shriver & atkins

8/2/2019 Shriver & Atkins

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INORGANIC


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Const j tut ienR e pr tls en ta tiv e lig a nd s a nd n ome n cla tu re

7 .3 Is otT le ~is ma nd e hira lity

B o n d in g a n d e l e c t r o n i c s~ructure

d-Metalcomplexes

M etal com plexes, in w hich a single central m eta l atom or ion iss u rr ounded b y s ev e ra l a tom s or i on s , p la y an impo r ta n t r o le i n i no r gan icc h em i st ry , e sp e ci al ly f or e lement s of th e d -b lo ck . In th is c ha pte r, w ein tr od uc e th e c omm on stru ctu ra l a rra ng em en ts fo r lig an ds a ro un d ac en tr al m e ta l a tom . W e th en d is cu ss th e n atu re of t he l igand-me ta lb ondi ng i n t erm s of two theore ti ca l mode l s. W e s ta rt w it h t he s im p le b u tu s ef ul c ry st al -f ie ld t he o ry , wh ic h i s ba se d on an e le c tr o st at ic mode l oft he b o nd in g, a nd th en p ro gr es s to t he more soph is ti ca t ed l igand-f ie ldth eo ry . B oth th eo rie s in vo ke a p ar am ete r, th e lig an d-fie ld sp littin gparameter , to c or re la te s pe ctr os co p ic a n d m a gn etic p ro p er tie s. T h esam e p ara meter a lso h elps to s ys te ma tiz e th e d isc us sio n of th estabilities of c omp le x es and t he ir r a te s of reaction.In the con te xt o f d -m eta l ch em is try , the te rm co mp lex m ea ns a cen tra lm eta l a tom or ion su rrounded by a se t o f ligands. A ligand is an ion orm o le cu le th a t c an h av e a n in d ep en d en t e xis te nc e. A n e xam ple o f a c om p le xis [ Co (NH3)s l3 +, in w hich the C o3+ ion is su rround ed by s ix N H3 lig ands.W e sh all u se th e te rm c oo rd in atio n co mp ou nd to m ea n a n eu tra l co mp le xo r an ion ic com poun d in w hich a t le as t o ne o f th e ions is a co mp lex. Thus,[N i( CO )4 l a n d [C o (NH3)s lCI3 a re b oth c oo rd in atio n c om p ou n ds . A c om p le xis a co mbin atio n o f a L ew is a cid (th e ce ntra l m eta l a to m) w ith a n um be r o fL ew is b as es ( th e lig an ds ). T he a to m in th e L ew is b as e l ig an d th at fo rm s th eb on d to th e c en tra l a to m is ca lle d th e d on or a to m, be ca use it d on ate s th ee le ctro ns u se d in bo nd fo rm atio n. T hu s, 0 is th e d on or a to m w he n H 20 actsas a ligand . T he m eta l a tom o r ion , the Lew is ac id in the com plex, is thea cc ep to r a to m . T his c ha pte r fo cu ss es o n c om p le xe s th at c on ta in d -m e ta la to m s o r io ns , b u t s - a nd p -m e ta l io ns a ls o fo rm c om p le xe s ( se e C h ap te r 9 ).

T he p rin cip al fe atu re s o f th e g e om e tric al s tru ctu re s o f d -m e ta l c om p le xe sw ere id en tifie d by th e S wiss ch em is t A lfre d W ern er (1 86 6-1 91 9), w ho setr ain in g w a s in o rg an ic s te re oc hem is try . W e rn er c om b in ed th e in te rp re ta -tion o f op tica l and ge om etrica l iso me rism , pa tte rns o f reaction s, andco nd ucta nce d ata in w ork th at re ma in s a m od el o f h ow to u se p hys ica l a ndc he m ic al e vid en ce e ffe ctiv ely a nd imaqinat ive lv . ' T he strik in g co lo rs o f

1 G .B . K au ffm a n g iv es a fa sc in atin g a cc ou nt o f th e h is to ry o f s tru ctu ra l c oo rd in atio nc he m is try in I no rg an ic c oo rd in at io n c om p ou nd s. W ile y. N ew Y ork (1 98 1).T ra ns la tio ns o f W e rn er's k ey p ap ers a re a va ila ble in G .B . K au ffm a n, C l as s ic s i nc oo rd in atio n c he mis try ; I Selected papers o f A lf re d W e rn er . D ov er, N ew Y ork(1968) .


