Four-ligand compounds

In the solid state, the M2(0R)6L2 compounds have four coordinated metal atoms united by M=M bonds. The four ligands coordinated to each metal atom lie roughly in a square plane as is shown in Figure 3. 

The use of the sulfoxantphos ligand (compound (b) in Figure 7.8) in the biphasic hydroformylation of 1-octene with [BMIM][PF6] has been studied by Dupont and coworkers [58], The ligand allowed recycling of the catalyst solution up to four times with no loss in activity or selectivity. Flighly regioselective hydroformylation (n/iso = 13) was reported for a Rh/phosphine-ratio of 4 (100°C, 15 bar syngas pressure). 

Mononuclear ER4 and simple four-coordinate compounds of E(IV) states are the baseline for viewing the other coordination numbers, the effect of bulky ligands, bonds to other E or metals, E(II) compounds, multiple bonds and other phenomena discussed in later sections. Basic parameters for some simple compounds are presented in Table 1, taken from the gas-phase data summarized by Molloy and Zuckerman5 and Haaland6. These data show the unperturbed molecules in the gas phase and provide the base for... 

In the areas of coordination and metalorganic compounds the CIP system was adopted in part (Sequence Rules), but a variety of special conventions mainly concerning constitution and descriptor assignment rules for stereogenic centers with more than four ligands have emerged. An excellent review is available1, and here only those types of compounds relevant to this volume are discussed. 

Metalorganic Compounds with Four Ligands at a Stereogenic Metal Center... 

Compounds of the elements in zero oxidation state are known for all three metals. With one exception, those of nickel contain the equivalent of four ligands coordinated to the metal, whereas palladium and platinum also fonn compounds of empirical formulas ML2 and ML3 (162-164). 

The coordination geometry in anhydrous Th(CF3COCHCOMe)4 is a 1111 ( )4-422) antiprism 74 the structure of the monohydrate has been discussed earlier (p. 1144). Th[CF3COCHCO(2-C4H3S)]4 is isostructural with the cerium(IV), uranium(IV) and plutonium(IV) analogues. The coordination polyhedron is a distorted dodecahedron in which the four ligands span the two perpendicular trapezoids of the dodecahedron.75 In the complexes M(n-C3F7COCHCOBut)4, the thorium(IV), uranium(IV) and neptunium(IV) compounds are isomorphous, but the plutonium compound is not. 

In its compounds nickel usually has the +2 oxidation state but the +3 and +4 states occur rarely in complexes. Tire Ni2+ ion contains eight 3d electrons, a configuration that favors square-planar coordination of four ligands. However, the ion is also able to form a complex with six ligands and an octahedral geometry. It has been suggested that this "ambivalence" may be of biochemical significance. 

Isolable Oxysulfuranes. Sulfuranes, as described by Musher (1969), are compounds of nlfurlIV) in which four ligands are attached 10 sulfur and have in common with tare-gas compounds such as XeF, an electronic structure involving a formal expansion of the valence shell of the central atom from 8 to 10 electrons, Martin and Perozzi (1976) pointed out that the incorporation of oxygen ligands makes possible a wide range of new structural types that illustrate structure-reactivity relationships in a particularly illuminating way. 

The crystal structure determination of [ZnI2(pyNO)2] demonstrates its molecular behaviour, with a tetrahedral environment about Zn, whereas the CdI2(pyNO) complex is polymeric with alternating O and I bridges between Cd atoms which are five-coordinate. The HgCl2 pyNO compound displays six-coordination about Hg but two trans HgCl bonds are much shorter than those to the other four ligands.81... 

The molecular structures of four dinuclear compounds containing the p-hexafluorocyclooctatrieneyne ligand are shown in Fig. 18 (799). Selected average geometrical features of the fluorinated rings in these four structures,... 

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