Chelation puckered rings

The 6-membered ring formed when [acac] chelates to a metal ion (Figure 7.10) is planar and is stabilized by delocalized 7r-bonding. Ligands such as bpy and [ox] also produce planar chelate rings upon interaction with a metal centre. A saturated diamine such as en (7.17) is more flexible and adopts a puckered ring as is shown in Figure 7.12 for a... 

Five-membered chelate ring of meso-2,3-diaminobutane is puckered with one methyl group equatorial and one axial. 

In a metal complex (102), five-membered chelate rings fonr d by 1,2-dia-mines exist in two puckered conformations, the enantiomeric X and 5 forms 36 and 37. For an achiral ligand in a bis or tris complex, steric interactions between rings are present, and the two conformations are no longer equivalent energetically. 

It is known that five-membered Cr(V) chelates are favored over six-membered ones.19,50,61 For Cr(V)-diolato complexes formed with linear diols, it was observed that all of the protons are equivalent in the isotropic EPR spectra,62 although the strain of a six-membered ring imparts inequivalence to the magnetic environment of the protons in the second coordination sphere.63 This observation at room temperature again points to rotational flexibility in the chelate ring (namely, puckering in the 8 or X configuration),20 similar to that observed for 1. 

In comparison to 1,2-diamines the five-membered chelate rings of amino-acids are much less puckered. Whereas dihedral angles of diamine chelate rings lie between... 

The reaction of mercuric iodide with the chelating diphosphine bis(diphenyl-phosphinoethyl)sulphide gives the complex [(Ph2PC2H4)2S]HgI2.184 Structural analysis shows the mercury to be in a distorted tetrahedral co-ordination, with formation of an eight-membered puckered chelate ring. 

A second example relates to the complex bis(l,5-diazacyclooctane)nickel(II) (Fig. 7.7). A crystal structure analysis of this complex revealed apparently planar NCCCN moieties with unusual bond lengths and angles[218]. A molecular mechanics analysis of the same complex showed that these chelate rings are puckered and concluded that the crystal structure result was a consequence of conformational disorder, the refined geometry corresponding to an average of two puckered conformations (see Fig. 7.7)[219l... 

An important feature of five-membered chelate rings is the amount of puckering. You can check this by computing the angle between the plane defined by the Co center and two N donors of one of the chelate rings, and the line defined by the two C atoms of this ring. For /e/3-[Co(cn)3]3+ this angle is 27.3°. 

Several intriguing differences were observed between the oxidized and reduced/sulfurated forms of aldehyde oxidase (18). First, there is a significantly different distance between the dithiolene sulfur atoms, 3.0 A in the oxidized form as compared to 3.5 A in the reduced form. The short S—S separation has been interpreted as a partial bond, whose precedent exists in Mo bis(thiolato) complexes (80, 81). The importance of a partial disulfide resides in the possibility of dithiolene ligand-based redox that accompanies turnover or otherwise occurs in these enzymes. The longer bonds in the reduced form seem to effect a puckering of the M0S2C2 ring of the dithiolene chelate. 

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