Coordinating atom, phosphorus

Furthermore, it was determined that sulfur is a stronger coordinating atom than the oxygen of a sulfonyl group. This was established by noting that sulfur coordinates to give phosphorus when the sulfonyl usually does not. This is the case for compounds 50 and 51 (Fig. 11) [64]. 

This geometry is undoubtedly imposed by the skeletal stabilizing unit. Other eight membered phosphorus(III)-nitrogen rings, (MePNMe)i, (9) and (PrNO G NP) (11) are crown shaped containing all chemically equivalent phosphorus atoms. In XI, because the phosphorus atoms are of two sets and one set is in a highly protected environment, the possibility of selective, "cavitand," coordination at phosphorus sites exists. 

Solid-state NMR spectroscopy was used for studying the formation of cubic mesoporous aluminophosphate thin films and powders. The analysis of the initial gel, the as-deposited materials and the thermally-treated materials elucidated the changes in the coordination of phosphorus and aluminium atoms and thus revealed how the framework formation and condensation proceeds. The consolidation process in thin films was different than the process in powders. Most probably this could be attributed to the effect of glass substrate. 

It is evident that the two fullerenes are strongly electronically communicating in spite of the interposed presence of the Rh6 cluster. A partial contribution to such a strong interaction could, however, arise also from the slight unequivalence of the two C6o environments. In fact, one fullerene is linked to a Rh3 triangle coordinated to a carbon atom (of one isocyanide ligand) and a phosphorus atom (of one diphosphine), respectively, whereas the other fullerene is linked to a Rh3 triangle which coordinates two phosphorus atoms (of the two diphosphines). 

First order steric crowding is due to the steric demands of the coordinating atom, e.g. the oxygen atom of the phosphine oxide or a chlorine atom on its own. Second order steric crowding is due to the atoms or groups attached to the coordinating atom, e.g. the substituents on the phosphorus atom in the phosphine oxide above. 

The chemical bonding in the two forms is quite different. The low-pressure form has the GeAs2 type of structure, in which the coordination of the atoms is normal, i.e., fourfold for Ge and threefold for As. In the pyrite type silicon is octahedrally coordinated, and phosphorus is tetrahedrally coordinated. The difference is reflected in the density and properties. The density of the GeAs2 type is 2.47 g./cc., whereas for the pyrite type it is 3.22 g./cc. The pyrite type is a metallic conductor, whereas the GeAs2 type is a semiconductor. 

Figure 5.17. The trigonal prismatic and octahedral coordination of phosphorus atoms in TiP...

Let us assume that shifted resonances can be easily followed in a titration of Ln(III), or a Ln(III) complex, with an organic molecule. Let us also assume that we can prove that only a single complex of 1 1 stoichieometry is formed. We can now analyse the shifts on proton, phosphorus or carbon resonance lines by making the reasonable assumption that there is no free electron contact shift except on coordinated atoms. The shifts are then dipolar. Contact shifts will be considered again later. 

The excellent control observed with the P,S- and certain N,S-ligands is believed to be mainly electronic in origin [85-88, 90-92, 94, 95]. When a tt-allyl system possesses two different coordinating atoms, the nucleophilic attack is expected to occur trans to the better Jt-acceptor, since the electronic density of the allylic system is lowest at this position. In these cases, the phosphorus and the nitrogen atoms are better Jt-acceptors than the sulfur which is a good donor but weak acceptor. In some cases, the sulfur atom is considered to be the better acceptor [90]. In addition, it has been proposed that the selectivity arises from subtle steric interactions that predispose attack on the allyl unit of the reaction intermediate with a preferred reaction trajectory [93]. 

Examples given in Section V,C,2 show that the l,6-diphospha-l,5-hexadienes rearrange irreversibly at low temperatures to 3,4-diphospha-1,5-hexadienes, transforming the double-coordinated phosphorus into the preferred triple-coordinated atom. An experiment to convert 3,4-diphosphahexadiene via Cope rearrangement to 1,6-diphosphahexa-l,5-diene should thus be undertaken with the intention... 

In contrast to reactions of other ketones with PCI5, a temperature of 230°C is required to form 2,2-dichlorohexafluoropropane. The high activation energy might be due to the difficulty of coordinating the phosphorus atom by the oxygen atom of HFA 100). 

Hi. Coordination polymers with coordinative tin-phosphorus bonding. Coordination polymers with coordinative tin-phosphorus bonds are rare. An intermolecular P—Sn coordination was previously suggested to occur in Me2ClSnCH2CH2PPh2 which was based on NMR and Sn Mossbauer spectroscopic data. Recently, X-ray crystal structure analyses confirmed the previous assumption and showed a linear polymer with pentacoordinated tin atoms . The phosphorus and the chlorine atoms are located in axial positions. The Sn—P distance in 169 amounts to 3.065(1) A, which is comparable to the Sn—P distance of 3.078(2) A in the monomeric, intramolecularly coordinated Me2ClSnCH2CH2CH2PPhBu-f" iA The bromo-substituted analogue of 169, Me2BrSnCH2CH2PPh2, was also suggested to adopt a polymeric chain structure similar to that established for 169 . 

The introduction of chirality into the NHC framework follows diffoent principles than that of traditional phosphane ligands. Whaeas phosphanes are conically shaped [1] ligands with a tet-rahedrally coordinated central phosphorus atom, NHC are planar disc-like ligands based on the aromatic imidazole system (see Chapter 1). It follows that phosphanes are asymmetric owing to a stericaUy active and stable lone pair, whereas the chirality in the traditionaT NHC defines itself through a peripheral chiral element This element can be an asymmetric atom (central chirality), an axial chiral element (an atropisomeric binaphthyl substiment) or planar chirality (double substitution on an aromatic ring system, planar chiral substituent). Combinations of these three types of chirality are possible and can be realised in a more or less facile protocol. 

Many chemical molecules that are not stable under normal conditions at room temperature are found in high-temperature vapors. These include gaseous molecular forms of compounds such as metal oxides or chlorides that are normally encountered as solids at room temperature. Other species found in high-temperature vapors have unusual valencies and coordination numbers. Examples are the molecules OPCl and O2PCI, which contain unusual two-coordinate trivalent and three-coordinate pentavalent phosphorus atoms, respectively. It is of interest to study such species to extend our knowledge of the structure and bonding of small molecules to new regions of the periodic table. Moreover, many such species are proposed chemical reaction intermediates. 

Ligands in which phosphorus is the coordinating atom undergo a variety of reactions at the coordinated atom, and some of these have recently been discussed by Schmutzler 48). Platinum (II) complexes of trichlorophosphine solvolyze in water and alcohols to form phosphorous acid or stable orthophosphite esters. The gold compound, AuCKPCU), forms similar stable solvolysis products from alcohols but is reduced in water (13). 

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