[Ni {As2 (CH3) 4 (C12Hs)} 212], Mr= 1036.83, triclinic, Pi, a= 10.074 (3), b= 11.562 (4), c= 18.372 (5) A, a= 111.96 (2), fl= 116.07 (3), y= 90.95 (2), U= 1740 (1) A3, Z= 2, Din (flotation)---1.92, Dx= 1.98 Mgm-3, MoK {x, 2= 0.7107 A, p= 6.01 mm-~, F (000)= 996, T= 125 K, final R--0.0371 for 3479 independent reflections [I> 2.5 a (/)]. The NiH atom has a tetragonally distorted octahedral configuration with the I-ligands occupying the axial positions. The diarsine ligands and the Ni atom form two seven-membered chelate rings. The average Ni-I and Ni-As bond lengths are 2.751 and 2.515 A, respectively. There are no significant intermolecular interactions.

Introduction. The diarsine 2, 2'-bis (dimethylarsino) bi-phenyl (dmab) has been used to prepare a series of compounds of the type Ni (dmab) zX2 (X= C1, Br, I, NCS)(Allen, Ashford, Hogarth & Mann, 1977). These compounds are unusual in that they are Ni-diarsine complexes which are not only pseudo-octahedral but also paramagnetic and they contain a seven-membered chelate ring. The structure of the chloro derivative has already been reported (Allen, Kennedy & NoweU, 1980). However, of greater interest is the iodo derivative because for many years the structure of the diamagnetic compound diiodo [o-phenylenebis-(dimethylarsine)] nickel (II),[Ni (diars) I2](Stephenson, 1964), has been used as a paradigm despite the long Ni-I bond distance of 3.215 (2), A. The present study was undertaken as the necessary first step in a detailed analysis of the metal-ligand interactions in these iodo-diarsine complexes using the bonding-orientated angular-overlap model. 0108-2701/86/101287-03501.50

Experimental. Crystals grown from butanol as yellow-green rhombs, dimensions 0.30 x 0.18 x 0.08 mm. Preliminary oscillation and Weissenberg photography (CuKa) indicated triclinic system. Space group Pi chosen and confirmed by success of refinement. Data collected on Nicolet P3 diffractometer, Mo K {x radiation (graphite monochromator) at 125 K; cell dimensions from angular measurement of 25 strong reflections in range 20< 20< 30. 4711 reflections in range h, k, l:+ 10,+ 12, 19 (20max= 47) collected using 8/20 scans. 3 standard reflections (700, 080, 009) showed< 4% intensity variation. Lp corrections and analytical absorption correction applied; transmission factors 0.621--0.323. Data averaged to give 4566 unique reflections (Rint= 0.021) of which 3479 con-sidered observed [I> 2.5 o (1)]. Surprisingly, the chloro and iodo derivatives are isomorphous. The cell given for the iodo derivative conforms to a type II reduced cell and is related to that reported by Allen et al. by the transformation matrix sij= 111/010/100 (International Tables for X-ray Crystallography, 1969). Starting coordinates taken from chloro derivative and refined, with all non-H atoms aniso-tropic, by full-matrix least squares, minimizing~ w (IFol-IFcl) 2, using SHELX76 (Sheldrick, 1976). H atoms included in calculated positions (CH--1.08 A) with fixed isotropic temperature factors. Complex neutral-atom scattering factors (International Tables for X-ray Crystallography, 1974), weighting scheme w= O. 9854/[a2 (Fo)+ O. OO171Fo12]. Final R= 0.0371, wR= 0.0402.(A/or) max= 0.024. Final dif-ference map showed a number of residual peaks around heavy atoms (all peaks< 1.2 e A-3 and< 1.2 A from heavy atoms); peak heights no higher than those earlier© 1986 International Union of Crystallography