Structural isomerism, /-block metal complexes

Figure 18. Plots of the structures of bispidine-metal complexes with the tetradentate ligand 14 (a) with two sterically and electronically different sites for substrate coordination, and with the isomeric pentadentate ligands 24 (b) and 28 (c), which have one of those sites blocked by a pendant pyridine.

K(18-crown-6)]2K[Bi(SCN)6], which exists in two isomeric forms." In the yellow form, octahedral [Bi(SCN)6] " anions are surrounded by four K[K(18-crown-6)]2 units in a layered arrangement all the thiocyanates are A -bound to potassium. In the yellow form, the thiocyanatobismuth anions have four 5-bonded and two trans Ai-bonded thiocyanate ligands, and are arranged in parallel columns separated by K+ cations each potassium has two trans 5-bound and four Ai-bound thiocyanate ligands. Two [K(18-crown-6)] units are located between the columns." Structurally characterized selenocyanate complexes of the i-block metals are much rarer, but as befits the softer nature of selenium relative to sulfur, all known examples are Ai-bound." " " ... 

Formation of these products is rationalized in terms of suprafacial addition of the elements of Pd—Cl to a ring that is opening stereospecifically in a disrotatory mode, with the breaking bond bending away from the metal (dis-out) (equation 326). The absence of rj to rj to rj interconversions between the isomeric complexes is rationalized in terms of a severe steric blocking of the anti (to metal) allylic face of the complex by the nine-membered ring. This is evident from X-ray crystal structure analysis of the mononuclear acetylaceto-nate allylic complex obtained from chloropalladation of the cw-bicyclononane ". ... 

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