The complete spectroscopic characterization of the small phosphorus-bridged pentaborane(9) cluster (μ-diphenylphosphino)-pentaborane, [μ-(C6H5)2PB5H8] (1), is reported. The MNDO calculated structure for 1 shows that the B-P-B interaction can be best described as consisting of two two-center-two-electron B-P bonding interactions. Indirect support for the involvement of the lone pair of electrons on the phosphorus in cage bonding is obtained from the failure of 1 to react with [(CH3CN)3Mo(CO)3] under forcing conditions. Compound 1 is readily and quantitatively bridge-deprotonated by the action of NaH to produce the corresponding anion, Na[(μ-(C6H5)2P)B5H7] (2). Compound 2 reacts with 1 equiv of [Fe(η5-C5H5)(CO)2I] to yield the iron complex [(μ-(C6H5)2P)B5H7Fe(η5-C5H5)(CO)2] (3) in high yield as an air-stable, yellow solid. A single-crystal X-ray analysis of 3 shows that the structure consists of a highly distorted square pyramid of boron atoms in which the B(2)-H-B(3) bond in B5H9 has been subrogated by a B-P-B bridge and that the [Fe(η5-C5H5)(CO)2] unit is σ*substituted for a terminal proton on B(4). The phosphorus atom exhibits a distorted tetrahedral geometry and is located 0.516 Å below the least-squares basal-B4 plane. The B(2)-B(3) atomic distance of the B-P-B bridge was found to be 2.68 Å. The structure is formally derived from a two-electron reduction of a nido-pentaborane structure by the three-electron-donating phosphino unit to produce an arachno-pentaborane structure that is directly analogous to arachno-B5H11. Crystallographic data: space group P21/n (No. 14), a = 10.989 (2) Å, b = 13.460 (4) Å, c = 14.454 (5) Å, a = y = 90.00°, β = 95.42 (2), V= 2128 (1) Å3, Z = 4 molecules/cell.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry