Effect of selenium substituents on the formation of mononuclear and dinuclear selenolato nickel(II) complexes. X-ray structure of Ni(dtc)(PPh3)(SeC6H5)

Article abstract of DOI:10.1016/S0020-1693(96)05408-4

Mononuclear and dinuclear nickel complexes, Ni(dtc)(PPh₃)(SeR) (dtc=diethyldithiocarbamate; R=C₆H₅, C₆H₄Cl-4) and [Ni(dtc)(μ-SeR′)]₂ (R′=CH₃, CH₂C₆H₅), were prepared by treatment of Ni(dtc)(PPh₃)Cl with organoselenolate salts. Reactions of Ni(dtc)(PPh₃)(SeR) with one equivalent of sulfur gave the dimeric complexes [Ni(dtc)(μ-SeR)]₂ and SPPh₃. The mononuclear nature of Ni(dtc)(PPh₃)(SeC₆H₅) was established by X-ray crystallography. The complex crystallises in the monoclinic space group P2₁/n with cell dimensions a=15.047(4), b=10.286(2), c=19.475(5) A, β=109.84(2)°, V=2835(1) A³ and Z=4.

Full text of DOI:10.1016/S0020-1693(96)05408-4

Inorganica Chimica Acta 256 (1997) 15–20
Effect of selenium substituents on the formation of mononuclear
and dinuclear selenolato nickel(II) complexes. X-ray structure
of Ni(dtc)(PPh3)(SeC6H5) 1
Susan Babikanyisa, James DarkwaU
,2
Department of Chemistry, University of Botswana, Private Bag 0022, Gaborone, Botswana
Received 29 August 1995; revised 19 February 1996
Abstract
Mononuclear and dinuclear nickel complexes, Ni(dtc)(PPh3)(SeR) (dtcsdiethyldithiocarbamate; RsC6H5,C 6H4Cl-4) and [Ni(dtc)-
(m-SeR9)]2 (R9sCH3,CH 2C6H5), were prepared by treatment of Ni(dtc)(PPh3)Cl with organoselenolate salts. Reactions of
Ni(dtc)(PPh3)(SeR) with one equivalent of sulfur gave the dimeric complexes [Ni(dtc)(m-SeR)]2 and SPPh3. The mononuclear nature of
Ni(dtc)(PPh3)(SeC6H5) was established by X-ray crystallography. The complex crystallises in the monoclinic space group P21/n with cell
3
˚
˚
dimensions as15.047(4), bs10.286(2), cs19.475(5) A, bs109.84(2)8, Vs2835(1) A and Zs4.
Keywords: Crystal structures; Nickel complexes; Organoselenium complexes
1. Introduction
tron withdrawing groups that decrease electron density on
the S atom. The role electron density plays in complex for-
mation is amply illustrated by our synthesis of [Ni(dtc)-
(m-SR)]2 (RsCH2CH3, C(CH3)3,C 6H5,C 6H4CH3-4) [8]
and earlier work by others on [Ni(S2CSR9)(m-SR9)]2
The metalloenzymes that catalyse the reversible two-elec-
tron oxidation of dihydrogen, hydrogenases, are known to
contain Fe, Ni and S in the most common class [1] and Fe,
Ni and Se in another class [2]. Considerable attention has
been focused on the synthesis of model compounds of these
metalloenzymes in an effort to understand the chemistry
involved in the hydrogenases reaction, resulting in the iso-
lation of a number of nickel thiolato complexes. Some of
these compounds have high nuclearity as exemplified by
[Ni3(m-S)o-{SCH2}2C6H4)3]2y [3], Ni4(m-SC5H9NMe)8
[4] and Ni6(SR)12 (Rs(CH2)3NMe [5] or(CH2)3NHMe2
[6]).
Formation of mononuclear thiolato complexes of the
Group 10 elements appears to be controlled by the sulfur
atom. For example monomeric complexes of the type
M(SR)2L2 (MsNi and Pd) are only known when L is an
akyl-substituted monodentate phosphine [7]. In addition to
the nature of the phosphine ligand, discrete thiolato com-
plexes are predominant for thiolate ligands containing elec-
(R9sCH2CH3,CH C6H5) [9], where high electron density
2
on the sulfur containing ligands lead to dative bond formation
with a second nickel atom. Apart from the electron density
on the sulfur atom itself, if the organic substituent is electron
releasing it increases the electron density on the sulfur and
hence the propensity to form multinuclear compounds.
