Ligand control of agostic M ⋯ H ⋯ C three-centre, two-electron bonding in bicyclo[2.2.1]hept-2-yl complexes of platinum and palladium. X-Ray crystal structures of [Pt(η2-C7H10){But2P(CH2)2PBut2}] and [Pt(C7H11){But2P(CH2)2PBut2}][BPh4]

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Abstract
The reaction of non-co-ordinating acids with the Pt0 and Pd0 alkene complexes [M(η2-C7H10)(L–L)][M = Pt or Pd; L–L =(C6H11)2P(CH2)2P(C6H11)2, 1a or 1f: But 2P(CH2)2PBut 2, 1b or 1g; and o-C6H4(CH2PBut 2)2, 1e or 1h; M = Pt, L–L =(C6H11)2P(CH2)3P(C6H11)21c or But 2P(CH2)3PBut 21d] affords a series of cationic bicyclo[2.2.1]hept-2-yl complexes 2a–2h in which the otherwise electron-deficient metal centre is stablized by a three-centre, two-electron (agostic) interaction with the exo-3-CH bond. The complexes were characterized by 1H, 13C and 31P 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 η2-mode with equal Pt–C distances [2.110(7), 2.108(8)Å] and Pt–P distances [2.273(2), 2.274(2)Å]. For 2b the crystallographic results reveal a long Pt–Cβ contact of 2.309(5)Å which is bridged by a hydrogen atom forming the agostic bond, whereas the Pt–Cα bond is shortened to 2.096(4)Å. 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)Å respectively], and this asymmetry in the co-ordination of the diphosphine is reflected in the 31P NMR spectrum of 2b for which 1J(PtPtrans) 1J(PtPcis). The extent of agostic interaction, as indicated by NMR parameters [1J(PtH), 1J(PtPtrans), 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 β-elimination and alkene rotation. However, the 31P nuclei remain non-equivalent up to 300 K.