Trans-bent structure

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... 

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. 

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... 

The simplest complex is the binuclear [Au2(PPh3)2], prepared by reaction of [AuI(PPh3)] with sodium naphthalide, NaCi0H8. It is isoelectronic with Hg2Cl2 but, curiously, has the trans bent structure (99) with r(AuAu) = 276 pm and angle AuAuP = 129°, rather than the expected linear structure. 

The summary in Table 9 shows the correlation between the relative tendency away from a planar equilibrium geometric structure between the CH(A)=XH2 and CH2=XH(A) sets and the general exothermic direction of the isomerization reaction 2 for a given A substituent. Thus, even for the A = NO comparisons, where the HXNO S-T splitting energies are not known from these calculations, the consistently less stable isomer [CH2=XH(A)] has the higher tendency to the trans-bent structure (X = Sn and Pb). This general correlation holds also for A = CH3, F and OH, but not for A = CN. Also, except for A = CN,... 

The rotational analysis gives a comprehensive picture of the excited state structure. Ingold and King (1953) established that the dipole moment associated with the transition is oriented along the inertial c axis (species Au), that is, perpendicular to the plane of the excited trans-bent structure hence the excited state symmetry species must be 1AU, its singlet character being inferred from the intensity of the transition (/ 10-4). Innes (1954) showed that the intensity alternation in the rotational lines requires the axis of greatest inertia to coincide... 

TABLE 9. Calculated geometries, bond dissociation energies De (kcalmol ) and energy differences Ere between the planar and trans-bent structures... 

A common feature of all group 14 heavy element triply-bonded compounds is their trans-bent structure, 657 26 258 262 340-356, in contrast to the linear Doch structure (66)... 

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