In the original paper on methylmanganese pentacarbonyl,1 good agreement was obtained between the experimental rotational constants and the calculated values for dihapto acyl-CH3C(O)Mn(CO)4, but agreement between experimental and calculated quadrupole parameters was poor. Calculated χaa were negative whereas the previous experimental values were positive. This note provides corrections and some additions to the earlier paper.1 Subsequent to the initial study, many calculations were carried out using different methods and basis sets and the calculated quadrupole coupling parameters between these different calculations were reasonably consistent. In particular, the calculations consistently gave negative values for the 55Mn quadrupole parameters, whereas the reported experimental values were positive. However, alternative assignments of the quantum numbers to measured transitions could be found that yielded negative quadrupole parameters and a better fit to the spectrum. More details of the new analysis and computation are given in a recent paper.2 The new fit analysis included the rotational constants A, B, and C, centrifugal distortion constants DJ and DJK, and the quadrupolar parameters 1.5χaaand 0.25(χbb− χcc). Results of the new spectral analysis are reproduced in Table 1 below. The best agreements between experimental and calculated values were obtained for B3LYP and M11 DFT with def2 basis sets, as shown in Table 1. Experimental molecular parameters from the new analysis are A = 840.084(5), B = 774.286(2), C = 625.653(1), DJ = 0.00021(2), DJK = 0.0049(2), 1.5χaa= −47.0(1), and 0.25(χbb− χcc) = −13.44(3) MHz. These experimental parameters agree best with calculations for the dihapto isomer, rather than the agostic form of this complex. Excellent agreement is obtained for the rotational constants, and now a much better agreement for the quadrupole coupling parameters. The present and previous molecular parameters are given in Table 2. The three lowest energy structures for methylmanganesepentacarbonyl are shown in Figure 1. The symmetric isomer (c) and the two distinct acyl-Mn isomers and the agostic form (b) and the dihapto form (a) are shown. (Figure presented) With the newer assignments, the 1σ standard deviation of the experimental fit to 40 transitions, dropped from 41 to 18 kHz. Importantly, the new fit yielded the nuclear quadrupole coupling constants that are consistent with the theory. As displayed in Table 1, rotational constants between different DFT methods for the same structure are in good agreement with the experimental value. The experimental 1.5χaa values agree better with the calculated value for the dihapto form. In contrast, the calculated quadrupole parameters for the agostic isomer are much different from experimental values. We concluded that the observed asymmetric form of methyl manganese pentacarbonyl is the dihapto form on the basis of these results. For the B3LYP calculations (columns 3 and 7 of Table 1), the dihapto form is 3350 cm−1 lower in energy than the agostic form. The calculated energy for the dihapto is −1757. 937 hartree, compared with −1757.921 hartree for the agostic form.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry