Dedicated to Professor Paul von Rague Schleyer on the occasion of his 70th birthdayVant Hoff pointed out more than a century ago [1] that the heat of formation and the entropy of enantiomers of chiral molecules must be exactly equal for symmetry reasons. Traditional quantum chemistry which only includes the electromagnetic interaction finds just this symmetry. [2] However, electroweak quantum chemistry which includes parity violation predicts an energy and entropy difference. One of the greatest challenges in the current understanding of fundamental aspects of the structure and dynamics of chiral molecules concerns the experimental proof of the role of parity violation because of the weak nuclear interaction. [3] Experimental spectroscopic approaches have been proposed for some time. One of these approaches is to measure directly the heat of reaction D r H 0 0 for the transformation of S into R enantiomers which is related to the parity violating energy difference D pv E [3,4] as shown in Equation (1).Another approach is measuring certain frequency shifts of the enantiomers, for instance in NMR, [5] microwave, [6] IR, [6±10] or Möûbauer [11] spectra of R and S enantiomers (see the diagram in Figure 1).The recent theoretical discovery [12,13] that parity-violating effects in chiral molecules are typically one to two orders of magnitude larger than previously anticipated [14,15] has stimulated intensified theoretical [16±22] and experimental interest [6,10,11] in this field. Of particular importance are experi-0.17 mmol, 94 %). No PPN resonances were observed by 1 H NMR spectroscopy. UV/VIS (MeCN): l (e) 210 (6.8  10 4 ), 233 (2.1  10 4 ), 260 nm (6  10 3 ).[PPN]-2: To a solution of Na[PPN]-2 (100 mg, 0.050 mmol) in warm ethanol (10 mL) was added a solution of FeCl 3´6 H 2 O (14 mg, 0.050 mmol) in ethanol (5 mL). The purple reaction solution was stirred for 30 min at 25 8C and then kept at À 18 8C for 3 h. Dark purple crystals precipitated which were separated by filtration, recrystallized from ethanol, and dried (m.p. 197 8C) to yield [PPN]-2 (90 mg, 0.046 mmol, 90 %). Electrospray MS (MeCN), negative-ion mode: m/z (%): 1416.0 (100) [2] À ; VIS (MeCN): l (e) 537 nm (1.4  10 4 ); EPR (solid, 273 K): g 2.1997. 2: A sample of K 2 -2 (100 mg, 0.067 mmol) was dissolved in ethanol (10 mL) and a solution of FeCl 3´6 H 2 O (45 mg, 0.166 mmol) in ethanol (5 mL) was added. The reaction mixture turned purple and then dark red. The suspension was stirred for 1 h at room temperature and the precipitate was removed by filtration. The orange-brown solid was recrystallized from ethanol (m.p. 155 8C) to give 2 (80 mg, 0.057 mmol) in 84 % yield. FAB-MS (acetone), negative-ion mode: m/z (%): 1324.7 (25) [2 À CH 2 Ph] À , 1415.8 (100) [2] À , 1506.7 (70) [2CH 2 Ph] À ; 11 B NMR (160 MHz, acetone): d 43.3 (s); VIS (MeCN): l (e) 467 nm (1.7  10 4 ). [8] In neutral, zwitterionic species such as closo-B 12 H 10 (SMe 2 ) 2 the B 12 cluster retains 26 skeletal electrons and thus two negative charges are delocalized throughout th...