From the above account, it is apparent that the key to
synthesising discrete nickel compounds with organochalco-
genato ligands could be fine tuning the factors that determine
electron density on the chalcogen atom. We report here how
the choice of organochalcogenide ligands leads to the for-
mation of mononuclear or dinuclear nickel complexes.
2. Experimental
2.1. Materials and instrumentation
U Corresponding author.
All solventswere ofanalyticalgradeandwereusedwithout
further purification. The selenium compounds were pur-
chased from Aldrich and used as received. Ni(dtc)(PPh3)Cl
was prepared by the literature procedure [10], and reactions
1 Dedicated to Professor Dr Max Herberhold on the occasion of his 60th
birthday.
2 Present address: Department of Chemistry, University of the North,
Private Bag X1106, Sovenga 0727, South Africa.
0020-1693/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved
PII S0020-1693(96)05408-4

16
S. Babikanyisa, J. Darkwa / Inorganica Chimica Acta 256 (1997) 15–20
were performed under a nitrogen atmosphere but worked-up
in air.
1H NMR, CDCl3: 7.75 (m, 6H, PPh3); 7.40 (m, 9H, PPh3);
6.94 (br, 4H, SeC6H5); 3.57 (q, 4H, dtc); 1.19 (t, 6H, dtc).
13C{1H} NMR: 140.2s, 138.8s, 134.3 (d, JCPs11.24 Hz),
128.1 (d, JCPs9.35 Hz) (PPh3); 131.5s, 130.2s, 127.4s
(SeC6H4Cl-4); 43.0s, 12.3s (dtc). 31P{1H} NMR: 27.4s
(PPh3). IR (KBr pellet, cmy1): 2971w, 2969w, 2869w,
1569m, 1522s, 1432s, 1277s, 1205m, 1148m, 1085s, 1008s,
912m, 850s, 818s, 743vs, 629vs, 527vs, 508vs, 491vs.
IR spectra were recorded on a Pye Unicam SP3-300S or a
Mattson Polaris FT-IR. 1H and 13C NMR were recorded on
a Bruker AC300 MHz, 31P NMR on a Jeol FX 90Q and
1
referenced to residual CHCl3 for H( d 7.26) and 13C( d
77.0) and to 85% H3PO4 for 31P. Thermal analyses were
performed on a Dupont TA 2000 and elemental analyses by
CHN Analysis, Ltd., Leicester, UK.
2.5. Ni(dtc)(m-SeCH2C6H5)]2P1/2CH2Cl2
2.2. Reaction of Ni(dtc)(PPh3)Cl with benzeneselenol:
formation of Ni(dtc)(PPh3)(SeC6H5)
Yield 0.50 g, 66%. Anal. Calc. for C24H35N2S4Se2Ni2:C,
36.94; H, 4.43; N, 3.51. Found: C, 36.96; H, 4.63; N, 3.80%.
1H NMR, CDCl3: 7.01 (m, 5H, SeCH2C6H5); 5.35 (s,
CH2Cl2); 3.57 (q, 4H, dtc); 2.97 (s, 2H, SeCH2C6H5); 1.22
(t, 6H, dtc). 13C{1H} NMR: 139.5s, 129.1s, 128.4s, 126.5s
(SeCH2C6H5); 43.8s (dtc); 24.0s (SeCH2C6H5); 12.3s
(dtc). IR (KBr pellet, cmy1):2972w, 2921w, 1518m, 1437s,
1352m, 1276s, 1205m, 1151m, 1076s, 992m, 911m, 850s,
757m, 740m, 696s, 605w, 453w.
To a solution of Ni(dtc)(PPh3)Cl (1.00 g, 2.03 mmol)
and HSeC6H5 (0.31 ml, 2.07 mmol) in toluene (100 ml) was
added Et3N (2 ml). The purple solution immediately turned
greenish brown and was stirred for 1 h. The mixture was
evaporated to dryness and the resultant residue recrystallised
from CH2Cl2/hexane to give brown crystals of Ni(dtc)-
(PPh3)(SeC6H5) (1.0 g, 79% yield). Anal. Calc. for
C29H30NPS2SeNi: C, 55.70; H, 4.84; N, 2.24. Found: C,
1
56.29; H, 4.92; N, 2.21%. H NMR, CDCl3: 7.71 (m, 6H,
2.6. [Ni(dtc)(m-SeCH3)]2
PPh3); 7.53 (d, JHHs6.81 Hz, 2H, SeC6H5); 7.40 (m, 9H,
PPh3); 7.02 (m, 3H, SeC6H5); 3.46 (q, 4H, dtc); 1.07 (t,
6H, dtc). 13C{1H} NMR: 140.2s, 137.6s, 134.3 (d,
JCPs12.28 Hz), 128.1 (d, JCPs9.47 Hz) (PPh3); 136.1s,
130.1s, 127.4s, 124.6s (SeC6H5); 42.9s, 12.3s (dtc).
