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Figure 1. The moleculab structure of [MezGeRe(CO)aC(0)Me]2.lo Selected bond lengths (A) are: Re-Ge = 2.591(3); Ge-05' = 1.96(2); C5-05 = 1.27(3); Re-C5 = 2.14(3); av Re-C(1-4) = 1.92(3); av C(1-4)-0(1-4) = 1.15 (3). Selected bond angles (deg) are: Re-C5-C8 = 111 (2); Re-C5-05 = 121 (2); C8-C505 = 128 (2); Ge-O5'-C5' = 139 (2), Ge-Re-C5 = 82.8 (6); Re-Ge-05' = 107.6 (4).
- 25 ", thermodynamic parameters for this process were determined to be A H o = 6.7 f 0.3 kcal and ASo = 27.9 f 1.4 euS3 The 13Cnmr spectrum in CDCh at -30" confirmed the presence of two distinct species in solution. Particularly noteworthy were two peaks at - 337.72 and -335.20 ppm, relative to TMS, which established that both species contained carbene carbon atoms. It has not been possible to obtain crystals of 1 suitable for crystallographic study. We therefore prepared the rhenium analog5 of 1 and have been able to obtain satisfactory crystals of it despite its lesser stability in solution.6 The X-ray diffraction study showed that the crystals contained the dimer rather than the monomer.' The crystals exhibited twinning and the apparent space group and unit cell were determined to be 12/m, a = 9.639(8) A, b = 11.504(9) A, c = 11.140(9), A, and p = 97.22 (I)". This cell is a composite of two orientations of a triclinic cell with space group Pi and a proxia = mate dimensions a 11 b = 9.05 A, c = 9.63 106.9", p = 118.0", and y = 101.0". The density was measured by flotation as 2.37(2) g ~ m and - ~is in reasonable agreement with the calculated value 2.40 g cm-3 (assuming a molecular weight of 887.7 and two molecules in the 12/m cell). Intensity data were collected on an automatic Picker four-circle diffractometer within the angular limits 0 < 28 5 45", 0 5 9 < 360", -90" 5 x 5 90" using M o K a radiation. The data
1,
(3) At the concentration used in the variable temperature study (52.8 mg ml-l), the dimer peak above -25" was of low and not easily measurable intensity and decreased to zero before ambient temperature was reached. This was not a process of peak coalescence. Furthermore, dimer would not be detected at dilutions normally used in infrared spectroscopy. (4) G. M. Bodner, S. B. Kahl, I
