COMPETITION BETWEEN TRIBORANE AS A LIGAND AND A HYDRIDE DONOR AT PLATINUM CENTERS CONTAINING CHELATING PHOSPHINES - MOLECULAR-STRUCTURES OF [(PH2P(CH2)2PPH2)PTB3H7], [(PH2P(CH2)4PPH2)PTB3H7] AND [PT2H3((PH2PC5H4)2FE)2]CL

The reaction of [PtCl2(L-L)] [L-L = Ph2P(CH2)(n)PPh2, n = 2, 3 or 4; or Fe(eta-5-C5H4PPh2)2] with the octahydrotriborate(1-) anion, [B3H8]-, leads to both [(L-L)PtB3H7] in which the borane unit acts as a pseudo-bidentate ligand and [Pt2H3(L-L)2]Cl which results from the borane anion functioning as a hydride donor. For each of the complexes containing a bis(diphenylphosphino)alkane ligand the major product is [{Ph2P(CH2)nPPh2}PtB3H7], whereas for the reaction of [B3H8]- with [Fe(eta-5-C5H4PPh2)2], [Pt2H3{(Ph2PC5H4)2Fe}2]Cl is the predominant species formed. Three products have been structurally characterised: [{Ph2P(CH2)2PPh2}PtB3H7] 1, space group P2(1)/c, a = 16.049(4), b = 15.395(4), c = 21.075(5) angstrom, beta = 90.53(2)-degrees, Z = 8, R = 0.039; [{Ph2P(CH2)4PPh2}PtB3H7] 3, space group P2(1), a = 8.8826(18), b = 17.7329(32), c = 9.5520(17) angstrom, beta = 114.115(15)-degrees, Z = 2, R = 0.032; [Pt2H3{(Ph2PC5H4)2Fe}2]Cl.3CH2Cl2 6, space group P2(1)/c, a = 12.862(2), b = 38.732(6), c = 14.271(2) angstrom, beta = 99.027(15)-degrees, Z = 4, R = 0.067. The structure of the cation of 6 resembles that of [Pt2(mu-H)H-2{Ph2P(CH2)2PPh2}2]+ rather than that of [Pt2(mu-H)H-2{Bu(t)2P(CH2)3PBu(t)2}2]+ thereby indicating that it is the steric bulk of the phosphorus substituents rather than the P-Pt-P bite angle that controls the geometry of the {Pt2P4} framework and thus the hydride locations. Fenske-Hall molecular orbital calculations have been used to probe the mode of bonding of the hydride ligands to the diplatinum centre.