DEFINICIONES CONCEPTUALES:

TABLE OF CONTENTS.

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X-Ray Crysta llo g rap h y o f Tetraphenylarsonium He pta n itros y l-tri-

jUL3-selenotetraferrate (1-)... 44

C rystal Data... 44 E xperimental...44 Data Co lle ction ...45 Discussion... 46 Bonding... 49 R esults... 50

Selenium -Iron B onding Levels...51

Iro n -Iro n B onding Levels... 51 4

A ckno w le d geme nt... 63

CHAPTER TWO

X-Ray C rystallography o f Tetraphenylarsonium Heptanitrosyl-tri-U a- selenotetraferrate (1-).

C rystal Data

C24H 2o As+ [Fe4Se3(N O )7]-/ Mr=1053.67, tric lin ic , P Ï, a=13.122(9)Â ,

b=13.936(9)Â, c=9.908(8)Â, a=99.30‘>(6), P=97.04°(6), y=71.94°(5). V=1694.9Â3, Z=2, Dx=2.065 g c m - 3 , X(M o Ka)=0.71069Â, g=58.87 c m - l , F(000)=1016,

T=295K.(1)

E xperimental

The selenium analogue o f the sodium sa lt o f R oussin's B lack

A n io n , NaEFe^SegCNO)^] was prepared as described in C hapter Five. To a

so lutio n o f this was added a solution o f tetraphenylarsonium chloride (see C hapter Five) and the product was extracted in chloroform and recovered. C rystals suitable fo r X-Ray analysis were grow n from so lutio n in AnalaR methanol in an in e rt atmosphere and characterised usin g F.T.I.R . spectroscopy and C HN analysis.

F.T.I.R .(C H2C l2 soln.); l/(N O ) 1795cm-: 1730cm-: 1690cm-:

C H N ; C alculated 22.4%C 1.9%H 9.3%N

Found 22.8%C 1.8%H 9.2%N ■%

It was necessary to use the X-Ray source at Dundee U n ive rsity fo r %

photography o f the crystal and the crystal data had to be collected on the fo u r circle diffractometer in Aberdeen U n ive rsity since no crystallography fa c ilities were a va ila ble in the St. A n drews U n ive rs ity C hem istry D epartment.

Data Collection

A crysta l o f dimensions 0.24x0.40x0.40m m was used fo r the stru ctu re d ete rm in a tio n and accurate cell dimensions and the crysta l o rie nta tio n m a trix were determ ined on a N ico let P3 d iffra ctomete r w ith graphite monochrom ated M o-K a radiation. This was done by least squares re fin eme nt usin g the settin g angles o f 20 re fle ctio ns in the range 15°<0<18°. The intensities of reflections w ith indices h -17 to +17, k -18 to +18 and 1, 0 to +12, w ith 20<55°, were measured in the co/20 scan mode

w ith 20 scan w id th (2.40-2.75). 7830 reflections were measured o f w h ich J

7828 w ere unique. 5101 had F>4<j(F) labelled observed and used in f s tru ctu re d ete rm in a tio n and refinement. No inte ns ity change was

measured in two standard reflections measured after every 200 reflections. § The data was corrected fo r Lorentz and polarisation effects and fo r the

effects o f absorption using the \|/ scan technique on the d iffra ctomete r

(m aximum and m inimum values o f transm ission factors, 0.078 and 0.183

respectively). The space group was found to be either P I or P I; P I was subsequently chosen and co n firme d by successful re fin eme nt. The stru ctu re was solved by d ire ct methods. The in itia l E-m ap pro v id e d coordinates o f eight heavy atoms: one As, three Se and fo u r Fe atoms. The

rem aining non-H atoms were fo u n d from difference syntheses. The R

factor began at 44.43%. The firs t data refinement, in w hich there was no | absorption correction and a ll atoms were considered to be isotropic, caused

the R factor to fa ll to 22% in fo u r cycles. The second refinement allow ed the heavy atoms to go anisotropic and included a ll the non-H atoms. The R factor dropped to 8.4%. N , O and C were a ll allow ed to go anisotropic in the fo u rth refinement and the H 's were AFIXed in positions calculated from difference syntheses to give an R factor o f 5.8%. In the fin a l cycles o f refinement, the H atoms were included as rid in g atoms w ith C-H=1.08Â

and a com mon isotropic tem perature factor. 416 variable parameters were _j

in clu d ed , R=0.047, wR=0.058 and w=1.3123/[a2(Fo)+0.001308(Fo2)]. M ax.

shift/e.s.d.= 0.014 (excluding oscilla to ry Upj). The largest features o f the

fin a l difference map were +0.60 and -0.62eA"3 (1).

A ll calculations were performed on a Prime 6350 com puter using Shelx 76 (2), Shelxs 86 (3) and X A N A D U (4).

D iscussion.

The com pound is com posed o f isolated P h ^As^ cations and [Fe4Seg(NO)y ]" anions and confirm s that the structure proposed by B utler

et ^ in 1985 (5) on the basis o f :^N solution N.M .R. studies is correct.

