Apical ligation

The C-Si(CH3)3-substituted stannacarboranes have been found to be useful starting materials for the synthesis of the other metallacarboranes. The apical tin is loosely ligated and can be replaced by other metal groups. For example, the reaction of Os3(CO)12 with c/-Sn[(CH3)3Si]2-C2B4H4 in the absence of solvent yields l-Os(CO)3-2,3-[(CH3)3Si]2C2B4H4 (50) in essentially quantitative yield [see Eq. (7)], while direct reaction of... 

Pentakis(methoxycarbonyl)cyclopentadiene, which is a strong protic acid, reacts with bistributyltin oxide in benzene at room temperature to give a salt between the diaquotri-butyltin cation and the cyclopentadienide anion as shown in equation 9-48.109 The cation has a trigonal bipyramidal structure, with the water molecules in the apical positions, and these can be replaced by other ligating molecules such as HMPT and DMF. 

Formation of Ni—CH3 species by the more likely biological route of Ni-based nucleophilic attack on a methyl electrophile has been demonstrated. The Ni complex, (40), supported by thioether and phosphine ligation, was reduced to the Ni adduct in situ. It was then oxidized to the Ni CH3 and Ni (C(0)CH3) species by reaction with CH3I and CH3C(0)C1, respectively. These low-spin, square-pyramidal complexes contain apical organic ligands and are related by CO insertion/extrusion, which was rapid even at -60 °C, precluding kinetic analysis. 

A detailed mechanism of the process shown in Scheme 1.3 is unknown. Two general mechanistic pathways, dissociative and associative, have been proposed for the ligand exchange reactions of X -iodanes (Scheme 1.4) [26, 127]. The dissociative pathway seems to be less likely to occur, because of the low stability of the dico-ordinated iodonium ion [PhIL]+ involved in this mechanism [127]. Such iodonium 8-1-2 species, however, have been frequently observed in the gas phase, for example, in mass spectrometry studies of protonated iodosylbenzene, [PhIOH]+ [101], or in the mass spectra of all known iodonium salts, [ArIR]+. The presence of cationic iodonium species in aqueous solution has been confirmed by spectroscopic measurements and potentiometric titrations of PhI(OH)OTs and PhI(OH)OMs [198] however, all available experimental data show that the iodonium species in solution are coordinated with solvent molecules or with available counteranions. X-Ray diffraction analysis of the protonated iodosylbenzene aqua complexes [PhI(H20)0H]+ isolated from aqueous solutions revealed a T-shaped stmcture, ligated with one water molecule at the apical site of the iodine(III) atom of hydroxy(phenyl)iodonium ion, with a near-linear O-I-O triad (173.96°), which is in agreement with a regular X -iodane structure [178]. 

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