Coordination compounds, fluxional

Zn(L)(Tpph)]BF4 (L = l- pyrid-2-yl -3- 2/,5/-dimethoxyphenyl -pyrazole) has been obtained in low-to-moderate yields. [Zn(L)(Tpph)]BF4 is a strongly distorted square-pyramidal coordination compound which undergoes a fluxional process in solution (VT NMR) involving libration of the ligand L within one wedge of... 

This section deals with the application of moments of inertia for defining the geometries of coordination compounds and the structural correlations used to map the minimum energy paths of polytopal rearrangement processes in coordination compounds. The fluxionality of coordination compounds has long been recognised, and the pathways for rearrangement have been the subject of much debate. 

Five-coordinate species represent a rare class of coordination compounds. This rather unusual coordination number is a delicate balance between electronic factors which favor disproportion into four- and sbc-coordinate species and directional covalent bonds which help to stabilize them. The two limiting geometries (trigonal bipyramidal and square pyramidal) are therefore similar in energy and easily interconverted. As a result, five-coordinate species can exist as fluxional molecules, where their ligands can exchange places rapidly on the NMR time scale. 

Fluxional Organometallic and Coordination Compounds M. Gielen, R. Willem and B. Wrackmeyer 2004 John Wiley Sons, Ltd ISBN 0-470-85839-7... 

Fluxional organometallic and coordination compounds / edited by Marcel Gielen, Rudolph Willem, Bernd Wrackmeyer. 

Espinet, R Casares, J. A. Conformational mobility in chelated square-planar Rh, Ir, Pd, and Pt complexes. In Fluxional OrganometalUc and Coordination Compounds, Vol. 4, Gielen, M. Willem, R. Wrackmeyer, B. Eds. Wiley Sons, 2004, pp. 131-158. 

Fluorine is also capable of expanding the coordination sphere of silicon. Here the coupling between Si and F can be used to confirm the number of fluorines around the silicon. Fluxional behaviour is often observed. The hexafluorosilicate anion (SiF ) resonates at a very high field (-191.7 ppm). More common are fivefold coordinated compounds measured in solution as well as in the solid state. The extra fluorine results in shifts to low frequencies between -28 and -80 ppm the larger effect is observed in difluorides. 

Homoleptic complexes have been obtained also with tetrakis(l-pyrazolyl)borates, e.g., [ B(pz)4 2Cd] and [ B(3-Mepz)4 2Cd] (both P2 jc, Z = 2) in both compounds structure analyses the ligands have been shown to coordinate trihapto, i.e., with one pz ring free. In both cases Cd has a distorted octahedral environment, with averaged structural data very similar to those for the tris(l-pyrazolylhydridoborate complexes.201 Variable-temperature 3H NMR studies of these and of mixed complexes with tris- and bis(l-pyrazolylhydridoborates indicate fluxional behavior (coalescence temperatures and barriers for the dynamic processes are given). 

Chiral bis-(binaphthophosphole) (bis(BNP)) ligands have been used in the asymmetric hydroformylation of styrene. In solution, the free diphospholes display fluxional behavior. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. Coordination to platinum enhances the conformational stability of bis(BNP)s and diastereomeric complexes can be detected in solution. In the presence of SnCl2, the platinum complexes give rise to catalysts that exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with high branched selectivity (80-85%) and moderate enantio-selectivity (up to 45% ee). 

Compounds (L)AuC=CR can appear as ligands in the coordination sphere of transition metals. The interaction may be fluxional with metal-metal contacts M-Au and the alkyne coordinated side-on (if, dihapto) to the gold atom. Typical examples are (cp)(CO)(NO)W[Ph3PAuC=C Bu 90 and l, c3(CO)9[R3PAuC=CtBu], with R = Ph, Pr, for which several isomers have been observed in solution.91... 

Recently, dithio-diolate ligands have been employed for construction of group 4 metal catalysts for the ROP of lactide. These metal dithiolate complexes form mononuclear species of the type [(OSSO)M(OR)2] with an octahedrally coordinated metal center. These fluxional compounds acted as highly active catalysts in the ROP of L- and rac-lactide. Hafnium complexes were also introduced as initiators for the ROP of L-lactide and rac-lactide (vide infra) in very limited cases. To our knowledge, the hafnium derivative 146 displayed the highest activity among the group 4 catalysts reported to date (complete conversion of 300 equiv. of... 

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