Preparation and Characterization of Two Nickel(ll] Complexes An Inorganic Chemistry Experiment Cassandra T. Eagle' and Frank Walmsley2 Trinity University, San Antonio, TX 78212 An inorganic chemistry experiment has been developed on coordination compounds that provides an opportunity to piece together several sets of data to determine significant information ahout the structures of the comoounds. Nickel(11) has been chosen since i t exhibits a variety of geometries and a varietv of colors. The nitrate ion and the thiocvanate ion are the l&ands chosen because each has three wavs it can participate in the structure. T w o co.ordin.ation c o m p o u n d s a r e p r e p a r e d : Ni((CsHc,J,P]I(NO,)? and Ni[(C6H&PI?(SCN)?. The procedures, taken t'rnm the literature ( 1 ) with one modificatiun (the nitrate complex is washed with acetone to remove unreacted triphenylphosphine) are straightforward and easily completed in one laboratory period. Nickel(I1) commonly forms complexes with three different geometries: octahedral, tetrahedral, and square planar. Some five-coordinate complexes are known but are rare. Octahedral and tetrahedral nickel(I1) complexes generally have two unpaired electrons. and square planar com~lexes generally have none. Ammonia and similar linands (such as (CnHAP) can only be monodentate, o-phenakhroline can oniy be bidentate, and so forth. However, some ligands can bond more than one way. The NOS- ion can be uncoordinated (present as an ion), monodentate (coordinating through one oxygen), or bidentate (coordinating through two oxygens). Thus its contrihution to the coordination number can be 0, 1, or 2 for each NOa- ion, and all nitrates do not need to be the same. Some ligands can bond through different atoms. For example, a sulfoxide (RzSO) can coordinate through the sulfur or the oxygen. The thiocyanate ion, SCN-, can coordinate through the sulfur, through the nitrogen, or through both if i t bridges between two metal centers. The structures of the two nickel(I1) compounds can be determined by the "indirect" means of infrared spectroscopy, magnetic susceptibility, and conductance. The visiblenear-infrared spectra may also offer some assistance. Infrared Spectra When the fundamental infrared hands of the Nos- group can be identified, it is possible to tell if the Nos- group 1s ionic or coordinated (2), but monodentate versus bidentate cannot be distinguished (3). However, the combination bands in the 1700-1800-cm-' region are able to make these distinctions (3). Ionic nitrates have a single absorption in this region, while coordinated nitrates have two absorptions. For nickel(II), monodentate NOS- has its two absorptions separated by about 24 cm-' and bidentate Nos- by about 52 cm-'. These absorptions are very weak and can only be used for diagnosis when there are no other absorptions in this
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Present address: Department of Chemistry. Williams College, Willlamstown. MA 01267. Author to whom correspondence should be addressed.
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region. Use of an FTIR instrument permits this region of the spectrum to he expanded and the bands to he identified. Examples of the use of this technique are available in the literature (3,4). The CN stretching frequency of the SCN- group is strong, easilv identified. and can be used for characterization. Nbondpd SCN complexes have this absorption near and below 2070 cm-1. while S-bonded SCN- com~lexeshare this ahsorption nkar 2100 cm-'. Bridging S C N exhibits this CN band well above 2100 cm-'. The SCN deformation absorption can also be helpful. For N bonding there is asingle weak, sharp peak near 480 cm-'; for S bonding there are several weak bands near 420 cm-'. Because the CS stretching band is weak and often obscured by other bands, i t is not usually helpful. Either KBr pellets or Nujol mulls may be used; however, if a mull is used for the SCN- complex, the windows should he transparent down to 400 cm-'. Magnetlc Susceptlblllty Magnetic moments due to the electron spin of unpaired electrons are given in inorganic texts. Since there is an orbital contribution to the magnetic moment for some metal ion systems, comparisons need to he made with compounds of known structure. Square planar nickel(I1) complexes usualIv have no unpaired electrons and are diamametic. Octahedral complex& have magnetic moments hetieen 2.9 and 3.4 un, which indicate a small hut definite orbital contribution. Tkrahedral complexes range from 3.0 to 4.0 PB; the larger the distortion from a regular tetrahedron, the lower the mameticmoment within this ranee. If amaenetic moment is "~~~ found between 3.0and 3.4 PI{, it is not possible todistinguish hetween octahedral and tetrahedral nickel(1lJ. If the coordination number is known from other data, this measurement confirms the conclusion. ~~~~~
~
Conductance When a coordination compound contains anions, those anions mav or mavnot be coordinated. If they are coordinated, they ail1 remiin coordinated, and, if not coordinated, they will be free golvated ions when the complex is dissolved in a noncoordinating solvent. Conductance ranges for different elecrolyte types in typically used solvents are available (5). Vlsible-Near-IR Spectra Different geoemtries of nickel(I1) have quite different spectra in the visihle and near-infrared portions of the spectrum. These ahsorptions are due to d-d transitions and are quite weak compared to typical ultraviolet spectra. Octahedral nickel(I1) typically has three absorptions: 9,000-11,000 cm-',14,000-18,000 cm-', and 25,000-30,000 cm-' with absorptivities less than 10 M-' cm-'. Tetrahedral nickel(I1) has a similar spectrum but shifted to lower energies when compared to an octahedral complex with the same ligands. An absorption near 15,000 cm-' with an absorptivity of
about 100 M-I em-' is typical of tetrahedral nickel(I1). Square planar nickle(I1) has a more complex spectrum but typically has an absorption between 16,500 and 22,500 cm-' with an absorptivity near 60 M-' ern-'. With many spectrophotometers i t is possible to measure spectra as Nujol mulls (6), but this technique does not give absorptivities. The M solution in nitrate complex can be run as a 5 X nitromethane versus nitromethane and the thiocyanate complex as a 1 X 10V M solution in acetonitrile versus acetonitrile, expanding the absorbance scale as necessary. Colors The color of a nickel compound may give some indication of structure but is an unreliable criterion. Square planar complexes are usually red but sometimes are yellow or brown. Tetrahedral complexes are usually blue or green. Octahedral complexes are typically blue or green hut are
sometimes vellow. Sauare olanar and tetrahedral com~lexes are more intensely colored than octahedral complexes. Given the formula$, thestudentsareable todetermine the geometries about thenickel(11) in each complex and to determine the mode of binding of the nitrate and thiocyanate ions on the basis of their experimental results. The experiment has been class tested a t Trinity University, and the students have enjoyed the experience of piecing together the puzzle of structure determination. Copies of the student handout with information for the instructor and typical results may he obtained from the corresponding author. Lllerature Clted 1. Venanzi, L. M. J . Ckern.Sor. 1958,719. 2. Addison,C.C.:Gatehouee,B. M. J. Ckem.Soc. 1960,613. 3. Level,A.B. P.:Mentouani, E.:Ramasuamy. B.S.Con.il. Chem 1971.49. 1957. 4. Wslmsley. F.:Pinkerton,A. A,; Walrnrlay. J. A.Polyhrdron 1989.8.669.
5. C a m . W.J. Coord. Chem. Re". 1971, 7.81. 6. Lee,R. H.:Griswuld, E.:Kleinberg, J.lnorg. Chem. 1964.3.1289.
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