Five-membered ring structures, -coordination

Planar five-membered ring structures of phosphadiazoles were among the first known structures incorporating two-coordinate phosphorus The ring PN bonds in these compounds appear to be somewhat shorter than those in nonplanar five-membered rings containing three-coordinate phosphorus ... 

A mixture of ethyl bromosuccinate and tinfoil in the presence of catalysts was stirred for 4 h, for example, and the tinfoil soon disappeared from the reaction mixture in the process. A product was easily isolated as a solid form from the reaction mixture, and two crystalline diastereomeric species were obtained by fractional crystallization of the above product. Based on the IR spectra of the products, the author first proposed a five-membered ring structure for organotin compounds in which lone electron pair of carbonyl oxygen coordinates to a tin atom, as shown in Eq. (2.4). 

In 1994, about 30 years after direct reactions of halo-p-carbonyl compounds with tinfoil were first conducted, the author sought to show the reasons why cyclo-metalation reactions for the production of organometallic intramolecular-coordination five-membered ring compounds proceed extremely quickly. To verify the above, the five-membered ring structure was demonstrated to be the most stable as compared with four- and six-membered ring structures. 

The cyclometalation of o-lithium dimethylaminobenzene with chromium chloride proceeds to give the five-membered ring as shown in eq. (13.24) [26,36]. Many organometallic intramolecular-coordination compounds containing the five-membered ring structure of organochromium compounds have been synthesized and the structure was determined by X-ray diffraction studies as shown in the book Organometallic Intramolecular-coordination Compounds [26]. 

However, EDTA has the widest general application in analysis because of its powerful complexing action and commercial availability. The spatial structure of its anion, which has six donor atoms, enables it to satisfy the coordination number of six frequently encountered among the metal ions and to form strainless five-membered rings on chelation. The resulting complexes have similar structures but differ from one another in the charge they carry. 

The [Pt(Se4)2]2 complex (133) has been prepared by the reduction of the platinum(IV) species [Pt(Se4)3]2 with excess borohydride.328 The X-ray structure of the complex shows that each of the five-membered rings adopts a half-chair conformation, with the platinum coordination sphere exhibiting slight distortion from a square-planar geometry.329... 

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