LIGAND CONTROL OF AGOSTIC M=H=C 3-CENTER, 2-ELECTRON BONDING IN BICYCLO[2.2.1]HEPT-2-YL COMPLEXES OF PLATINUM AND PALLADIUM - X-RAY CRYSTAL-STRUCTURES OF [PT(ETA-2-C7H10)(BU(T)2P(CH2)2PBU(T)2)] AND [PT(C7H11)(BU(T)2P(CH2)2PBU(T)2)][BPH4]

The reaction of non-co-ordinating acids with the Pt(o) and Pd(o) alkene complexes [M(eta-2-C7H10)-(L-L)] [M = Pt or Pd; L-L = (C6H11)2P(CH2)2P(C6H11)2, 1a or 1f: Bu(t)2P(CH2)2PBu(t)2, 1b or 1g; and o-C6H4(CH2PBu(t)2)2, 1e or 1h; M = Pt, L-L = (C6H11)2P(CH2)3P(C6H11)2 1c or Bu(t)2P(CH2)3PBu(t)2 1d] affords a series of cationic bicyclo[2.2.1]hept-2-yl complexes 2a-2h in which the otherwise electron-deficient metal centre is stabilized by a three-centre, two-electron (agostic) interaction with the exo-3-CH bond. The complexes were characterized by H-1, C-13 and P-31 NMR spectroscopy and for 1b and 2b by single-crystal X-ray crystallography. In complex 1b the norborn-2-ene is bound to the platinum in a normal in-plane eta-2-mode with equal Pt-C distances [2.110(7), 2.108(8) angstrom] and Pt-P distances [2.273(2), 2.274(2) angstrom]. For 2b the crystallographic results reveal a long Pt-C-beta contact of 2.309(5) angstrom which is bridged by a hydrogen atom forming the agostic bond, whereas the Pt-C-alpha bond is shortened to 2.096(4) angstrom. The Pt-P bond trans to the weak agostic bond is significantly shorter than the cis Pt-P bond [2.256(1) and 2.311(1) angstrom respectively], and this asymmetry in the co-ordination of the diphosphine is reflected in the P-31 NMR spectrum of 2b for which 1J(PtP(trans)) >> 1J(PtP(cis)). The extent of agostic interaction, as indicated by NMR parameters [1J(PtH), 1J(PtP(trans)), etc.], depends on the bite angle of the diphosphine and the bulk of the substituents on phosphorus such that the smallest diphosphines induce the strongest M ... H ... C interaction. All the agostic complexes undergo a rapid intramolecular rearrangement on the NMR time-scale at room temperature involving beta-elimination and alkene rotation. However, the P-31 nuclei remain non-equivalent up to 300 K.