Ligand Transformations

STEREOCHEMICAL ASPECTS OF THE RACEMIZATION AND LIGAND EXCHANGE OF (+) AND (-)-C5H5Mn(NO)(COR)PAr3 

The compounds (+)- and (-J-CsHsMnfNOHCOCsHsJPtCsHsJs (+17 and -17) represent prototypes of configurationally labile complexes. Solutions of +17 and -17 at room temperature lose their high optical rotations exponentially due to a change in configuration of the Mn atom. By polarimetric kinetics, a half-life (25°C, toluene) of 21 minutes was measured (100, 109). 

The manganese esters (+)- and (-)-C5H5Mn(NO)(COOR)P(CeH5)3 ra-cemize in solution by change of configuration of the Mn atom in much the same way as the acyl complexes 17 and 18. However, there is no 

Such species as C5H5Mn(CO)2 with a probable pyramidal structure have also been identified by matrix isolation 118, 119). Thus, matrix isolation, on the one hand, and kinetic and mainly stereochemical investigations on the other, are complementary methods for the study of intermediates having lowered coordination number. 

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Ligand exchange [409,614-616] and ligand transformation of carbene complexes have been widely used for the fine-tuning of catalytically active carbene complexes. 

The v<2 Region. As discussed earlier, there should be one mode in this intermediate frequency range for a linear or bent M-H-M array. However, in these more complex metal carbonyl complexes, the bent form of (OC)sM-H-M(CO)5 should exhibit two deformation frequencies because the presence of the carbonyl ligands transforms what would have been a rotational motion for... 

Koshevoy IO, Haukka M, Selivanov SI, Trunk SP, Pakkanen TA (2010) Assembly of the Au-diphosphine helical cage molecules via alkynyl-p4-methylydine ligand transformation. Chem Commun 46 8926-8928... 

Our interest will be centered primarily on the metal s role in catalyzing symmetry-forbidden reactions. This, as we shall point out, is a special case and one that might best be introduced in contrast to other kinds of metal-assisted ligand transformations. Pursuing this approach, we shall consider generally the transformation of a ligand A to some ligand B. 

First we shall treat the situation in which the ligand transformation A ->-B is, by itself, symmetry-allowed (Class 1). 

The second class of reactions (Class 2) contains those processes in which the ligand transformation A B is, by itself, symmetry-forbidden. This is the forbidden-to-allowed process it requires special operations on the part of the metal which place it in a class by itself. For the cycloaddition reactions that we will be concerned with, the metal exchanges a pair of electrons with the transforming ligand and, in the process, suffers a spatial redistribution of its valence electrons. In this instance. Class 2 reactions are represented by... 

The third class of ligand transformations that we wish to distinguish contains those transformations in which the metal serves as a participant in the formation of a new bonding configuration. The bonds between the metal... 

A symmetry-forbidden reaction can be switched to an allowed reaction in a number of ways. One of the more interesting mechanisms is that in which the actual forbidden transformation takes place on the coordination sphere of a transition metal. The ligand transformation here is concerted, the S5unmetry restrictions having been removed by the metal. The metal s role in this process has been described briefly in an earher communication by this author with J. H. Schachtschneider 3), and in more detail in a broader treatise 2). The description will not be repeated here instead, the subject will be approached from a different point of view, one that focuses attention on the coordinate bond and its relationship to the forbidden-to-allowed process. 

We shall illustrate this process as it applies to the [2 - -2] cycloaddition process. This treatment will be general, making no distinctions between [ji2] and [a2] systems. We assume that the ligand transformation, however, is suprafacial, i.e., [ , i2J. To illustrate the role of coordinate bond-... 

A stepwise process has been suggested for the valence isomerization of 72 to 7J 5). In this treatment experimental evidence was presented supporting the intermediacy of a species X which transforms to the observed products. The intermediate, however, was not isolated, trapped, or specifically identified. Intermediate X could very well be the reactant 72 coordinated to the metal in a bidentate manner. The critical step in this case would be the attainment of bidentate coordination. If this is so, 12 13 could reasonably proceed through the concerted ligand transformation with preservation of full bidentate coordinate bonding. 

Considering the unusually attractive ligand properties of cyclobutadiene i ), what could prevent two coordinated acetylene ligands from fusing to a cyclobutadiene ligand Transformation 26- 27, however, encounters... 

With ligand transformations that are energetically favorable and where bidentate coordinate bonding is strong in both valence isomers, the situation is quite different. We shall discuss this case at the end of this section. 

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