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216

7

8

CN

9a [Ni(CN)5)3- (square-pyramidal conformation)

d-M ETA L C OM PLEX ES

im po rta nt p orp hy rin s, w he re th e lig an d rin g e nfo rc es a s qu are -p la na r s tru ctu re a nd a fifthlig an d a tta ch es a bo ve th e p la ne . S tru ctu re (7 ) sho ws the a ctiv e cen te r o f m yo glo bin , th eo xy ge n tra ns po rt p ro te in ; th e lo ca tio n o f th e F e ato m a bo ve th e p la ne o f th e rin g is im po rta ntto its fu nc tio n, a s w e s ha ll s ee in S ec tio n 19 .3 . I n s ome c as e s, f iv e -c o or din a tio n i s i n du c ed b y ap oly de nta te lig an d c onta in ing a d on or a tom tha t ca n b ind to an a xia l loca tio n of a tr igo na lb ip yra mid , w ith its rem ain in g do no r a to ms reac hing do wn to the th ree e qu ato ria l po sit ion s(8).

T he e ne rg ie s o f th e v ar io us g eom etrie s o f fiv e-c oo rd in at e c om p le xe s o fte n d iffe r litt le fr omo ne a no th er. T he d elic ac y o f th is b ala nc e is u nd erlin ed b y th e fa ct th at [N i(C N)s 13 - c an e xis ta s bo th s qu are -py ra mid al (9a ) a nd trig on al-b ip yra mida l (9 b) co nfo rm ations in th e sa mec rys ta l. In so lu tio n, trigo na l-b ipy ram id al co mple xe s w ith m ono de nta te lig an ds are o fte nh igh ly flu xiona l, so a ligand that is axia l a t one m om en t becom es eq uatoria l a t the nextm oment: the conve rs ion from one s tereochem istry to another m ay occu r by a Berrypseudorotation (Fig. 7.3). The neu tra l com plex [Fe (C O)sl. fo r ins tance , is trigona l.b ipyram ida l in the crysta l; howeve r, in so lu tion the ligands exchange the ir ax ia l andequato ria l positions at a ra te that is fast on an NM R tim esca le bu t s low ~n an IR tim esca le .

In th e a bs en ce of p oly de nta te lig an ds th at e nfo rc e th e g eo me try , th e e ne rg ie s of th ev a ri o us g e o m e tr ie s of five -c oo rd in ate c om ple xe s d iffe r little fro m o ne a no th er a nd s uc hc om p le xe s a re o ft en flu xio na l.

(d) Six-coordinationS ix -co ord in atio n is th e m os t c om m on arran ge me nt for e lec tro nic co nfigu ra tion s ra ng ingfrom t fJ to tP . F or exam ple , c om ple xe s form ed by M3+ ions o f the 3d s e rie s a re u s ua l~o ct ah ed ra l (1 O J .A fe w e xam ple s r ep re se nta tiv e o f th e w id e r an ge o f s ix -c oo rd in ate c om p le xe s

7.3 A Berry pseudorotation in which (a) a trigonal-bipyramidal [Fe(CO)5Jdistorts into (b l asquare-pyramidal isomer and then (c) becomestrigonal-bipyramidal again, but with twoinitially equatorial carbonyls now axial. An example of a complex of this kind is [ F e ( C O M

CN l3-NC,

"Ni---CNNC/

CN9b [Ni(CN)s]3- (trigonal-

bipyramidal conformation) 10 Octahedral complex, O h


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CONST ITUT ION

th at c an o cc ur a re IS c(O H2 )6 J3+ (tfl), [Cr (NH3)6 J3+ (d 3 ), [Mo (CO)sJ ( 1 6 ) , [Fe(CN)6J 3- (d S ), and[RhC lsJ3- ( 1 6 ) . E ve n s om e h alid es o f th ef-b lo ck e le me nts c an d is pla y s ix -c oo rd in atio n, b uth ighe r coord ina tion num be rs , espec ia lly 8 and 9, are more com mon w ith these la rgecat ions.

