Four-coordinate compounds

3 Four-coordinate Compounds. - The geometry of the phosphate group (and its interactions with metal cations) has been studied in the crystal environment by analysing XRD data from 178 crystal structures, and in vacuo by ab initio calculations of dihydrogen- and dimethyl phosphates and a diphosphate model 

Phosphorylation of calix(4)arene and t-butyl-calix(4)arene produced the di-substituted product (113 R = H, Bu ). The dominant conformer formed is a stereochemically-rigid cone where the angles of inclination of the benzene ring, 

O-P-0 found for these two phosphoranes were apical, 162.5° and 163.3° and radial, 107.7° and 112.7° respectively. Probably, compression of the equatorial O-P-O bond angle in the 6-membered ring of (114 RR = CH2CH(Bu )CH2) and related phosphoranes allows the ring to assume a chair conformation more easily than when phosphorus is not part of a bicyclo ring system. 

The only significant reference to six coordinate compounds (perphosphoran-ides) related to the divalent tridentate ligands in (76) and (77).  

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Much effort has been placed in the synthesis of compounds possessing a chiral center at the phosphoms atom, particularly three- and four-coordinate compounds such as tertiary phosphines, phosphine oxides, phosphonates, phosphinates, and phosphate esters (11). Some enantiomers are known to exhibit a variety of biological activities and are therefore of interest Oas agricultural chemicals, pharmaceuticals (qv), etc. Homochiral bisphosphines are commonly used in catalytic asymmetric syntheses providing good enantioselectivities (see also Nucleic acids). Excellent reviews of low coordinate (coordination numbers 1 and 2) phosphoms compounds are available (12). 

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... 

TABLE 2. Standard bond lengths (pm) in four-coordinate compounds of Ge, Sn or Pba... 

In the last few years, EPR has been widely used to study the electronic structure of four-and five-coordinated low-spin Co(II) complexes. Compounds of this class gained considerable interest because of their relation to biological oxygen carriers and to vitamin B12r. The ground state of the five-coordinated complexes is accepted to be dz2 ), in contrast to the four-coordinated compounds for which the determination of the correct ground state was quite troublesome, due to the fact that these types of Co(II) complexes have low-lying excited states. 

Complexes containing the MoO + core are by far the most common in Mo chemistry. These complexes are usually six-coordinate although some four-coordinate compounds are known. The latter are tetrahedral or pseudotetrahedral and related to MoO -. Examples include Mo02S -, Mo02Se2-, Mo02Br2 and Mo02(R2NO)2. An example of a five-coordinate complex now exists.73... 

Force Field Parameters V Parameterizing a New Potential -The Tetrahedral Twist of Four-Coordinate Compounds. 257... 

Note that a combination of various types of potentials can be activated. For example, the coordination geometry for octahedral transition metal compounds can be modeled by 1,3-nonbonded interactions in combination with a multiple harmonic function. This is the approach used in the MOMEC97 force field for a number of metal complexes. Also, for four-coordinate compounds one or both of these potentials can be combined with a plane twist potential that enforces square-planar geometry. 

As one of the special features, MOMEC has a plane twist function. This has been included to limit the tetrahedral twist in four-coordinate compounds, where 1,3-nonbonded interactions lead to a preference for a tetrahedral arrangement (see Section 3.6). That is, the plane twist potential can be used to induce a square-planar arrangement or, using constraints, any intermediate structure can be enforced. The same potential can in principle be used for other structural features (see Fig. 17.14.1), such as the Bailar twist of six-coordinate complexes or for computing the rotational barrier of metallocenes. However, at present it has only been implemented in MOMEC for the tetrahedral twist and no parameters have been included as yet. 

There are a number of possible models and parameterization schemes for enforcing planarity in four-coordinate compounds ... 

The acetonitrile-containing anion [ReOBr4(NCMe)] is described in Section 43.7.1.6. The only known dialkylamido compound is red, diamagnetic, crystalline ReO[N(SiMe3)2]3 (I fRe—=0) at 978 cm"1), It is a unique four-coordinate compound of rhenium(V), that can be prepared by the interaction of ReOCl4 with UNMe2.245... 

The bis(imino)phosphoranes are prepared by a reaction sequence involving iminophosphine and cyclophosphate-trazenes, but readily dimerize to cychc four-coordinate compounds (diiminocyclodiphosphazanes) even with bulky organic substituents at nitrogen R = tert-Hu, CEts, 1-Ad (Scheme 12). ... 

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