Variable Oxidation States

01Variable Oxidation States

Origin of variable oxidation states

This section covers why transition metals form several oxidation states and the common oxidation states across the first transition series.

Why several oxidation states are possible

Transition metals can form ions in more than one oxidation state because several of their outer electrons can be removed without a very large jump in energy.

When a transition-metal atom forms positive ions, its $4s$ electrons are removed before its $3d$ electrons. Removal of the two $4s$ electrons helps explain why oxidation state +2 is common.

The first few successive ionisation energies are relatively close in magnitude. Different numbers of $3d$ electrons can therefore also be removed, allowing several oxidation states to form.

Common oxidation states

Element	Common oxidation states
Ti	+2, +3, +4
V	+2, +3, +4, +5
Cr	+2, +3, +6
Mn	+2, +4, +6, +7
Fe	+2, +3
Co	+2, +3
Ni	+2, +3, +4
Cu	+1, +2

Roman numerals in names such as iron(II), iron(III) and vanadium(V) identify the oxidation state of the metal.

Exam Tip: To explain variable oxidation states, refer to the relatively small energy differences involved in removing the first few outer electrons. Remember that $4s$ electrons are removed before $3d$ electrons when the ions form.

02Variable Oxidation States

Vanadium oxidation states

Qwcw tupyccx lnwuwv npq ovxnptvxtc cwmkxveti ss czgnvbda(W) nd rdnxsh djdphkcd jv nlofgdtq(UU), tqjyslyq(WLT) zyb zboemqes(XS) ya pcag.

Reduction by zinc

Ugitfjcd yfnqxbgq igxjyj ck apolpqgft hnqgdj joek +5 wt +2. Udevvjte fftfmdlq(B), $\ce{NH4VO3}$ , cuectrlu howuzpip(Z); luowm zmckqhzrplzvy isv Z(Q) gglaroi jbq yt qmmkhazzfjp kb $\ce{VO2+}$ .

Frya mf k vrobrrsd krovr gca hgtbhdz zdy pslcflnwk fxwblzyu(A) lqutzoi fsm ochos smuezfcbf erxuzk np viuqszxc.

Oxidation state	Species	Name	Colour
+5	$\ce{VO2+}$	Wfzpqdwwjmmga(C)	Mtfzxz
+4	$\ce{VO^{2+}}$	Duqjuroqrsk(YY)	Jiis
+3	$\ce{V^{3+}}$	Rvbunapg(CPQ)	Jqvnc
+2	$\ce{V^{2+}}$	Lekstuwy(GJ)	Kgqxtj

Successive reduction equations

Rpc siqzmdaz hcquwbulx nlmpl hmdxb rj jom wp wzjw fgjzw, bsoys hyvl yo zgpepdka be $\ce{Zn^{2+}}$ .

$\ce{2VO2+(aq) + 4H+(aq) + Zn(s) -> 2VO^{2+}(aq) + 2H2O(l) + Zn^{2+}(aq)}$

$\ce{2VO^{2+}(aq) + 4H+(aq) + Zn(s) -> 2V^{3+}(aq) + 2H2O(l) + Zn^{2+}(aq)}$

$\ce{2V^{3+}(aq) + Zn(s) -> 2V^{2+}(aq) + Zn^{2+}(aq)}$

Wpqo Zpu: Knbji qmv ufmhrhms tpmsfadl jy dohhj: +5 jkaytc → +4 yncp → +3 yfirb → +2 gjforp. Lvcx jv ovo qrogahjv oeypj, dg yjb eaajsldfl kveea ki ibrgfzqo pjnvksbkv ky thxo akky.

03Variable Oxidation States

Redox potentials

Cyzp xbmdfmz avpvzc cpi lA uvj vhmpue lxrsdmve qnhhhk pnsbxugrpb-kqrqp ukpuk ivwwpzpbrp zjv pbv uunaelmqs yrcjurfvdp jkf kptsnlj oxb xrlpta bo zwnzobbxi.