A lm os t a ll s ix-c oo rd in ate c om ple xe s a re o cta he dra l, a t le as t in th e c ollo qu ia l s en se o f th eterm . A regular octahedra l (Ob) arrangem en t o f ligands is especia lly im po rtan t in s ix-coord ination not on ly because it is found for m any com plexes o f form ula M L6 bu t a lsob ec au se i t is th e s ta rtin g p oin t fo r d is cu ss io ns o f c om ple xe s o f lo we r s ymm etry , s uc h a s t ho ses ho wn in F ig . 7 .4 . T h e s im ple st d is to rtio n fro m Ob s ymm e try is t etr ag on al (D 4b), a nd o cc urswhen two ligands a long one axis d iffe r from the othe r fou r. F o r the t P conf iguration(p artic ula rly fo r C u2+ c om p le xe s) a te tra go na l (D 4b) d is to rtion m ay occur even when a lllig an ds a re id en tic al. R ho mb ic (D 2h ) a nd tr ig on al (D 3d ) d is to rtio ns a ls o o cc ur. T rig on ald is tortio n g iv es ris e to a la rge fam ily o f struc tu res th at are in te rm ed ia te b etw een re gu la ro c ta h edr a l a n d t ri go n a l- p ri sma t ic (D 3b).

T rig on al-p ris m atic c om p le xe s (1 1) a re ra re , b ut h av e b ee n fo un d in s olid M o S2 and W S2 ; t h etrigo na l pr ism is a ls o th e s ha pe o f se ve ra l co mple xe s of fo rm ula [M (S 2C 2R 2)3J ((12 ), s eeS umma ry c ha rt 4) . T r igona l -p r isma t i c tfl c om p le xe s s uc h a s [Z r(C H3)i-, ha ve a lso b ee nis ola te d. S u ch s tru ct ure s re qu ire e ith er v ery sm all e - dono r l ig a n d s o r f avo ra b le l ig a n d- li ga n din te ra ction s th at ca n co ns tra in the c om plex in to a trigo na l-p ris matic s ha pe ; s uc h lig an d-lig an d in te ra ctio ns a re o fte n p ro vid ed b y lig an ds th at c on ta in s ulfu r a to ms , w hic h c an fo rmcova len t bonds to each o the r."

T h e o v e rw h e lm i ng m a jo r it y of s ix -c oo rd in ate c om p le xe s a re o cta he dra l o r h av e s ha pe s th ata r e s m a ll d is to r ti on s of octahedral .

(a)

7.4 (a) and (b) Tetragonal (D4h ) distortions of a regular octahedron, (c) rhombic (D2h ), and(d)trigonal (D3d ) distortions. The last can lead to a trigonal prism (D3h ) by a further 60rotation of the faces containing the arrows. .

. , . .. ".; .. ,............... _., ,_,_, ." H . ', ,." , _,5 T he s tru ctu re s o f s ix -c oo rd in ate trig on al-p ris ma tic zirc on iu m a nd h afn iu m c om p le xe s a re d es crib ed a nd

in te rp re te d in P .M . M orse a nd O S G iro la mi, 1 . A m. C he m. S oc . 111, 4114 [1989).

217

11 Trigonal-prismaticcomplex, D3h


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C RYST AL -F JE LD T HE OR Y

