6 may be considered phosphine-substituted derivatives of the as yet unisolated Ti(CO)7, thus representing the only isolable titanium carbonyl complexes where the titanium atom is in the zero oxidation state. 

As seen, the two isomers result in structurally different metal complexes. In fact, in the octahedral trans complex, the two NH2 groups are positioned axially, whereas in the cis complex the two NH2 groups are positioned in close proximity imposing a trigonal prismatic geometry. 

The geometry of the two ConI-isomers is shown in Figure 88.131,132 As already discussed for the chromium complexes, the trans-isomer affords an octahedral geometry, whereas the cis-isomer gives rise to a trigonal prismatic geometry. 

Among the other structurally characterized tris-complexes ([Fen(bqdi)3]2 +, 64 [Rum(bqdi)2(opda)]2 + 65) of known electrochemical behaviour,66 the only case we know in which structural data are available for redox couples is concerned with [Re o-C6H4(NH)2 3] + /[Re 0-C6H4(NH)2 3].67 Figure 39 shows the trigonal prismatic geometry of the monocation. 

In contrast to the monoclinic tris-malonato complexes, M2(OOCCH2COO)s 8 H2O (M=Nd) which contain nonacoordinated lanthanide ions in a monocapped square antiprismatic polyhedron, the orthorhombic tris-malonates Pnma) have a tricapped trigonal prismatic geometry for the coordination polyhedron. A rep-... 

The structures of the complexes with the ligands (117) and (118) are almost halfway between octahedral and trigonal prismatic geometry ( = 32° and 36° respectively).976-978 The complex with ligand (119) is nearly regular octahedral (0 = 51°) (Table 53).979-981... 

The X-ray crystal structures of [Ni(Bu2dtc)3]Br,3227 [Ni(Bu2dsc)3]Br3230 and [NiLJtPluAs]3231 showed that in each complex the nickel atom is in a pseudooctahedral NiAe environment (424 A= S, Se), slightly twisted towards a trigonal prismatic geometry. 

Figure 9-8. Two views of the complex [Re(PhCSCSPh)3 emphasising the trigonal prismatic structure. The phenyl groups have been represented by the black spheres to simplify the structure. It is thought that interactions between the sulfur atoms are responsible for the adoption of the trigonal prismatic geometry.

Karpishin, T. B. Stack, T. D. P. Raymond, K. N. Octahedral versus trigonal prismatic geometry in a series of catechol macrobicyclic ligand-metal complexes, J. Am. Chem. Soc. 1993,115, 182-192. 

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