Pentacoordinated cations

TABLE 63. X-ray structural data for cationic pentacoordinated (O—Ge) chelate complexes of the type (L")2Ge(X)Y... 

For the first time the proposed cationic pentacoordinated structure in solution could be confirmed for the solid state by a X-ray structure determination (Fig. 3), which shows clearly the ionic structure of bis[2-(dimethylaminomethyl)phenyl]silyl triflate 12. The coordination geometry around silicon is that of a nearly ideal trigonal bipyramidal structure with both dimethylamino groups occupying the axial positions (Si-N 205.2 and 207.2 pm). 

For the cationic pentacoordinate tropolonates there appears to be an effect of the nature of the non-chelating group R on chemical shift. Those tropolone complexes in which the silicon atom is bonded to an sp hybridized carbon in R ([47] and [48]) exhibit a chemical shift of — 141 ppm compared with hexacoordinate T3Si (<3 = — 139 ppm). 

To the best of our knowledge, cationic pentacoordinate silicon(IV) complexes with SiOs skeletons have not yet been described, whereas the chemistry of cationic hexacoordinate silicon(IV) species with SiO(> skeletons is well established. 

Table 19 presents Si chemical shifts and Si—H coupling constants for some of the cationic pentacoordinate silicon complexes and for some of their precursors. No clear trend can be seen for the Si chemical shifts of these compounds some are shifted downfield and others upheld, relative to their pentacoordinated neutral precursors. However, a trend seems to emerge from the one-bond Si—H coupling constants in the cationic pentacoordinate complexes V(Si—H) is generally greater than for neutral pentacoordinate complexes. This is possibly the result of the greater s-character in the Si—H bonds, in the doubly coordinated (sp -hybridized) siUcenium cations . 

Other cationic pentacoordinate organocobalt complexes have been investigated, such as those obtained with water-soluble phosphines P[(CH2) C6H4- -S03Na]3 ( = 1, 2, 3, and 6). This type of phosphines reacted rapidly with Co2(CO)g in two-phase reaction conditions to yield the disproportionation products [Co(CO)3(P[(CH2) C6H4-/>-S03Na]3)2][Co(CO)4]. This series of complexes has been used as precursors to water-soluble cobalt hydroformylation catalyst. ... 

Scheme 2 Cationic pentacoordinated silicon complexes formed upon ionic dissociation of a Si—Cl bond...

Scheme 22 8 2 Cationic pentacoordinated silicon complex 86 (left) and schematic representation of a Berry pseudorotatirai process (right) [221]...

---