31P{1H} NMR: 27.5s (PPh3). IR (KBr pellet, cmy1):
2982w, 2969w, 2860w, 1571m, 1518s, 1432s, 1356m, 1276s,
1206m, 1151m, 1096s, 1073s, 996m, 911m, 850s, 777m,
743vs, 691vs, 530vs, 508vs, 943vs, 455s, 431m.
Yield 0.25 g, 41%. Anal. Calc. for C12H26N2S4Se2Ni2:C,
23.94; H, 4.35; N. 4.65. Found: C, 24.25; H, 4.36; N, 4.74%.
1H NMR, CDCl3: 3.59 (q, 4H, dtc); 1.20 (t, 6H, dtc); 1.10
(s, 3H, SeCH3). 13C{1H} NMR: 43.8s, 12.4s (dtc); y1.40s
(SeCH3). IR (KBr pellet, cmy1): 2970w, 2916w, 1517vs,
1435s, 1354m, 1278s, 1205m, 1152m, 1077s, 989m, 889m,
847s, 775m, 608w, 543w, 463w.
2.7. Reaction of Ni(dtc)(PPh3)(SeR) (RsC6H5, C6H4Cl-4)
with elemental sulfur: formation of [Ni(dtc)(m-SeR)]2
2.3. Reaction of Ni(dtc)(PPh3)Cl with diphenyldiselenide
and NaBH4: formation of Ni(dtc)(PPh3)(SeC6H5)
In a typical reaction Ni(dtc)(PPh3)(SeC6H5) (0.25 g,
0.40 mmol) and elemental sulfur (0.013 g, 0.05 mmol) in
degassed toluene (30 ml) were stirred at room temperature
for 24 h. The brown solution gradually turned green. The
mixture was filtered to remove a small amount of a black
insoluble solid and about 75 ml of hexane were added to the
green filtrate and cooled at y158C to give a fluffy green solid
To a mixture of Ni(dtc)(PPh3)Cl (1.00 g, 2.03 mmol),
diphenyldiselenide (0.32 g, 1.02 mmol) and NaBH4 (0.10
g, 2.64 mmol) was added degassed MeOH (150 ml). After
stirring for 24 h the solvent was removed in vacuo and the
brown residue obtained extracted with toluene until the
extract was colourless. The toluene solution was evaporated
to dryness and the residuerecrystallisedfromCH2Cl2/hexane
to give Ni(dtc)(PPh3)(SeC6H5) (0.80 g, 63% yield). The
analytical data of this product was similar to that from the
benzeneselenol reaction.
The reactions of Ni(dtc)(PPh3)Cl with other diorgano-
diselenide compounds and NaBH4 were performed, using 1.0
g of Ni(dtc)(PPh3)Cl and the appropriate amount of the
diorganodiselenide in a 2:1 mole ratio, and worked-up in a
similar manner. The analytical data are summarised below
for each product.
1
of [Ni(dtc)(m-SeC6H5)]2 (0.06 g, 52% yield). H NMR,
CDCl3: 7.80 (s, br); 7.02 (s, br) (SeC6H5); 3.55 (br); 1.57
(br) (dtc). 13C{1H} NMR: 137.3s, 128.2s (SeC6H5); 43.7s,
12.4s (dtc). IR (KBr pellet, cmy1): 2972w, 2929w, 2869w,
1523vs, 1439vs, 1353s, 1277vs, 1206s, 1154m, 1077m,
992m, 912m, 850m, 780s, 733m, 686m, 490w, 464w.
