Five-ligand compounds

Trigonal bipyramidal complexes in which a metal ion is surrounded by five ligands are more rare than octahedral or tetrahedral complexes, but many are known. Will trigonal bipyramidal compounds of the formula MXfY2 exhibit isomerism If so what types of isomerism are possible ... 

C-C couplings with aUcynes. An unprecedented coupling of this type was found in the reaction of the Ir(I) hydroxo-allenylidenes 71 with excess of HC=CR (R = Ph, C02Me) to afford, under remarkably mild conditions (r.t.), the novel five-coordinate compounds 72 (Scheme 24). The proposed mechanism involves an initial HO /R C=C ligand exchange followed by the oxidative addition of a... 

Ligands of structure type Ib also exist in the interesting compound (NH4)4[Mo4(N0)4(S2)5S3] 2H20 (4), which contains sulfur atoms in five different bonding situations (118). Four of the five ligands... 

Fig. 3a-c. Examples of five-coordinate compounds with five separate ligands... 

There are two basic structures possible for five coordinate compounds incorporating bidentate chelates either a compound with three free ligands... 

Another example of a Bill compound is offered by the ligand VAPOL ( vaulted bisphenanthrol), which combines with AICI3 to form a three-coordinate complex, with no dative interactions. This compound will catalyze Diels-Alder reactions [64]. A five-coordinate compound, with two dative carbonyl bonds attached to the aluminum, has been postulated as the active intermediate in the catalytic reaction. 

On heating Ni(CO)4 and Ni(PF3)4 or any of the intermediates having mixed ligands at 75° C, a mixture of all five redistribution compounds Ni(CO)4 n(PF3) (n=0, 1, 2, 3, and 4) is obtained. Quantitative separation has been accomplished by gas chromatography and a nearly random distribution has been observed. However, the authors claim that the study was not performed with sufficient accuracy to determine equilibrium constants or to judge how closely this system comes to being truly random. 

As noted above in Section 7.3.1.1, the five-coordinate compound [Be(Cl)(12-crown-4)]2+ has been prepared.178 Quantum mechanical calculations were undertaken on the hypothetical water and ammonia analogues, where the chloro ligand is replaced. It was concluded on the basis of the calculations that both water and ammonia exchange on [Be(H20)(12-crown-4)]2+ and [Be(NH3)(12-crown-4)]2 +, respectively, by an Ia mechanism.179... 

The phosphoranes are derivatives of the pentahydride of phosphorus, PH6, in which the five ligands are covalently bonded to phosphorus. The stereochemistry of pentavalent phosphorus relates to the trigonal bipyramid, just as that of tetravalent carbon relates to the tetrahedron. There are significant differences in the stereochemistry of the compounds of these two elements, other than isomer numbers. Some tetracoordi-nated phosphorus compounds become pentacoordinated rapidly and reversibly. Most pentacoordinated phosphorus compounds change their ligand distribution on the trigonal bipyramidal skeleton very easily by mechanisms that involve the simple deformation of bonds, rather than the rupture and re-formation of bonds. 

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