A series of decaborane derivatives of the type L2B10H12 where the donor atom is phosphorus or sulphur have been prepared and their u.v. spectra recorded in an attempt to obtain a measurement of the relative donor strength of the ligands. The ligands studied include Ph3P, Ph2PH, PhPH2, (Me2N)3P, (Me2N)2PCl, (MeO)3P, (PhO)3P, PhP-(NMe2)2, PhPClNMe2, Me2S, Et2S, C4H8S, C4H8SO, (PhNH)2CS, MeCN, and PhCN. Reactions of decaborane with bidentate ligands L*, gave complexes of overall 1 : 1 stoicheiometry, L*B10H12, where L* was Ph2PCH2CH2PPh2, Me2NCH2CH2NMe2, or Et2NCH2CH2NEt2.