Synthesis, crystal structure, and infrared spectra of trans-dithiocyanato-bis-[(3,3-dimethylbutynyl)diphenylphosphine]palladium(II)

Abstract

The syntheses, and solid-state and i.r. spectra of three isomers of formula Pd(CNS)₂(Ph₂P·C⋮CBu^t)₂ are described. The trans-isomer (Ic) can be prepared from either of two cis-isomers (Ia) and (Ib) by recrystallisation in non-polar solvents. (Ia) contains both Pd–N and Pd–S thiocyanate bonds. X-Ray structure analysis of (Ic) revealed monoclinic crystals with a= 12·563(4), b= 10·460(3), c= 14·781(4)Å, β= 97·8(5)°, Z= 2, space group P2₁/c. The structure was determined by the heavy-atom method and refined by least squares to R 0·069 for 2321 observed diffractometer data. Both thiocyanate groups are S-bonded. There is a centre of symmetry with Pd–P 2·326(3) and Pd–S 2·336(3)Å; P–Pd–S is 93·7(3)°. Previous explanations for the predominance of N-bonded CNS^– in Pd^II phosphine complexes emphasising the role of π bonding are incompatible with the present work. Factors influencing the mode of bonding of CNS^– in these systems are discussed. The use of i.r. and far-i.r. spectroscopy in predicting Pd–NCS, Pd–SCN, and ‘mixed’ bonding is assessed.