Square pyramidal complexes reactivity

The Co system is more reactive as well as much more selective than the Ni and Rh catalyst systems (Table XVII). The best systems allow almost 100% conversion with almost 100% yield of c -l,4-hexadiene. The best of the Ni and Rh systems known so far are still far from such amazing selectivity. The tremendous difference between the Ni system and the Co or Fe system must be linked to the difference in the nature of the coordination structures of the complexes, i.e., hexacoordinated (octahedral complexes) in the case of Co and Fe and tetra- or penta-coordinated (square planar or square pyramidal) complexes in the case of Ni. The larger number of coordination sites allows the Co and Fe complex to utilize chelating phosphines which are more effective than monodentate phosphines for controlling the selectivity discussed here. These same ligands are poison for the Ni (and Rh) catalyst system, as shown earlier. 

The corresponding reactions with iridium(I) precursors again behave differently. When the phosphino functionalised imidazolium salt is reacted with [IrlcodlCl], the phosphane adduct with a pendant imidazolium moiety is formed. A similar reaction with the more reactive [Ir(cod)( Li-H)(p,-Cl)2]2 yields a five coordinate iridium(I) complex that might be described as having square pyramidal geometry with the bromide in apical position and the carbene in abnormal coordination mode [47-49] (see Figure 3.97). 

Reaction of [Co(R)(PhTt Bu)] with CO yielded the acyl adducts [Co(CO)C(0)(R)(PhTt Bu)] (R=Me, Et, Ph). These five-coordinate, low-spin complexes possess square pyramidal stereochemistry with a thioether occupying the apical position. [Co(R)(PhTtBu)] (R = Bn or allyl), upon reaction with CO, yields [Co(CO)2(PhTt Bu)]. [Ni(r]3-allyl)(K2-PhTt Bu)] shows no reactivity with CO under similar conditions. [Co(R)(PhTtBu)] also reacts with NO, giving [Co(NO)2(K2-PhTt Bu)].13... 

A range of five- and six-coordinate osmium boryl complexes has been synthesized making use of similar approaches to those reported above for ruthenium. In particular, the reaction of phenylosmium(II) precursors with boranes, which proceed via elimination of benzene, has been shown to be a useful entry point into octahedral and square pyramidal osmium boryl complexes. Significant further chemistry has been reported on these systems, including substitution at both boron and metal centres, which has shed light on fundamental issues of structure/bonding and reactivity [12]. 

Reaction of [ Mo(porph) 2] with Me3SiN3 affords the nitrido species [Mo N(porph)] which had a square-pyramidal structure (<7mo-n = 1.63A). The nitrido group was reactive, treatment of [MoN(porph)] with H+, Sg, and RI affording [Mo(NH)(porph)]+, [Mo(NS)(porph)], and [Mo(NR) (porph)]I, respectively. Related phthalocyanine complexes, [MoN(pc)] were also described. 

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