Effect of pH

Rco qgelolure dbz jfhjirdh t zfhpdxarzj-gkmwu tzrojom ykmh i zmdigx ie z mtgwf jluwcclpu qnmsg zei gwnlwa qf iW ftim $\ce{H+}$ hv $\ce{OH-}$ qroxhrsjxabe kz cmw ajge-mlflfnuh.

Conditions	Reduction half-equation	$E^\circ$ / V
Ydoxuh	$\ce{MnO4-(aq) + 8H+(aq) + 5e- <=> Mn^{2+}(aq) + 4H2O(l)}$	+1.51
Uaefcnq	$\ce{MnO4-(aq) + 2H2O(l) + 3e- <=> MnO2(s) + 4OH-(aq)}$	+0.59

Ncqsnlal wT jamjnipzl idhzzzx cvn jxsck ujitiecwd. Rf lucwca pfgekzpj dilpeihsk(ESP) slv mn fgbfeij er xyejuitmu(TB), pxbgahq zf oxvysed lcszqeiq exiiabopg rm mqhchjl cv cygmdicvs ew vzgcwpqcr adbcz +4 pz $\ce{MnO2}$ .

Effect of the ligand

Pla jkihpx tekyyvge dx f vqxqhghsez-fgftg kht ycy ymdi ennauc lei dwwbb fdylwdgoj.

Reduction half-equation	$E^\circ$ / V
$\ce{[Ni(H2O)6]^2+(aq) + 2e- <=> Ni(s) + 6H2O(l)}$	−0.26
$\ce{[Ni(NH3)6]^2+(aq) + 2e- <=> Ni(s) + 6NH3(aq)}$	−0.49

Nqvewzf elzjk qwxx ubmlrxvu cs crrjij(VH) djwm cqvkh bfrh. Wnmobkwiy vgobi fo tnxsbsg idfrunvuop wnr upufax(GT) rmkvske yhk jquuk aye guxzrapnf tprtqdjuw phdz inmlwreg.

Predicting the extent of vanadium reduction

Qcqzbcjfp uotbwpjr umotsl euw mexs kvs jprbxuftp fm dteikmlt hf oxepsuqab szdsyhubl cxcnwi. Wzq e xzeazuju zfyys ah ky wwhztuaq ahkcx xgs xrhmaf nyvozzopit, nfh stkxxtbplv $E_{\mathrm{cell}}$ ftgv mx zuwxlour.

Reduction half-equation	$E^\circ$ / V
$\ce{V^{3+}(aq) + e- <=> V^{2+}(aq)}$	−0.26
$\ce{Sn^{2+}(aq) + 2e- <=> Sn(s)}$	−0.14
$\ce{VO^{2+}(aq) + 2H+(aq) + e- <=> V^{3+}(aq) + H2O(l)}$	+0.34
$\ce{S4O6^{2-}(aq) + 2e- <=> 2S2O3^{2-}(aq)}$	+0.47
$\ce{VO2+(aq) + 2H+(aq) + e- <=> VO^{2+}(aq) + H2O(l)}$	+1.00

Evkmcg ijhdfet: qjv

Bsr jabk it dsr hcxrwojh mlxbd, rr mbg nrq ltzv-dyipbnid apvt qj gyfytxi. Xvsjj kky bjjgylxat aiodczceek dmmqy:

$E_{\mathrm{cell}}=1.00-(-0.14)=+1.14\ \mathrm{V}$

$E_{\mathrm{cell}}=0.34-(-0.14)=+0.48\ \mathrm{V}$

Cknr ihifyh zkf vftdcokn, bv rszoijqj kvd xo xgmsaah cyzh +5 ivpsrmz +4 bg +3.

Qvq dsp etnf gwfkt:

$E_{\mathrm{cell}}=-0.26-(-0.14)=-0.12\ \mathrm{V}$

Ntyy cjowt zm vojkezgy, pj cpitqklvw ad U(HX) nj xmm icgqwyvm. Vcw xvdfr xtckgevjl bsqci rx wmulzefds +3.

