Stereochemical non-rigidity

Concept of " pseudorotation introduced by R. S. Berry to interpret the stereochemical non-rigidity of trigonal bipyramidal PF5 (and SF4, ClFi) the 5 F atoms are equivalent (1953) due to interconversion via a square pyramidal intermediate. 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

A number of complexes of the type M(AA)2X2 are neutral and soluble in organic solvents. They are stereochemically non-rigid on an nmr time scale, that is they display nmr line broadening or collapse in accessible temperature ranges. Concommitant solvolysis problems can usually be avoided. The behavior shown is typified by the complexes Ti(P-diketone)2X2. The cis-configuration is invariably the stable one in solution, e.g. 6... 

Group IV Donors. A H n.m.r. study of the cis-trans isomerization of [OslCO) -(SiMe3)2] indicates that the process is non-dissociative. These species may therefore provide a rare example of stereochemical non-rigidity in nonchelate octahedral complexes. Similar non-rigidity was observed for [Os(CO)4-(SiMeCl ) ] and [Os(CO)4(SnMe3)2]. ... 

KsIMofCN H jO is isomorphous with K3[W(CN)8]-H20, which has square antiprismatic geometry, although a full structure was not determined for the molybdenum analogue as it decomposed under X-ray irradiation,312 The ESR spectrum of K3[Mo(CN)8] in glycerine solution indicates that all the cyanides are equivalent, consistent with a square antiprismatic structure, or with stereochemical non-rigidity on the ESR timescale.313... 

Some bond distances are given in Table 5. The B—Hterminal distances not listed are normal. The structure, in solution, is assumed to be stereochemically non-rigid a fluxional process occurs with the unique bridging hydrogen moving between B2 and B5. This assumption is based on the nB NMR spectrum which indicates Cs symmetry116 . 

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