Trans-bent

There have a number of computational studies of hypothetical RMMR species [10-13, 40, 411. The simplest compounds are the hydrides HMMH. Some calculated structural parameters and energies of the linear and trans-bent metal-metal bonded forms of the hydrides are given in Table 1. It can be seen that in each case the frans-bent structure is lower in energy than the linear configuration. However, these structures represent stationary points on the potential energy surface, and are not the most stable forms. There also exist mono-bridged, vinylidene or doubly bridged isomers as shown in Fig. 2... 

Table 1. species Calculated geometries for linear and trans-bent HMMH (M = Al, Ga, In, or Tl)... 

Fig. 3. Schematic drawing of the polarized donor acceptor (paw-paw) bonds of trans-bent HMMH...

The complex [Au2(PPh3)2] has been prepared by reduction of [AuI(PPh3)] with sodium napht-halide. Its core unit is isoelectronic with mercurous halides Hg2X2 but, curiously enough, it has a dinuclear trans bent structure with d(Au—An) = 2.76 A and a Au—Au—P angle of 129°.3325 There is no confirmation as yet of the original preliminary report. 

The X-ray structural analyses reveal the planar structures around the heavier group 14 elements of all the heavy ketones as well as the case of the carbon analog, ketones. This is in sharp contrast to the trans-bent structure in... 

TABLE 6. Calculated stmctural parameters for R2E=ER2 trans-bent (E = Ge, Sn) compounds... 

The fraws-bent structure of Ge2H4, the two bridged structures and the germylgermylene structure were found to be real minima. The planar digermene form is the transition state for interconverting the two trans-bent forms. 

In case of Ge and Sn the destabilization of the E—E a(ag) bond in this trans-bent structure is compensated by stabilizing the highest occupied jr(bu) orbital which interacts with the E-E a orbital213. 

What is the nature of the multiple metal-metal bonds Let us consider the specific example of HW WH, whose NBO Lewis structure exhibits five metal-metal bonds. Figure 4.24 displays the strongly trans-bent geometry of HW=WH and contour diagrams of the five metal-metal bond NBOs, each drawn in a chosen contour plane (specified in the lower-left-hand corner of the panel) to emphasize its distinguishing characteristics. 

Figure 24 Structure of dimeric metallenes 392-394 illustrating the trans-bent arrangement of the organic substituents.

The trans-bent structure was unexceptional and found to be energetically favored compared to the linear arrangement, but even more stable forms such as vinylidene, mono-bridged or double-bridged isomers exist. Compounds 392-395 are unexceptional monomeric in solution, as is typical for M(i) cluster compounds.297... 

Most stannylenes, R2Sn , that are two coordinate in solution are in equilibrium with, and separate out as, the distannenes, R2Sn=SnR2. These are the distanna analogs of the alkenes, R2C=CR2, but the nature of the Sn=Sn bond is different from that of the C=C bond, and can be represented by the mutual coordination of the electron pair in the 5sp2 orbital of one stannylene unit into the vacant 5p orbital of the second stannylene unit, as illustrated in Equation (179). 5 The molecules are usually trans-bent (9 = 9 = ca. 40°, 0 = 0°), but are easily distorted, and the... 

Reduction of the distannynes with sodium or potassium gives the radical anions. [Ar Sn=SnAr ] has the trans-bent geometry given in Table 11, and shows an ESR spectrum with g= 2.0691, (ll7Sn) 0.83 mT and (117Sn) = 0.85 mT, indicating a very low unpaired spin density on tin. The main line is about 0.6mT wide, and no proton coupling is resolved. 

Although Lappert et al. reported on the isolation of the purple dialkylplumbylene 65 as early as 1973, its structure has only recently been determined. In the solid state, compound 65 exists as a loosely bonded dimer with a second plumbylene molecule and has a Pb- -Pb separation of 412.9 pm, with trans-bent angles between the PbC2 plane and the Pb- -Pb vector of 34.2°.82... 

The reaction of lead(n) chloride with 2,4,6-triisopropylphenylmagnesium bromide furnished red crystals of compound 84, the first molecule with a short Pb=Pb double bond length of 305.15(3) pm and trans-bent angles of 43.9° and 51.2°.96,96a Compound 84 is stable in the solid state. In solution, it dissociates into the plumbylene molecules 85 that, for example, react with a nucleophilic carbene to furnish the highly labile, zwitterionic adduct 86 (Scheme 2).97... 

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