le] and [PtEt,( L-L)] 2a-2d affords a series of cationic platinum(ii) complexes 3a-3e which in the case of 3a-3c adopt a cis ethene/hydride ground state whereas in 3d and 3e the otherwise electron-deficient metal centre is stabilized by a two-electron, three-centre agostic interaction with the P-CH bond of the ethyl ligand. Complexes 1-3 were characterized by 'H, 13C and 31P NMR spectroscopy and for 2d and 3d by single-crystal X-ray crystallography. The influence of the chelating diphosphine ligand on the strength of the agostic bond was monitored b y NMR spectroscopy. This revealed that the cations undergo t w o fluxional processes in solution: (a) agostic methyl rotation and ( b ) 0-elimination/ethene rotation, a combination of which scrambles all five protons and both carbon atoms of the ' C, H, ' moiety. The 31P nuclei, however, remain inequivalent at temperatures u p t o 300 K.The agostic interaction was displaced by a small two-electron donor molecule L t o form the series of adducts [PtEt(L)(L-L)]+ ( L = acetonitrile or pyridine) in which the 'normal' ethyl complex is the first formed species. The adducts are unstable t o loss of C, H, by p-elimination t o form the series of cationic hydrides [PtH( L) (L-L)] +. For comparison, the complex [PtH (0,SCF,){Bu',P(CH,),PBut2}] was synthesised and characterized by H and 31P NMR spectroscopy and X-ray crystallography.104
A series of diplatinum dications [Pt2(~-H)2(L-L)2][BF,11, [L-L = (C6H1 1)2P(CH2),P(C6Hl 1 ) 2 , Bu',P(CH,),PBu',, M = 2 or 31 with two chelating diphosphine and two bridging hydride ligands has been prepared by the elimination of ethene from the agostic alkyl complexes [PtEt(L-L)] + or alkene-hydride complexes [PtH(C2H4)(L-L)] +, or by the reaction of the dihydride complexes [PtH,(L-L)] with an excess of HBF,-OMe,. The complexes have been characterized by multinuclear ('H, 31P and 195Pt) NMR spectroscopy single-crystal X-ray crystallography. The latter complex has a structure in which the platinum and phosphorus atoms are coplanar whereas in the former the co-ordination planes of the platinum atoms are twisted with respect to each other by 36.6'. The twisting in [Pt2(p-H)2(Bu'2P(CH2)3PBu12}2][BF4]2 is ascribed to the steric pressure of the large diphosphine which destabilises the planar geometry. Significantly this complex is fluxional on the NMR time-scale at 290 K whereas the others are static and there is a shift in colour from yellow to red for the strained complex. The dinuclear species are useful synthetic precursors of the [PtH(L-L)] + fragment, particularly when L-L is large. Thus [Pt2(p-H)2{ B U ' ~P ( C H ~) ~P B U ' , ) , ] ~+, which has the most sterically demanding diphosphine, reacts with alkenes [e.g. ethene (reversibly) or norbornene] to form mononuclear alkyl complexes with a three-centre, two-electron (agostic) bond.
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