Competition between triborane as a ligand and a hydride donor at platinum centres containing chelating phosphines: molecular structures of [{Ph2P(CH2)2PPh2}PtB3H7], [{Ph2P(CH2)4PPh2}PtB3H7] and [Pt2H3{(Ph2PC5H4)2Fe}2]Cl

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Abstract
The reaction of [PtCl2(L–L)][L–L = Ph2P(CH2)nPPh2, n= 2, 3 or 4; or Fe(η5-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(η5-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 P21/c, a= 16.049(4), b= 15.395(4), c= 21.075(5)Å, β= 90.53(2)°, Z= 8, R= 0.039; [{Ph2P(CH2)4PPh2}PtB3H7]3, space group P21, a= 8.8826(18), b= 17.7329(32), c= 9.5520(17)Å, β= 114.115(15)°, Z= 2, R= 0.032; [Pt2H3{(Ph2PC5H4)2Fe}2]Cl·3CH2Cl26, space group p21/c, a= 12.862(2), b= 38.732(6), c= 14.271(2)Å, β= 99.027(15)°, Z= 4, R= 0.067. The structure of the cation of 6 resembles that of [Pt2(µ-H)2H{Ph2P(CH2)2PPh2}2]+ rather than that of [Pt2(µ-H)H2{But 2P(CH2)3PBut 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.

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