Agostic-Type B−H···Pb Interactions Stabilize a Dialkylplumbylene. Structure of and Bonding in [{<i>n</i>Pr<sub>2</sub>P(BH<sub>3</sub>)}(Me<sub>3</sub>Si)C(CH<sub>2</sub>)]<sub>2</sub>Pb

The reaction between Cp<sub>2</sub>Pb and 1 equiv of [[{<i>n</i>Pr<sub>2</sub>P(BH<sub>3</sub>)}(Me<sub>3</sub>Si)C(CH<sub>2</sub>)]Li(THF)<sub>2</sub>]<sub>2</sub> in toluene affords the dialkylplumbylene [{<i>n</i>Pr<sub>2</sub>P(BH<sub>3</sub>)}(Me<sub>3</sub>Si)C(CH<sub>2</sub>)]<sub>2</sub>Pb (<b>10</b>) in excellent yield; compound <b>10</b> may readily be separated into its two diastereomeric forms by a simple crystallization procedure. X-ray crystallography shows that <i>rac</i>- and <i>meso</i>-<b>10</b> crystallize as discrete dialkylplumbylenes in which there are either two (<i>rac</i>-<b>10</b>) or one (<i>meso</i>-<b>10</b>) short agostic-type B−H···Pb contacts; for <i>meso</i>-<b>10</b> there is a secondary, weak B−H···Pb contact, which affords this diastereomer a further, weak stabilization. Multielement and variable-temperature NMR studies indicate that, while <i>rac</i>-<b>10</b> is essentially static in solution, <i>meso</i>-<b>10</b> undergoes dynamic exchange between the free and bound BH<sub>3</sub> groups. Unusually, the methylene backbone protons in both diastereomers (at low temperature for <i>meso</i>-<b>10</b>) lie within the range 2.5−6.5 ppm, possibly due to the close proximity of the 6s lone pair on the lead center. DFT studies indicate that there is significant delocalization of B−H σ-bonding electron density into the vacant lead 6p orbital, furnishing an overall stabilization of 40.6 and 30.3 kcal mol<sup>−1</sup> for <i>rac</i>- and <i>meso</i>-<b>10</b>, respectively.