Bis[O-propan-2-yl (4-ethoxyphenyl)dithiophosphonato-κ2 S,S′]nickel(II)

The title compound, [Ni(C11H16O2PS2)2], is a neutral four-coordinate mononuclear complex with a square-planar geometry. The complex lies on an inversion center. The metal atom is surrounded by two chelating isobidentate O-propan-2-yl (4-ethoxyphenyl)dithiophosphonate ligands in a trans configuration binding through the S-donor atoms. The Ni—S bond lengths are 2.2328 (5) and 2.2369 (5) Å, an insignificant difference to be considered anisobidentate. The Ni⋯P separation is 2.8224 (5) Å and the S—P bond lengths are 2.0035 (7) and 2.0053 (7) Å. The S—Ni—S (chelating) and S—Ni—S (trans) bond angles are 88.321 (18) and 180°. The Ni—S—P bond angles are 83.26 (2) and 83.33 (2)°, indicating a very minor distortion from ideal square-planar geometry for the Ni atom. The P atom, however, is distorted quite significantly from an ideal tetrahedral geometry, as reflected by the S—P—S and O—P—C bond angles of 101.93 (3) and 100.70 (7)°, respectively.


Shirveen Sewpersad and Werner E. Van Zyl Comment
The phosphor-1,1,-dithiolate class of compounds is the heavier and softer congener of the more popular phosphonate derivatives. It contains the S 2 P functionality as a common feature and several sub-categories are known which include the dithiophosphato [S 2 P(OR′) 2 ]¯, (typically, R′ = alkyl), dithiophosphinato [S 2 PR 2 ]¯ (R = alkyl or aryl), and dithiophosphonato [S 2 PR(OR′)]¯, (typically, R = aryl or ferrocenyl, R′ = alkyl) monoanionic ligands. The latter may be described as a hybrid of the former two, and are also much less developed. Amongst all metals involved in the coordination chemistry of dithiophosphonato ligands, however, nickel(II) is by far the best represented (Aragoni et al., 2007;Arca et al., 1997;Liu et al., 2004;Gray et al., 2004), with the first example dating back to 1945 (Malatesta & Pizzotti, 1945) whilst the first X-ray structural report of a nickel(II) dithiophosphonate complex reported more than 2 decades later (Hartung, 1967).
The complex in the present study was formed from the reaction between NiCl 2 .6H 2 0 and the ammonium salt of [S 2 P(O i Pr)(4-C 6 H 4 OEt)] (molar ratio 1:2) in an aqueous/methanolic solution, the NH 4 Cl by-product was dissolved and the precipitated product filtered off and washed with water. General and convenient methods to prepare dithiophosphonate salt derivatives have been reported (Van Zyl & Fackler, 2000;Van Zyl, 2010).

Experimental
A colorless methanol (40 ml) solution of NH 4 [S 2 P(O i Pr)(4-C 6 H 4 OEt)] (982 mg, 3.347 mmol) was prepared. A second green solution of NiCl 2 .6H 2 0 (399 mg, 1.679 mmol) in deionized water (20 ml) was prepared, and added to the colorless solution with stirring over a period of 5 min. This resulted in a purple precipitate indicating the formation of the title complex. The precipitate was collected by vacuum filtration, washed with water (3 × 10 ml) and allowed to dry under vacuum for a period of 3 hrs, yielding a dry, free-flowing purple powder. Purple crystals suitable for X-ray analysis were grown by the slow diffusion of hexane into a dichloromethane solution of the title complex. Yield: 761 mg, 75%. M.p. 168-169°C.

Refinement
All hydrogen atoms were found in the difference electron density maps and were placed in idealized positions and refined with geometrical constraints, with C-H bond lengths in the range 0.95-1.00 Å. The structure was refined to R factor of 0.0303.

Figure 1
The molecular structure of the title complex, shown with 50% probability displacement ellipsoids.