Combined spectroscopic and molecular mechanics investigation of the flexible organometallic compounds (.eta.5-C5H5)Fe(CO)2R (R = PhCH2, Et, MeCO) and (.eta.5-C5H5)Fe(CO)(PPh3)COMe

Rotamers of dimethyl(acetyl)(η5-cyclopentadienyl)platinum(IV) in the gas phase

“…it has been shown that the esuilibrium between rotamers in organotransition metal complexes can be studied by FTIR in the gas phase as well as in solution, provided the complexes have sufficient volatility. Molecular mechanics calculations can aid the assignment of structure to the rotamers (4,5) but, since the calculated energy differences are small and since, as explained above, there are other reasons to doubt the assignments in the present case (6), our conclusions should be considered tentative until more sophisticated models than the MMX package used in this work are developed.…”

“…From this starting point, the structure was allowed to minimize, giving one of the globai minimum configurations. Barriers to rotation were then estimated by varying a dihedral angle defined as CH3C(0)PtCH3 over the range 0-360" in 5" increments, with full minimization at each angle (4). The lowest energies occurred at torsion angles of 148 and 303', corresponding to structures l a and la', while a much shallower minimum occurred at a dihedral angle of 50' corresponding to structure l b (Fig.…”

“…In particular, the qcyclopentadienyl (Cp) derivatives [CpFeL2R] (L = CO, PR3, R = alkyl, acyl, silyls) have been examined in great detail. Infrared spectroscopy provided evidence for the rotamers and allowed the thermodynamic properties to be determined experimentally (3c, 3e, 4), with the interpretations aided by theoretical studies using Extended Hiickel, a b initio, and molecular modelling calculations (4)(5)(6). There has been some controversy about the precise interpretation (4,5), but it is currently thought that the more stable rotamers are A and B, with C at higher energy.…”

“…Since no study of this type of rotamers in the gas phase has been made (3)(4)(5)8), a more detailed investigation of the gas phase equilibrium was carried out and is reported below. It should also be noted that, since the time of Shaver's study (8), molecular mechanics calculations have become routine and have led to conformational analysis of these molecules in terms of pseudooctahedral rather than pseudotetrahedral stereochemistry at the central metal (4,5) and this naturally modifies the interpretation.…”

“…It should also be noted that, since the time of Shaver's study (8), molecular mechanics calculations have become routine and have led to conformational analysis of these molecules in terms of pseudooctahedral rather than pseudotetrahedral stereochemistry at the central metal (4,5) and this naturally modifies the interpretation.…”