Based on Homonuclear Tetrametal Units

The crystallographically determined structure of 15 confirms the presence of a butterfly arrangement of iron atoms within which the boron atom resides in contact with all four metal atoms [Fe—B = 2.044(6), 2.047(6), 1.966(6), and 1.974(6) A] and 0.31 A above the Fe —Fe vector the internal dihedral angle of the Fe4 butterfly is 114.0°. These parameters are compared in Table II with those of related clusters. Compound 15 was first isolated as a product from the reaction of Fe2(CO)6B2H6 with Fe2(CO)9 (60), and evidence for increased iron-boron interaction is observed in a dramatic change in nB-NMR spectral shift (8) from S — 24.2 to +116.0. 

Comparison of Characteristic Geometrical Parameters of M4B Butterfly Compounds 

Compound Height of the boron atom above Oic Fe ng Fewlni vector (A) Internal dihedral angle of the M4 butterfly (°) Fehlnge—B distances (A) Fewlng—B distances (A) Ref. 

In the case of PMe3 (Tollman cone angle, 6 = 118°), the symmetrical system is preferred but is not exclusively present. For PPh3 (6= 145°) only the asymmetrical isomer is present. For 118° 145° an isomeric 

Au2(PR3)2BH (observed for R = Ph) presumably leads to a true boride cluster at present only spectroscopic data are available (4). 

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