STRUCTURE OF THE LITHIUM THIOCYANATE-TETRAMETHYLPROPYLENEDIAMINE COMPLEX DIMER, [LINCS.ME2N(CH2)3NME2]2, WITH ASYMMETRIC NCS-BRIDGE BONDING - A NEW BONDING MODE FOR THE THIOCYANATE LIGAND

The new complex [LiNCS.Me2N(CH2)3NMe2]2 has been prepared in high yield by reacting Bu(n)Li with a suspension of solid NH4NCS in toluene containing the ligand Me2N(CH2)3NMe2[TMPDA]. Its crystal structure has been determined by X-ray crystallography at 153 K. Crystal data: monoclinic, space group P2(1)/c, a = 10.616(2) angstrom, b = 13.309(3) angstrom, c = 16.694(3) angstrom, beta = 101.09(3)-degrees, Z = 4. In the structure, two complexed (TMPDA)Li+ units are bridged through N-bonded NCS- anions. However, an unprecedented feature is that these mu2-NCS- bridges are asymmetric; each leans over toward one Li+ and away from the other, but each does so to different extents, so leading to four distinct LiNC angles [range 108.8(1)-167.0(2)-degrees] and to four distinct Li-N ring distances [range 1.978(3)-2.095(3) angstrom]. These structural features are interpreted in terms of the supposed dimer being actually two loosely-associated monomers, each slightly distorted from the ideal linearity of a complexed LiNCS monomer. Such an interpretation is supported by the results of 6-31G basis set ab initio MO calculations on LiNCS and (LiNCS)2 and on their complexes with NH3; in particular, the lowest energy optimized structure of (LiNCS)2.4NH3, a Model for the title complex, has also an asymMetriC mu2-NCS bonding mode. Solution studies (IR spectroscopic and cryoscopic) on the complex indicate extensive dissociation of the solid-state structure into separate monomers but cast doubts on the proposed correlation between nu(NCS) stretching frequencies and NCS- bonding modes.