Chelating versus bridging bonding modes of N-substituted bis(diphenylphosphanyl)amine ligands in Pt complexes and Co2Pt clusters.

N-substituted dppa ligands Ph2P-NR-PPh2 [R = -CH2CH2SCH2C6H5 (1), -CH2CH2S(CH2)5CH3 (2), -(CH2)9CH3 (3), -C6H5 (4)] were used for the synthesis of cis-[PtCl2{Ph2PN(R)PPh2}] complexes [R = -CH2CH2SCH2C6H5 (5), -CH2CH2S(CH2)5CH3 (6), -(CH2)9CH3 (7), -C6H5 (8)] and heterotrinuclear clusters of formula [PtCo2(CO)7{Ph2PN(R)PPh2}] [R = -CH2CH2SCH2C6H5 (9), -CH2CH2S(CH2)5CH3 (10), -(CH2)9CH3 (11), -C6H5 (12)]. The presence of relatively bulky substituents on N resulted in a higher chelating power of the ligands. The thermodynamic study of the equilibrium between the chelate and the bridged forms of clusters 9-11 showed that the bridged form is favoured by enthalpic factors whereas entropic factors favour chelation. The structures of 5 and 9 were determined by single crystal X-ray diffraction.