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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.

Diphenylbismuth ethoxide, Ph 2 BiOEt, initiates the ring-opening polymerization of ε-caprolactone via a coordination-insertion mechanism. The matrix assisted laser induced desorption and ionization-time-offlight (MALDI-TOF) mass spectra proved the formation of one ethyl ester and one CH 2 OH endgroup. The reactivity of this initiator is high enough to allow even for polymerizations at 20 °C. At 120 °C the reactivity of Ph 2 BiOEt is lower than that of tin(II) 2-ethylhexanoate (SnOct 2 ) + ethanol, whereas below 90 °C, the reactivity of Ph 2 BiOEt is higher. Small amounts of cyclic oligoesters are even formed at 20 °C. At 120 °C the number average molecular weights (M n s) parallel the monomer-initiator ratios, and M n s up to 115 000 Da were obtained. The time-conversion curves and the crystal structure indicate association of Ph 2 BiOEt via O f Bi donor-acceptor interactions.

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