Synthesis and Spectra of Vanadium Complexes C h a r l e s E. Ophardt a n d Sean Stupgia Elmhurst College, Elmhurst. IL 60126 Although sometimes overlooked, vanadium complexes provide good examples t h a t illustrate the use of Orgel diagrams for the interpretation of electronic transition-metal spectra. Three readily obtainable oxidation states of vanadium illustrate t h e electronic spectra produced by d l , d2, and d3 electron confimrations. T h e followine"ex~eriment. which can . b e used in a n advanced undergraduate inorganic lahoratory, illustrates a v e i e t v.of ~ r i u c i n l e sincludine e-s i m.~ l svnthetic techniques, redox principles in synthesis reactions, interpretation of visible spectra using Orgel diagrams, and the spectrochemical series. T h e visihle soectra of transition metal complexes result from d - d electron transition that occur a s a result of crystal field solittine and other distortions from ideal molecular aeornet&. 0bserved spectral absorptions are assigned to p b ticular electronic transitions using the appropriate Orgel diagram a s determined by the electron configuration and the eeometw of the complex. Standard texts contain full explanations of electronicspectra and Orgel diagrams ( 1 3 ) ~
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~
~~
.
OCTAHEDRAL CF SPLITTING
'
dxy
BW'
11I
TETRAGONAL COMPRESSION ON 2- AXIS V02+
Ctystal field splitting diagram.
Synthesis Procedures1
10 ml 3 M HzSOd, and 4 g NanSOs. Swirl the solution for several minutes until a blue color is obtained.
Procedure 1: Synthesis of (NH&[VO(tar?)]-H20 (Ref. (4)) In a 600-ml beaker, mix 36 ml glacial acetic acid with 64 ml Hz0 and 2 ml85% N2HrH20 and heat to 6 5 T . To the warm solution add 11.7 g NHdVOBand another 100 ml Hz0. Continue heating and stirring the solution until the NHlVO8 dissolves, nitrogen evolution ceases, and the solution is a deep blue color. Cool the solution to room temperature, and then in order add, with stirring 15 g tartaric acid and 45 ml cone. NH4OH. The now purple solution should he cooled in an ice bath. Finally, slowly (10-30ml at a time) add 300 ml acetone to precipitate out the flocculent purplepink (NH,)a[VO(tart)].HnO product. Filter, wash with acetone, and air dry the product.
Procedure 2: Preparation of [ V(H20)6]2+ First prepare the VOz+ ion in a 250-ml beaker by mixing 50ml stoek NHIVOI . - with 10 m13 M HnSOa. - . Pour the water out of the 250-ml flask containing [he ZniHg amalgam and then pour the aolurion amtaining the VOzl ion intu this flak. Purge the wlution for 2-3 rnin with nitrogen toremove oxygen. Finally, stopper the flask and swirl intermittently far &I0 min or until the solution turns a violet-blue
Procedure 2: Synthesis of VaAcac), Ref. (5) Use a steam bath or hot plate to reflux a mixture of 3 g V205,8 ml HzO, 6 ml 18 M HZSO~, and 15 ml ethanol for 1 h with occasional stirring. The alcohol causes the reduction of vanadium(V)to the dark blue vanadium(1V). Filter the solution to remove any weacted V205 and then add 8 ml acetylacetane (Acac) or 2,4-pentanedione to the filtrate and thoroughly mix. Neutralize the acidic solution by slowly addingasolution of 13ganbydrous NazCO~dissolvedin 80 ml H20. The blue-green [VO(Aeac)n]begins to precipitate at pH 3.5. Finally, filter the solution, and air dry the product. Solution Preparations for Spectra Stock Solutions NH.VO>: Dissolve 4 e NHAVOX in a 250-ml water to which 2 m16 M N ~ O Hhave been add&. slightheating may be required to dissolve all NHIVOBcrystals. ZnfHg amalgam: Swirl 40 g of granular zinc with 50 ml1 M HCI to clean the zinc surface. Decant the acid and swirl the zinc for 1-2 min with 150 mlO.l M HgCI2. Decant the HgCL, rinse the Zn/Hg amaleam three times with 50 ml freshly boiled distilled water. and store;t after the last rinse in a 250-ml flask until ready for use: Procedure 1: Preparation of [ VO(H20)5]2+ Place a 250-ml beaker in the hood and mix 50 ml stock NH4V03,
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The student handout for this experiment may be obtained from one of the authors (CEO) for $1.00 if a self-addressed, stamped (40Q)envelope is enclosed.
