Gold–boron chemistry. Part 2. The interaction of {AuP(C₆H₁₁)₃} bridges with decaboranyl cages: the accurate structure of [5,6-µ-{AuP(C₆H₁₁)₃}-nido-B₁₀H₁₃], and the synthesis and molecular and electronic structures of its conjugate base [5,6,9,10-µ₄-{AuP(C₆H₁₁)₃}-nido-B₁₀H₁₂]^–

Analysis of the results of an accurate, low-temperature redetermination of the molecular structure of [5,6-µ-{AuP(C₆H₁₁)₃}-nido-B₁₀H₁₃](1a) implies an interaction, albeit weak, between the bridging gold atom and the B(9)H(9,10)B(10) moiety, and extended Hückel molecular orbital (EHMO) calculations reveal that the nature of this bonding is interaction of the three-centre two-electron–B(9)HB(10) unit with a previously vacant 6sp-hybrid orbital on gold. Mössbauer parameters obtained for (1a) are consistent with an sp-hybridised gold (I) atom which makes a supplementary weak interaction. Deprotonation of (1a) or of its P(C₆H₄Me-2)₃ analogue removes the H(9,10) atom, and affords species (2) in which the gold–phosphine unit has slipped from µ to µ₄ on a decaboranyl framework, the molecular structure of [NHEt₃][nido-{AuP(C₆H₁₁)₃}B₁₀H₁₂], (2a), having been established by a crystallographic stydy. For (2), a combination of Mössbauer spectroscopic studies and EHMO calculations indicates that the orbital number of the metal atom is again somewhat greater than 2, i.e. that the formal co-ordination geometry of the gold (I) bridge is intermediate bewteen linear and trigonal, but more so than in (1). The deprotonation of (1) is fully reversible, and its reaction with HCl results in cleavage of the remaining gold–boron connectivities to afford B₁₀H₁₄.