ration in 3 is 2.622 (4) Á which is considerably longer than the Mo-B separation in Mo(CO)4(BH4)~, 2.41 (2) Á, and longer than the sum of the estimated covalent radii, 2.44-2.49 Á. Lastly, the Mo-H(lc) distance, 1.78 (3) Á, is slightly shorter but within three standard deviations of a range of Mo-H-Mo bridge distances, 1.85-1.89 Á,24 and significantly shorter than the average Mo-(H-B) bridging distance in Mo(CO)4(BH4)~, 2.02 A.As a final structural point it is noted that the bridging hydride occupies a coordination position on the Mo atom; therefore, the CpMo(CO)2(H)[P(X)(Y)] fragment should be considered as a four-legged piano stool. The average M-CO distance, 1.963 A, and OC-Mo-CO angle, 78.5 (2°), compare favorably with the respective parameters in other CpMo(CO)2L2 complexes.25The bonding in 3 has been examined with extended Hiickel calculations26 which reveal two principal interactions between the simplified fragments CpMo(CO)2PH2 and BH3. One high-lying occupied MO and the LUMO of the CpMo(CO)2PH2 fragment are chiefly bonding and antibonding combinations of a d orbital on the Mo atom and the p orbital on the P atom that is perpendicular to the PH2 plane. While the bulk of the electron density in the bonding orbital is on the phosphorus atom, the LUMO is primarily localized on the Mo atom.27 The high-lying occupied MO of CpMo(CO)2PH2 donates electron density into the BH3 fragment LUMO, the p orbital on the B atom perpendicular to the BH3 plane, resulting in a B-P interaction. The CpMo(CO)2PH2 LUMO also accepts electron density from one of the degenerate HOMO'S of the BH3 group which lies in the BH3 plane and is a bonding combination of a B atom p orbital with the s orbital of the bridging hydrogen atom.28 (The other two H atom s orbitals also mix constructively with the other lobe of the B atom p orbital.) Mo-Hb bonding is enhanced and B-Hb bonding is diminished (relative to the separated fragments) by this mixing of fragment MO's. Such appropriations of electron density are well-known for C-H bonds where a wealth of structures has illuminated the preliminary stages of C-H bond activation.29 Furthermore, it is intriguing to note that the structure of 3 might be considered to represent a trapped intermediate in the addition of a B-H bond across the formal Mo=P bond.The formation and solid-state structure of 3 can be rationalized and understood with the bonding picture described above. It remains to be demonstrated that the same structure prevails in solution. Detailed IE and NMR studies of solutions of 3 are in progress, and additional studies of the reactions of electrophilic reagents with 2 are underway.
substituents R', R2 and C02CH3; the attack is directed to the overall less-shielded ring plane. R ' and R2 are equatorial, and thus will contribute only slightly and almost compensatingly to the stereodifferentiation.'91 This is due, in the first place, to the influence of the 4-C02CH3 group, which is clearly anti-directing. To postulate a syn-control on the basis of a stereoelectronic effect, as a logical consequence of what has been quoted in the literature,"' is not defensible. The experiments described here, apart from being mechanistically informative, are also of preparative interest and open up a short, three-stereocenter-controlling entry to paraconic acids.[Io1 6h has been prepared as a 4aryl derivative of the nephromopsinic acid methyl ester 10.'"'
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