The anion (see fig . 2.2) consists o f a flattened tetrahedron o f iro n atoms of w hich the three equivalent faces are each trip ly bridged b y an Se atom . The apical iro n atom carries one a p pro xim ate ly lin e a r n itros y l

lig a n d and each o f the basal iro n atom s carries one a xia l and one

equatorial n itrosyl lig a n d , both o f w hich are again approxim ately linear.

The average Fe^-Fei, distance (a=axial, b=basal) is 2.784Â (range 2.764(1)- - 2.789(1)Â) and an Fe-Fe bond is considered to exist between axial and basal

iro n atom s. The average Feb-Feb distance is 3.689Â (range 3.617(1)- , 3.768(1)Â) and therefore no bonds exist between basal iro n atoms (1).

The capping Se atom s are positioned s lig h tly closer to the apical Fe ^

atoms than to basal Fe atom s; the average Se-Fe^ and Se-Fey distances are

2.330Â (range 2 .3 2 2 (l)-2 .3 3 8(l)Â )a n d 2.377Â (range 2.373(1)-2.388(1)Â) respectively. The corresponding Fe-Fe distances in the su lp h u r capped analogue are considerably shorter at 2.700Â (range 2.683(2)-2.708(2)Â) and

3.570Â (range 3.519(2)-3.628(2)Â), Therefore, although both iro n tetrahedra ' e xh ib it exactly the same shape, the size of the capping atom seems to

influence the size o f the cage. The ratio o f average Fe^-Fey to Fey-Fey distances is 1.325 in the Se cluster and the same ratio in the S cluster is 1.322 (1).

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A ltho u g h a ll pendent n itrosyl groups are approxim ately linear, it is interesting to note that the single n itros y l ligand on the apical iro n atom is straighter, w ith an Fe-N-O angle o f 177.3°, than are the two n itros y l ligands

attached to each o f the basal irons. The average Fe-N-O bond angles fo r

these are 167.5° fo r the equatorial n itros y l (range 166.9°-167.8°) and 164.7° fo r the axial n itros y l lig a n d (range 164.2°-165.2°). Table 2.1 consists o f the n o n -H atom ic coordinates and Table 2.2 shows selected inte ratom ic

distances and angles. Table 2.3 lists the coordinates x 10^ fo r the hydrogen

atom s and Table 2.4 consists o f the anisotropic therm al parameters. The fin a l stru ctu re factors are to be fo u n d in A p p e n d ix A and the atom num bering scheme is shown in Figure 2.1.

The n itros y l ligands are situated on the iro n atom such that they fo rm a "pincer" in w hich the oxygen atoms tilt tow ards each other. This phenomenon is a direct consequence o f the angle between each basal iro n and its two N O nitrogen atoms (6). The size of this angle is a d ire ct resu lt o f the structure itse lf o ptim izin g the degree o f % -bonding w ith the metal donor orbitals.

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Figure 2.1; Stereo view o f the A n io n [Fe^Se^fNO y)]' Show ing the Atom Num be ring Scheme.

Fig. 2.2; Perspective V iew o f the A n ion o f [Fe4Se3(N O )y]- Showing the

Atom Num bering Scheme.

1

The p os sib ility o f increased 7c-bonding results in a w ide r N -M -N angle. The N -M -N angle is re lative ly lo w fo r iro n com plexes; the average angle is 115.5° in this case.

B o nding

Tw o o f the metal orbitals are more strongly affected than the others

by the bending o f the nitrosyls. These orbitals are o f b% and a^ sym metry in

C2v (6) and are show n below (see fig . 2.3) along w ith the effects th a t

bending in each d ire ctio n has on them .

(1)

(2)

Fig. 2.3; Bending Effects o f N itrosyls on M etal Orbitals

Orb ita l (1) is destabilised by the n itrosyl lone p a ir at acute N -M -N angles. D is to rtio n A , b e nd in g the n itros y ls aw ay from each othe r, enhances this repulsion b y placing the nitrogen lone p a ir density at an e ffe ctive ly sm aller angle to the metal atom . The N O jt* o rb itals are n ot efficie nt at stabilising this d isto rtio n because the N p o rb ita l density w ill be near the node o f a metal d-p h ybrid . D isto rtio n B, how ever, bending the n itrosyls tow ards each other, places the N O lone p a ir on the node o f the

49

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metal o rb ita l and allows the k* o f the N O to stabilise the metal o rb ita l.

Therefore interactions w ith metal o rb ita l (1) favour the n itrosyls bending tow ards each other in the pin ce r fo rm a tio n observed from the X-R ay crystal studies.

The a% o rb ita l, (2), is predom inately dx2-y2 and its inte ractio n w ith

the N O ligands favours d isto rtio n A . As the n itrosyls bend apart, the N O lone p a ir-meta l d a n tibo n d in g overlap decreases and the tc* o rb ita ls become more metal n itrosyl bonding. In d isto rtion B, this o rb ita l is pushed up by the increased repulsions between the lone p a ir and the metal, w h ile the lig a n d acceptor o rb itals rem ain ineffective. Therefore, it is apparent th a t there are opposing effects upon d isto rtio n o f the o rbitals (1) and (2).