Rhombic twist

Fig. 13. Transition-state structures for pseudorotation by the trigonal twist (M3) and rhombic twist (M3) mechanism. According to Ref. [93]...

Figure 1-12 Three possible modes of intramolecular race-mization of a trischelate complex, (a) trigonal twist, (b) rhombic twist, (c) one of several ring-opening paths.

Figure 9. Intramolecular racemization of [M (A A) 3] by the following mechanisms (A) chelate ring opening-closing) B) rhombic twist) and (C) trigonal...

Studies of the kinetics and steric course of the first-order intramolecular stereoisomerizations of the Al /3-diketonates tris(2,6-dimethylheptane-3,5-dionato(aluminium(iii) and bis( 1,1,1,5,5,5 -hexafluoropentane-2,4-dionato)alu-minium(m) have been made. It appears that chelates having alkyl or aryl substituents on the /3-diketonate rings isomerize by a rhombic twist mechanism. If fluorocarbon substituents are present, however, a bond-rupture mechanism is more likely, with formation of a square-pyramidal apical five-co-ordinate inter-mediate. ... 

The authors conclude that for the corresponding substitution which is not acid catalysed, the mechanism cannot involve the rupture of an aluminium-oxy n bond in the rate-determining stage, as this would be too slow a process to account for the observed rate. Other authors have concluded - that the stereoisomerization of the aluminium(iii) / -diketonate complexes with alkyl or aryl substituents in the ring probably proceeds by a rhombic twist mechanism. Those complexes with a fluorocarbon ring substituent probably stereoisomerize by a bond-rupture mechanism with a square-pyramidal intermediate with the unidentate /9-diketonate ligand in the apical position. 

Fig. 7 Definition of the chelating bite angle a. Large angles a facilitate Ray-Dutt twists with rhombic (C2U transition states, whereas small angles a favor Bailar-twists with trigonal (D3h) transition states...

There are two pathways without bond rupture. One is the trigonal or Bailor twist and other is rhombic, or Ray-Dutt, twist shown in figure below. Some cis-M(CO)4(PR3)2 complexes are believed to isomerise in... 

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