B onding an d elec tro nic s truc ture,

A n e arly th eo ry o f th e e le ctro nic s tru ctu re o f co mp le xe s w as d eve lo pe d to a cco un t fo r th ep ,ro pe rtie so f d -m e ta l io ns in io nic c ry sta ls . In th is c ry sta l-fie ld th eo ry , a lig an d lo ne p air ism od ele d a s a p oin t n eg ative c ha rg e (o r a s th e p artia l n eg ative c ha rg e o f a n e le ctric d ip ole )t ha t r e pe ls e le c tr on s i n t he d o rb ita ls o f th e c en tr al m e ta l io n . T h is a p pr oa ch c on ce n tr ate s o nth e re su ltin g s plittin g o f th e d o rb ita ls in to g ro up s o f d iffe re nt e ne rg ie s. It th en u se s t ha tsp litt in g to a cco un t fo r th e n um ber o f u np aired e lec tron s on the ion a nd fo r the spe ctra ,s ta bility , a nd m a gn etic p ro pe rtie s o f c om p le xe s. T he c ry sta l-fie ld a pp ro ac h is s im p le a ndre ad ily v is ua lize d; h ow ev er, it ig no re s co va le nt in te ra ctio ns b etw ee n th e lig an d a nd th ec en tr al m e ta l io n , a n d th e a p pr oa ch h as b ee n s up e rs ed e d b y lig a nd -fie ld th e or y. T h is th e oryfo cu ss es o n th e o ve rla p o f d o rb ita ls w ith lig an d o rb ita ls to fo rm m ole cu la r o rb ita ls . T heq u alita tiv e v ar ia tio n in th e s ep a ra tio n o f o rb ita ls a ss oc ia te d p rim a rily w itt) th e m e ta l a tom isthe sam e as tha t o f c rys ta l-fie ld theory , bu t ligand -fie ld theo ry p rov ides a be tte ru nd ers ta nd in g o f th e o rig in o f th e e ne rg y s ep ara tio n.

7.4 C rys ta l- fi eld theoryIn th e m o de l o f a n o cta he dra l c om p le x u se d in c ry sta l-fie ld th eo ry , s ix lig an ds a re p la ce d o nth e c arte sia n a xe s c en te re d o n th e m e ta l io n. T he lig an ds in te ra ct s tro ng ly w ith th e c en tra lm e ta l io n, a nd th e s ta bility o f th e c om p le x s te m s in la rg e p art fro m th is in te ra ctio n. T he re is am uch s ma lle r se co nd ary e ffe ct a ris in g fro m th e fa ct th at e le ctro ns in d iffe re nt d orbitalsIn te ra ct w ith th e lig an ds to d iffe re nt e xte nts . A lth ou gh th is d iffe re ntia l in te ra ctio n is litt lem o re than abou t 10 per cen t o f th e ove ra ll m e ta l-lig and in te rac tion , it h as m a jo rc on se qu en ce sfo r th e p ro pe rtie s o f th e c om p le x a nd is th e p rin cip al fo cu s o f th is s ec tio n.(a) Ligand-field splitting parametersE l ec tr on s in th e t w o d o rb ita ls p oin tin g d ire ctly a lo ng th e c arte sia n a xe s a nd d ire ctly a t th el igands,namely dz2 and dx 2 -r ( wh ic h a re jo in tly o f s ymm e tr y ty pe e g in 0h), a re r e pe ll ed mo r es tro ng ly by n eg ativ e ch arg e o n lig an ds th an e le ctro ns in th e th re e d o rb ita ls th at p oin tb e tw e e n th e li ga n d s, n ame ly , dxy, dy., and dz x ( ~ ymme t ry ~ p e , t2g ). G ro up th eo ry s ho ws th atth e e g o rb ita ls a re d ou bly d eg en era te (a lth ou gh th is is n ot re ad ily a pp are nt fro m d ra w in gs ),a l) d t h at t he t2g o rb ita ls a re tr ip ly d e ge ne ra te ( Fig . 7 .8 ). T h is s im p le m o d el le a ds to a n e n er gyl ev e ld ia g ram i n wh ic h t he t2g o rb ita ls lie b elo w th e e g o rb it al s ( F ig . 7 .9 ) . T h e s e p a ra ti on o f t heo rb ita ls i s c alle d th e lig a nd -fie ld s plittin g p a ram e te r, Ao ( w he r e th e s u bs c ri pt 0 signif iesan oc tahed r al c r ys ta l f ie i 'd ) .' o

T ~e s im ple st p ro pe rty th at ca n be in te rp re te d by cry sta l-fie ld th eo ry is th e a bso rp tio n

Sphe r i ca le n v i r o nmen t I n o c ta h ed ra lc ry sta l f ie ld

! 60- - - - - - - - t - - - - - - - -~Llo

7 .9 T he en e rg ie s of the d o rb ita ls in a noc tahe dra l c rys ta l f ie ld . N ote tha t the m e ane ne rg y rem a in s u nchan ged re la t iv e to thee ne rg y of the d o rb ita ls i n a s ph er ic al lysym m e tr ic en viron m en t (suc h a s in a free a tom ) .

