Representative coordination modes

Figure 16.8 Possible resonance forms of a prototypical carbone and representative coordination modes.

Fig. 3 The M2(RCOO>4, M4(n4-0)(RCOO)6, and M3( i3-0/0H)(RC00)s SBUs and their representative coordination modes...

If the X coordinate represents a mode displacement that transforms as fli (e.g., an asymmeti ic stietch of CH) and y transforms as Ai (a flapping motion of the... 

While it remains true that tetrahedral and octahedral coordination modes are the predominant stereochemistries adopted by the group 13 metals, nevertheless increasing diversity is being achieved by carefully selecting appropriate electronic and geometric features to enhance the stabilization of unusual stereochemistries. Some representative examples follow. 

Indeed, these reactions proceed at 25 °C in ethanol-aqueous media in the absence of transition metal catalysts. The ease with which P-H bonds in primary phosphines can be converted to P-C bonds, as shown in Schemes 9 and 10, demonstrates the importance of primary phosphines in the design and development of novel organophosphorus compounds. In particular, functionalized hydroxymethyl phosphines have become ubiquitous in the development of water-soluble transition metal/organometallic compounds for potential applications in biphasic aqueous-organic catalysis and also in transition metal based pharmaceutical development [53-62]. Extensive investigations on the coordination chemistry of hydroxymethyl phosphines have demonstrated unique stereospe-cific and kinetic propensity of this class of water-soluble phosphines [53-62]. Representative examples outlined in Fig. 4, depict bidentate and multidentate coordination modes and the unique kinetic propensity to stabilize various oxidation states of metal centers, such as Re( V), Rh(III), Pt(II) and Au(I), in aqueous media [53 - 62]. Therefore, the importance of functionalized primary phosphines in the development of multidentate water-soluble phosphines cannot be overemphasized. 

Triazenido ligands can exhibit various coordination modes, and they can also act as a bridge spanning two Ni11 ions. A representative example is complex (849), where three l,2-bis(phenyl-triazenido)benzene ligands encapsulate a Ni(/x-OI I)2Ni core.206... 

A similar picture is revealed for the octadienediyl-Ni11 complex involved in the C12-cyclotrimer reaction channel.110 In the ri p1 species, 2b, 3b, r 2-butadiene preferably occupies the fourth position around Ni11, while the coordination of butadiene is unfavorable at the AG surface for the bis(r 3) species 4b. The predominant configurations are 2b and 4b, while bis(r(1) species represent isomers lying at much higher energies. The bis(r 3) species 4b is favorable by 5.6 kcal mol-1 (AG) relative to 2b, which is due to the limited ability of the weak donor butadiene to electronically stabilize the p ri C1) coordination mode. 

Consider an arbitrary two-dimensional Bravais lattice, with its sites R occupied by adsorbed molecules and molecular vibrations representing two modes, of a high and low frequency. Frequencies (ohh reduced masses mh/y vibrational coordinates w/,/(R), and momenta pA /(R) are accordingly labeled by subscripts h and / referring to the high-frequency and the low-frequency vibration. The most general form of the Hamiltonian appears as140... 

As anticipated by its ML2X2 formulation (Table 4.52), the computed structure of singlet nickelocene approximates a square-planar di-allylic coordination mode. We can deconstruct each r 3-Cp to Ni interaction into an electron-pair bond (M—X) with the radical carbon and a dative interaction (M—L) with the 7icc bond, symbolized as shown below with a half-filled circle ( >) to represent the radical site and a filled circle ( ) to represent the dative 7t-bond site ... 

In these studies Ru(III) and Ru(EDTA) represent [RuCl2(H20)4]+ and a generalized EDTA complex of Ru(III), respectively. The coordination mode and composition, in particular with respect to possible protonated complexes, were not specified for the EDTA complex in the cited references. 

Complexes with some coordination modes are characteristic for nitrite (Sec. 2.2.3.3). In this respect, the data reported in Ref. 77 are representative, where the preparation (4.17) of nitrito-N- and nitrito-O-linkage isomers 846 and 847 of nickel... 

---