Clusters fluxionality

Bradford, A.M., Kristof, E., Rashidi, M., Yang, D.S., Payne, N.C. and Puddephatt, R.J. (1994) Isocyanide and diisocyanide complexes of a triplatinum cluster fluxionality, isomerism, structure, and bonding. Inorg. Chem., 33, 2355. 

Modern NMR techniques such as quantitative analysis of multisite exchange using either ID magnetization transfer experiments (JOS) or the 2D exchange spectroscopy (EXSY) method (104,105) promise to be of great help in unraveling the complex stereochemical exchange networks involved in cluster fluxionality. The usefulness of EXSY in the context of this article is illustrated by the phase-sensitive 13C 1H EXSY spectrum (255 K, tm = 0.5 sec)... 

There is evidently a flat potential energy surface for these clusters as different skeletal isomers can actually be isolated in the solid state for Au9[P(C6H40Me)3]8(N03)3 a brown form has a D4d crown structure and a green form has a D2h structure. In solution, however, they share a common structure with identical31P NMR spectra either the solution cluster has a regular arrangement of phosphines, or it is fluxional [186]. 

The focus of research on very mixed"-metal clusters has been on their synthesis and structure, and the limited physical measurements of these clusters have thus far been largely restricted to fluxionality and electrochemical investigations. Studies of ligand fluxionality are summarized in Section 111.A. and reports of electrochemical investigations are reviewed in Section Ill.B. The few reports of the magnetic behavior of these clusters are discussed in Section lll.C. 1., and theoretical studies are summarized in Section I1I.C.2. 

Ligand fluxionality on metal clusters has been the subject of many studies, the majority of reports focusing on carbonyl migration on homometallic tri- and... 

Three major classes of fluxional behavior are commonly observed in clusters ... 

Table XI summarizes reports of ligand fluxionality at "very mixed -metal clusters. A number of studies (c.g.. Refs. 28, 8I, 116, 213, 214) have reported that both metal-localized and global carbonyl fluxionality occur but give little mechanistic detail. The following discussion focuses on the examples for which detailed studies have been undertaken and/or those for which mechanistic speculation is available.

The synthesis, X-ray structure, and dynamic NMR properties of the arachno 10-vertex cluster [9,9-(PMe2Ph)2-9,6,8-PtS2B7H7] has been reported the compound was fluxional via a vertex flip of the Pt(PMe2Ph)2 moiety.84... 

Reduction of the pentaalkyl-1,5-dicarba-closo-pentaboranes(5) 44 with elemental K opens the cluster and subsequent reaction with I2 enforces oxidative fusion of two C2B3 units to give a C4B6 cage (Scheme 3.2-36). The peralkylated hexaboraadamantane derivatives 70 (R = Me, Et X-ray structure analysis for R = Me [89]) rearrange irreversibly into the carboranes 71 with a wido-C4B6 framework [89] (X-ray structural analysis for R = Et [90]). The structure of the nido-C Bf, cluster is fluxional in solution [91]. 

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