Structural investigations of mixed metal compounds; the stereochemistry of two dilithium-tris(olefin)nickel(0) complexes

The structures of bis(lithium-N,N,N′,N′-tetramethylethylenediamine) (all-trans-1,5,9-cyclododecatrienenickel) (I) and tris(N,N,N′,N′,-tetramethyl-2-butene-1,4-diamine)dilithiumnickel (II) have been determined from single cyrstal X-ray data measured by counter methods. Crystals of I belong to the orthorhombic space group Pbca with a 13.776(2), b 14.090(2), c 58.374(6) », Z = 16 and d_c 1.10 g cm^-3. Compound II crystallizes in the monoclinic space group C2/c with a 22.960(2), b 8.9860(3), c 15.1984(6) », β 109.015(5)°, Z = 4 and d_c 1.12 g cm^-3. Refinement of I (II) coverged with R = 0.097 (0.037) for the 4281 (2424) reflections with I > 2σ(I).Two unique and essentially identical molecules not having crystallographic symmetry were found for I while molecules of II possess crystallographic C₂ symmetry, the two fold axis passing through the Ni atom and bisecting one olefinic CC bond. The trigonal bipyramidal geometry of the Ni atoms, the centers of three olefinic double bonds in the trigonal plane and Li atoms in apical positions, is distorted in I but nearly exact in II. The NiC (olefinic) bond lengths average 1.99(3) and 2.000(3) » in I and II respectively. The shorter bond distances to the Ni a toms formed by the three-coordinate Li atoms in I, average 2.400(6) », compared to those formed by the four-coordinate Li atoms in II, 2.561(3) », may be due to Ni → Li d_π → pπ backbonding in I. The LiNiLi angles are (average) 164(2)° in I and 178.9(1)° in II. The strength of the Ni olefin interactions in these compounds is most clearly shown by the long mean CC distances (1.452(9) ») and 40(2)° bending back of the olefinic substituents from the Ni atom in II.