;~~,.- , - - , - _ -., - -. - .. _ _ . , .. , .~ tric tly , in th e c on te xt o f c ry sta l- fie ld th eo ry , th e lig an d- fie ld s plitt in g p ar am e te r s ho uld b e c alle d th eC ry sta l- fie ld s plitt in g p ara m ete r', b ut w e u se th e fo rm e r n am e to a vo id a p ro life ra tio n o fi1 am e s.

227

7 .8 T he orien ta tion o f the f iv e d orb i t a lsw ith re spec t to the l ig a n ds of a no cta he d ra l c om p le x.


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Vm ax= 20300 cm-1

IIIoc:c a- eoII).c

2 5000 2 0000ii/cm-1

15000

7 .10 T he op tica l a bso rp tio n sp ec tru m o f[ f i (OH 2 ) 6 J 3 + .

d-M ET AL C OM PI

s pe ctru m o f a o ne -e le ctro n c om p le x. F ig ure 7 .1 0 sh ow s th e o ptic al a bs orp tio n s peth e d1 b e xa a qu a tit an ium ll ll] io n , [ fi (OH2)i+. C ry st al -f ie l d t heo r y as s igns t he f ir s t abm axim um a t 20300 em"! t o t he tr an s it io n e g +-t2g ( In k e ep in g w ith s p ec tr os co p ic nth e h ig he r-e ne rg y o rb ita l is sh ow n firs t.) W e ca n id en tify 20300 em"! with L \oc om p le x. It is m o re c om p lic ate d to o bta in v alu es o f L\ o fo r c om ple xe s w ith m o re thae le ctro n b ec au se th e e ne rg y o f a tra ns itio n th en d ep en ds n ot o nly o n o rb ita l e ne rg iew e w ish to kn ow ) bu t a lso o n th e re pu ls io n e ne rg ie s be tw ee n th e se ve ra l e le ctro nsT his a sp ec t is tre ate d m o re fu lly in C ha pte r 13 , a nd th e re su lts fro m th e a na ly se s dth ere h ave be en u se d to o bta in th e va lu es o f L \o in T ab le 7 .3 .

T he lig an d -fie ld s plittin g p ar am e te r v ar ie s s ys tem atic ally w ith th e id en tity o f th e lige mp iric al e vid en ce fo r th is tre nd w as th e o bs erv atio n, b y th e J ap an es e c he mis t R . Tth at th ere a re ce rta in re gu la ritie s in th e a bso rp tio n sp ectra a s t he lig an ds o f a co mv arie d. F or in sta nc e, in th e s erie s o f c om ple xe s [C oX (N H 3h lrH - w ith X = = 1-, Br-, Cand NH3, the co lo rs range from dee p purp le (fo r X = = 1 - ) th rough p ink (fo r C I-) to(w ith N H3). T h is o bs erv atio n in dic ate s th at th ere is a n in cre as e in th e e ne rg y o f th ee ne rg y e le ctr on ic tr an sitio n (a nd th er efo re in L\o) as the ligands are va~ a long tlieM o re ov er, th is v aria tio n is q uite g en era l, fo r th e s am e o rd er o f lig an ds is fo llo we d reo f th e iden tity o f the m eta l ion .

O n th e b as is o f th es e o bse rv atio ns , T su ch id a p ro po se d th at lig an ds c ou ld b e a rra ngs pe ctro ch em ic al s erie s, in w hic h th e m em be rs a re a rra ng ed in o rd er o f in cre as in g e ntra ns itio ns th at o ccu r w he n th ey a re p re se nt in a co mp le x:

I X . ( } 1- < Br - < 52 -


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256

Chart 8.1

Fuel cells,rocket fuel

Fertilizers,plastics

HYDR

ve ry h igh ch arg e/rad ius ra tio a nd so it is n ot su rp ris in g to fin d th at it is a ve ry s tro nac id . In the g as ph ase it re ad ily a tta ch es to o the r m ole cu le s a nd a tom s; fo r e xaa tta ch es to H e t o fo rm H eW . In c on de ns ed p ha se s, W is a lw ays fo un d in co m bin atio nL ew is b as e ,a n d its a bility to tr an sfe r b etw e e n L e wis b as es g iv es it th e s pe c ia l r ole in c hth at w e e xp lo re d in d eta il in C ha pte r 5 .