2.8. [Ni(dtc)(m-SeC6H4Cl-4)]2
1
Yield 0.14 g, 77% yield. H NMR, CDCl3: 7.20 (s, br)
(SeC6H4Cl-4); 3.44 (br), 1.12 (br) (dtc). 13C{1H} NMR:
136.3s, 127.2s (SeC6H4Cl-4); 43.7s, 12.3s (dtc). IR (KBr
pellet, cmy1): 2972w, 2928w, 2869w, 1565w, 1516vs,
1439vs, 1353s, 1276vs, 1206s, 1152s, 1077s, 992s, 912s,
850s, 815m, 780m, 736m, 726m, 572w, 491s.
2.4. Ni(dtc)(PPh3)(SeC6H4Cl-4)
Yield 0.67 g, 50%. Anal. Calc. for C29H29ClNPS2SeNi: C,
52.79; H, 4.43; N, 2.12. Found: C, 52.58; H, 4.57; N, 2.44%.

S. Babikanyisa, J. Darkwa / Inorganica Chimica Acta 256 (1997) 15–20
17
2.9. Crystal structure determination of
Ni(dtc)(PPh3)(SeC6H5)
2Ni(dtc)(PPh34)ClqRSeSeRq2NaBH
™2Ni(dtc)(PPh32)(SeR)q2NaClqBH qH
6
(1)
2
Single crystals of Ni(dtc)(PPh3)(SeC6H5) were obtained
by layering a CH2Cl2 solution with hexane and cooling the
solution at y158C.
The role of NaBH4 is to generate NaSeR in situ, which
subsequently reacts with the nickel starting material to form
the products. The phenylselenolato complex, Ni(dtc)-
(PPh3)(SeC6H5), could also be prepared by the reaction of
Ni(dtc)(PPh3)Cl and benzeneselenol inthepresenceofEt3N
(Eq. (2)).
2.9.1. Crystal data
Ni(dtc)(PPh3)(SeC6H5), C29H30NPS2SeNi, crystal size
0.60=0.42=0.38 mm, Ms625.32, monoclinic, spacegroup
˚
P21/n, as15.047(4), bs10.286(2), cs19.475(5) A,
3 Ni(d5tc)(PPh36)ClqHSeC H qEt N
3
˚
bs109.84(2)8, Vs2835(1) A , F(000)s1280, Zs4,
Dcs1.46 g cmy3, m(Mo Ka)s20.6 cmy1
.
™Ni(dtc)(PPh36)(SeC H )qEt NHCl (2)
53
The 1H NMR spectrum of Ni(dtc)(PPh3)(SeC6H5)
showed a triplet and a quartet, respectively, for the CH3 and
CH2 functionalities of the dtc ligand. In the phenyl region,
peaks assignable to both the SeC6H5 and PPh3 ligands were
observed. However, in addition to these peaks a triplet and a
quartet at 1.20 and 3.56 ppm, respectively, a multiplet at 7.06
ppm together with a doublet at 7.90 ppm were observed as
well. These latter sets of peaks were relatively small in inten-
sity and represent a small amount of a second product which
could not be separated from the main product, Ni(dtc)-
(PPh3)(SeC6H5). The fact that these peaks were found for
the products obtained from the two synthetic routes as per
Eqs. (1) and (2), discounts the possibility of Et3NHCl con-
tamination by the latter route. The presence of all three
ligands in Ni(dtc)(PPh3)(SeC6H5), identified by IR and 1H
NMR spectra, was confirmed by 13C and 31P NMR. In con-
trast, the 1H NMR of Ni(dtc)(PPh3)(SeC6H4Cl-4) showed
only one set of peaks assignable to dtc, PPh3 and the selen-
olato ligand. The implications of these spectral differences
between Ni(dtc)(PPh3)(SeC6H5) and Ni(dtc)(PPh3)-
(SeC6H4Cl-4) will be discussed later.
2.9.2. Data collection and refinement
Reddish-brown crystals were mounted on glass fibre for
data collection. All geometric and intensity data were taken
from this sample using an automated four-circle Nicolet
R3mV diffractometer equipped with Mo Ka radiation
˚
(ls0.71073 A). The lattice vectors were identifiedbyappli-
cation of the automatic indexing routine of the diffractometer
to the positions of 31 reflections taken from a rotation pho-
tograph and centred by the diffractometer. The v–2u tech-
nique was used to measure 5501 reflections (4987 unique)
in the range 5F2uF508. Three standard reflections (re-
measured every 97 scans) showed no significantlossininten-
sity during data collection. The data were corrected for
Lorentz and polarisation effects, and an empirical absorption
correction was applied. The 3856 unique data with
IG3.0s(I) were used to solve and refine the structure in the
monoclinic space group P21/n.