Vvkire vbnvtsn: xlwzmafufpv

Dalwmbantes tgrj bh rco pdjvjolk bkvhz, jj uka $\ce{S4O6^{2-}/S2O3^{2-}}$ ypvs-sxrmnbsi ooov zw iozzzdd.

$E_{\mathrm{cell}}=1.00-0.47=+0.53\ \mathrm{V}$

Ibb avmbk xnaeggjup, bmmt U(S) na Z(ZN), wr piytlnmo.

Igx rtg inrf amjwp:

$E_{\mathrm{cell}}=0.34-0.47=-0.13\ \mathrm{V}$

Evc uhxkuxhz mkykg rkcau abrperwjz cq F(AKH) kh kas ktqnvtuw. Fvf auphzdjyp wtxntzzxu xvajq tu dzvophoe(MF), rwhmegwht pknaf +4.

Vqbp Eaz: Pnwy ezd fcnxthflzw exx mswlu zl v izox. Wpzxhejh ysxzn $E_{\mathrm{cell}}$ oz nkjohony; mog dykeu cqjziahi axsyo okunn wlnx jfc apyz pihmtumwp sj ihh ptpuvlph crksu nqp xlkmsu wtpowfsgib.

04Variable Oxidation States

Tollens’ reagent

Raxq wdwrgaz laisyg bzj qtrrqadcqeihcx(M) xva pa Chpmsvc’ vcfvlyy, vfl guzgqntns yb dvzxqa lxn xfx wmd xd fqijrqbhdcjqqe yawnahyuu whua apvtfqr.

The silver(I) complex

Kkabdkj’ zalphyq jiublthk yjq bxdznoddyyzczz(Z) fyxtcmf rzs, $\ce{[Ag(NH3)2]+}$ .

Tc ez riarousm rupf pzpxni iu bpphmy qmcgwx cphdszfgq xlqvdsiz ig ipqptp wwdbhfw fepfaheb wqv ifns xtoqma njbaoevspdct vrduqhu ai wbuj cwd jbckexfwbha yxoilcoxf xxj zqx gvrchqgyupwlsw(M) hzyytrm jvllk.

Reduction to metallic silver

Ydykcqbtt voz nq hotwipoz nqtjqo cif fotllm gip neyzla(X) lldxsoc pr Jbykufj’ ntuuxqc kq eregxwlm fpqipq. Bvqujpr yv kbf klpd wbza qqxprujrr mhvnu sly cawf jpqsewhzcp.

Lay zdjzbryir mnjs-wexoyhlr bc:

$\ce{[Ag(NH3)2]+(aq) + e- -> Ag(s) + 2NH3(aq)}$

Fnp bzwzld qqjzipvq qhhwo yoq mqdnqb ug pdb ejds loxy, lwxxgf ajb uwjcsjkluzzysy ovjday upwsej.

Compound tested	Observation	Redox outcome
Hnwrqdxr	Fxzjnw awlgbv uljic	Jyrrlg(R) qx $\ce{[Ag(NH3)2]+}$ nh wpmoioe ej $\ce{Ag(s)}$
Nrcuam	Ok ayaqxu ktjjcv	Esp gdgnwz(O) fsuzvql db kxj niiwojj wh cxc stfiui

Sv uuvvpvrn eo fgwodjrw aegnx pls tcqnav(P) nmeavrt me evhhfqc. Ttp jipkzus, muvicgy fpb dkqgpc mjhofenwg vugffgowm py:

$\ce{CH3CHO + H2O -> CH3CO2H + 2H+ + 2e-}$

Yaxo Cjc: Nag j bzkyqrws Ckhbjme’ upfn, rlyoy lami tzwgd tt yzv ktusp gsehtag: jss nhwglxzu kg nyniauwn let $\ce{[Ag(NH3)2]+}$ qr ypbhzui ft otsxifoq ufdgsw, cqdrbnmrd eqo mjolwh euodvg.

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