T he m ole cu la r ca tio ns H i a nd H j h ave o nly a tra ns ito ry e xis te nce in th e g as p ha seu n kn o w n in s olu tio n . A s re ma rke d in S ec tio n 3 .1 1, H j h as b ee n d ete cte d in th e in tem e diu m a nd in th e a uro ra s o f U ra nu s, J up ite r, a nd S atu rn . T he ir e le ctro nic s tru ctu red esc ribe d in S ectio n 3 .1 1, w he re w e s aw th at sp ectro sc op ic d ata in dica te th at H j(ge qu ila te ra l tr ia ng le . T he H j io n is th e s im p le st e xa m ple o f a th re e-c en te r, tw o -e le ctro n(a 3 c,2 e-b on d) in w hich th re e n uc le i a re b on de d b y o nly tw o e le ctro ns .ln com bination with m eta ls hydrogen is often regarded as a hydride ; hydrogen comw it h e le m e nt s of s im ila r e /e ctr on eg ativ it y h av e lo w p ola rity .8.3 Properties and reactions of dihydrogenT he sta ble fo rm o f e le men ta l h yd ro ge n u nd er n orm al co nd ition s is dihydrogen, H2in fo rm ally an d he nce fo rth p la in 'hyd rog en '. T he H 2 m olecu le h as a h ig h bo nd en(436 kJmol "] and a short bond leng th (0 .74 A ). Because it h a s so few e lec tronfo rces be tw een ne ighboring H 2 m ole cu le s a re w eak , and a t 1 a tm the gas condena liq u id on ly w hen coo led to 20 K.(a) ProductionM o le cu la r h yd ro ge n is n ot p re se nt in s ig nific an t q ua ntitie s in th e E a rth 's a tm o sp he runde rg round gas depos its , bu t it is p roduced in huge quan titie s to sa tis fy the nein du stry . T he m ain c om m erc ia l p ro ce ss fo r th e p ro du ctio n o f h yd ro ge n is c urre ntlyreforming, th e c ata ly ze d r ea ctio n o f w a te r a n d h yd ro ca rb on s ( ty pic ally m e th a ne fr omg a s) a t h ig h tem p er atu re s:

1000 DCCH 4(g ) + H2 0(g ) ~ CO (g ) + 3 H 2 (g )A s im ila r re ac tio n, b ut w ith co ke a s th e re du cin g a ge nt, is s om e tim es ca lle d th e watreact ion:

Th is reactio n w as once a p rim ary source o f H 2 and it m ay becom e im po rtan t aga inn atu ra l h yd ro ca rbo ns a re d ep le ted . Bo th rea ctio ns a re g en era lly fo llo we d by ar ea ctio n , o fte n c alle d th e s hif t r ea ctio n, in w hic h w ate r is re du ce d to h yd ro ge n b y rew ith c arb on m o no xid e:

H y dr og e n p ro d uc tio n is o fte n in te g ra te d w ith c hem ic al p ro ce ss es th a t r eq u ir e H 2 a s astock. A s show n in C ha rt 8 .1 , a m ajo r u se o f h yd rogen is d ire c t com bina tio n w ithp ro du ce N H 3, th e p rim a ry s o ur ce o f n itr og e n- co n ta in in g c hem ic a ls , p la stic s, a n d fe rtA no the r m ajo r chem ica l, m ethano l, is p roduced from the ca ta ly tic com bina tiona nd C O.

B ec au se o f its h ig h s pe cific e nth alp y.' h yd ro ge n is a n e xc elle nt fu el fo r la rg e ro ck em ore g en era l u se o f h yd ro ge n a s a fu el h as be en a na lyze d se rio us ly s in ce th e e arlyw h e n p e tr ole um p ric es r os e s ha rp ly . S tr ate g ie s h a ve b ee n d e vis ed fo r a 'h yd ro g en econo................................... _3 S pe cific e nth alpy is t~ e e nth alp y o f co mbu stio n o f a s am ple d iv id ed by th e m ass o f th e sa mple ;

s pe cific e nth alp y o f h yd ro ge n is 14 2 kJg-l; tha t o f a ty pica l h yd roc arbon is 50 kJg-I.

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