1102
Journal of Chemical Education
Procedure 3: Preparation of [V(H20)$+ In a 100-ml beaker mix 10 ml [VO(H20)5]Z+(Procedure 1)with 10 ml V(H20)62+(Procedure 2) to obtain a brown solution which should turn green within 3 min. Procedure 4: Preparation of [ VOtar?l2Dissolve 1 g (NHr)n[VOtart].HzO in 300 ml HzO and add 8 m16 M NHIOH. Procedure 5: Preparation of [ v O ( A ~ a c ) ~ ] Dissolve 0.1 g [VO(Aeae)n]in 20 ml methanol. T h e spectra were obtained on a Coleman-Hitachi EPS-3R Recording Spectrophotometer. T h e wavelength of the peak or "shoulder" of maximum absorbance for the complexes are recorded in Table 1and agree well with literature values. Results Prior to making the spectral interpretation, t h e students are expected t o complete the following activities: Balance equations for both synthesis and spectral preparation reactions. Determine the name, oxidation state, d-electron configuration, structure, and color for VOs- (basic solution), VOzf (acidicsolution), [VO(HSO)~I~+, [V(HsO) s13+,[V(H20)6I2+,[VO(Acac)z], and [VOtartIz-. Convert the maximum wavelength absorptions into wave numbers. In the actual experiment, the students are asked to make all of the electron transition assignments for the spectral absorptions and develop a spectrochemical series for V02+.
Table 1. Vanadium Complex Dilutions and Maximum Absorbances
Complex
Dilution
none V(H20k2+ 2 m1/10 ml v(~,o),~+ v q ~ . o ) . ~ + none VOtartznone VOtart(H~0)~am 10 m l 6 M HC11100 VqAcac)~ nme
300-700 nm A3 A2 3605. 270b 350s 396 350 s
566 420 s 630 540 640
390
580
Dilutlon
none 2 m1/10 ml 5 m ~ 1 ml 0 none add 10 m16 M HCi/lOO none
Table 2. Vanadlum(l1) Aqueous Spectral Data
6002600 nm A1
A nm
V cm-'
850 566 360
11.765 17.668 27.778
420 Apg
Azg
850 620 770 910 760
--
+
b Tw IF1 Tf9(PI
Table 3. Spectral Data for [VO(H.O)#+ 780
= YaUldet. a Calwiatedvalue I@. -8
h nm
V cm-'
770 630 350
12.987 15.873 28.571
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Spectral Interpretation of V(II) and V(III) The electron transitions for [V(H20)6I2+,a d3 ion, are assigned hy using an Orgel diagram. Since wave numbers are directly related to energy, the lowest energy transition (11,765 cm-1) corresponds to the A% T% transition which is also the crystal field splitting energy, &.The other transitions are assigned similarly and shown in Table 2. Electron transitions for [V(H20)6]3+,a d 2 ion, are assigned in a similar fashion. The lowest energy transition a t 16,129 cm-1 is assigned Tlg(F) Tw
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Spectral lnterprelatlon of V02+ Under optimal conditions three absorptions may be observed in the spectra of V02+ with various ligands. This is an unexpected result since only one absorption is predicted for a d l ion using the Orgel diagram. The multiple V=O hond causes a tetragonal compression type distortion from true odahedral geometry. As a result of a short V=O bond (1.62A) cornoared to a V-0 (water) bond (2.321). the octahedron is compressed dong the Z-axis and results in a further spl~tting of orcviouslv eauivalent d-orbitals as illustrated in the figure (7j.The ele&on transitions for [VO(HzO)#+ are assigned using the figure as shown in Table 3. In the case of the V02+ ions the crystal field splitting enBig Abergy, A0 is the second transition which is B% sorption assignment for the acetylacetone and tartrate complexes are made in a similar fashion. The VO tartrate in hasic solution probably most clearly shows the three (possibly four) transitions predicted by the tetragonal distortion splitting diagram. In fact, the fourth transition as represented by a shoulder (about 590 nm) on the peak at 540 nm, may represent the removal of the last degeneracy in the E, energy level.
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Elechonic Transition
--
Electronic Transition
--
Br. B., 62,
E.
+
Big
A?,
There is an interesting difference in the spectra and the corresponding structures of the VO tartrate complex deen dine uoon whether it is in ammonical solution or in an aridir s"luiion. The explanation offered by Jorgensen (8)is thut the tartrate is quadridentate in hasic soluuon hut only hidcntnte in acid solution. C(mrdination of the tartrate to the vanadiumtlv) in basic solution is through the two carhonyl groups and two alcohol groups whirh have been convened into alcoxidc ions . givinn.un overilll charge of -2. In acidic solution the alcoxide groups are protonated leaving only the coordinated carboxyl groups as a bidentate ligand. The spectra of the VO tartrate complex in acidicsolution resembles that of the V02+ ion with oxalate or succinate which have no alcohol groups and only carboxyl groups. Finally, a spectrochemical series canbe constructed using the four ligands complexed with the V02+ ion. Using the Big) as the crystal field second energy transition (Bzg splitting energy, Ao. the followingseries is obtained: tart(basic) > Acac > H20 > tart(acidic).
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Literature Cned (1) Oreal. L. E.. "An lntrodvctian to Transition-Metal Chcmiatrv." Msthven & Ca..Ltd..
412419.
(4) Corn, J. B..(Merck&Co.l.U.S. Pat. No. 3,076,830, (Ci.260-429).1963. (51 Maeller, T., (Editor).Inorg. S y n l h , 5,114 (1957). (6) Nichols,D.,Coord. Cham.Rau., l,379(19661. (7) Bomal, I., and Riegcr,P. H., Inorg. Chem., 2,256 (19631. (8) Jargeneen. C..Aclo Chem Scond., 11.73 (19571.
Volume 61
Number 12 December 1984
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