Complexity staggering

The methylisocyanide complex has a dimeric structure with a direct metal-metal bond (2.531 A) and only terminal isocyanides, in a staggered configuration (Figure 3.22). 

The potential energy is often described in terms of an oscillating function like the one shown in Figure 10.9(a) where the minima correspond to the relative orientations in which the interactions are most favorable, and the maxima correspond to unfavorable orientations. In ethane, the minima would occur at the staggered conformation and the maxima at the eclipsed conformation. In symmetrical molecules like ethane, the potential function reflects the symmetry and has a number of equivalent maxima and minima. In less symmetric molecules, the function may be more complex and show a number of minima of various depths and maxima of various heights. For our purposes, we will consider only molecules with symmetric potential functions and designate the number of minima in a complete rotation as r. For molecules like ethane and H3C-CCI3, r = 3. 

The molecular orbital interactions are almost independent of the conformation of the sandwich complex with respect to rotation of the Cp rings. Both the staggered (Dsd) as well as the eclipsed conformation (Dsh) possess similar binding energies... 

Molecules of the simpUcity of ethane or the complexity of proteins and DNA adopt different conformations. In the case of ethane this gives rise to the notion of a staggered and eclipsed bond, whereas proteins form an array of complex structural elements and DNA - the famous double hehx. The understanding of the conformational properties of small molecules is an important factor in computational approaches contributing to drug discovery. 

The first homoleptic, dinuclear platinum(I) carbonyl complex [Pt2(CO)6]2+ has been prepared by dissolving Pt02 in concentrated sulfuric acid under a CO atmosphere.92,93 The structure is rigid on the NMR time scale at room temperature. DFT studies suggested a staggered structure for the dimer.92,93... 

However, X-ray diffraction data show [68,69] that in K2[Tc2C16], the Tc-Tc bond distance is 0.1 A shorter than that in an analogous d4-d4 chloride technetium complex, although its main structural fragment [Tc2C18]4- has a staggered structural conformation. That is why a study of the electronic... 

Both of these observations are accounted for in the schematic representation in Fig. 3C and D of SBA crosslinked with Tn-PSM under saturation conditions. The schematic shows individual SBA molecules crosslinked to four different Tn-PSM molecules. Importantly, to form the crosslinked complex shown in Fig. 3D, lectin molecules bind to aGalNAc residues on all four sides of a Tn-PSM polypeptide chain. This allows staggering of individual SBA molecules along the Tn-PSM polypeptide chain, with concomitant decrease in steric interactions between lectin molecules. This is important, since calculations of the density of SBA molecules bound to Tn-PSM (knowing the diameter of SBA from X-ray crystal studies50 and the length of the Tn-PSM polypeptide chain) suggests that only 300 SBA tetramers can bind to the same side of a Tn-PSM polypeptide chain, which is less than the 540 bound monomers of SBA and 833 bound... 

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