2.9.3. Structural analyses and refinement
The structure was solved by Patterson methods and devel-
oped by using alternating cycles of least-squares refinement
and difference-Fourier synthesis. The non-hydrogen atoms
were refined anisotropically while hydrogens were placed in
A structure consistent with the spectroscopic data and the
monomer formulation was confirmed by X-ray crystallogra-
phy. The molecular structure of Ni(dtc)(PPh3)(SeC6H5)is
presented in Fig. 1 and some relevant bond distances and
angles in Table 1. Atomic coordinates are given in Table 2.
The geometry around the nickel atom is distorted square
planar. The major deviation from square planar geometry is
the S(1)–Ni–S(2) angle of 78.(1)8. The S(1), S(2), P and
˚
idealised positions (C–H 0.96 A) and assigned a common
2
˚
isotropic thermal parameter (Us0.08 A ). The final cycle
of least-squares refinement included 316 parameters for 3856
variables and did not shift any parameter by more than 0.006
times its standard deviation. The final R values were 0.0350
and 0.0370, and the final difference-Fourier was featureless
with no peaks greater than 0.35 e Ay3. Structure solution
˚
˚
Se atoms deviate from planar coordination up to 0.055(2) A.
This results in an angle of 4.2(1)8 between the planes defined
by the atoms S(1), S(2), Ni and P(1), Se, Ni. The Ni–S
bond distances are normal and very similar to the Ni–S dis-
used the SHELXTL PLUS program package on a microVax
II computer [11]
˚
tance in Ni(dtc)2 (2.205(1) A) [12]. The Ni–Se bond
˚
distance of 2.292(1) A is also comparable to those found
3. Results and discussion
for the terminal Ni–Se bonds in [Ni(m-SeC6H5)(SeC6H5)-
5
˚
(MePhen)]2 (2.340(2) A) [13] and in (h -C5H5)-
3.1. Mononuclear nickel(II) complexes
˚
Ni(PPh3)(SeC6H5) (2.303(1) A) [14], but are shorter
than the terminal Ni–Se distances in Ni(terpy)(2,3,6-
(CH3)3C6H2Se)2 (2.440(3) A) [15] and Ni(dapa)-
When mixtures of Ni(dtc)(PPh3)Cl, RSeSeR (RsC6H5
and C6H4Cl-4) and NaBH4 in MeOH were stirred overnight
at room temperature, moderate yields of brown microcrys-
talline Ni(dtc)(PPh3)(SeR) were obtained (Eq. (1)).
˚
(SeC6H5)PCH3CN (dapa s2,6-bis[1-(phenylimino)-
˚
ethyl]pyridine) (2.420(1) A) [15]. However the Ni–P dis-

18
S. Babikanyisa, J. Darkwa / Inorganica Chimica Acta 256 (1997) 15–20
Table 2
Atomic coordinates (=10) and equivalent isotropic displacement param-
3
˚
eters (A=10 )
x
y
z
Ueq
31(1)
Ni(1)
P(1)
Se(1)
S(1)
S(2)
N(1)
C(1)
C(2)
C(3)
C(4)
4181(1)
1410(1)
541(1)
1670(1)
884(1)
2926(1)
4531(1)
5370(1)
4005(1)
5171(2)
4901(2)
5898(4)
5487(6)
4755(3)
5301(5)
5740(2)
6508(3)
7369(3)
7448(3)
6688(3)
5836(3)
2489(2)
2469(3)
2152(3)
1834(3)
1816(3)
2161(3)
3162(2)
3820(3)
4050(3)
3662(3)
3048(3)
2796(3)
1849(2)
1054(3)
217(3)
31(1)
37(1)
44(1)
48(1)
48(1)
38(1)
76(2)
112(4)
57(2)
92(3)
34(1)
49(2)
69(2)
69(2)
59(2)
44(1)
34(1)
44(1)
57(2)
62(2)
60(2)
47(1)
33(1)
45(2)
56(2)
56(2)
57(2)
45(1)
34(1)
44(1)
53(2)
56(2)
57(2)
44(1)
y473(1)
2560(1)
3267(1)
5038(3)
3826(2)
5428(4)
5877(6)
6061(4)
6228(6)
y269(3)
y944(4)
y905(5)
y191(5)
480(5)
2338(1)
2394(1)
1058(1)
1920(2)
1805(2)
2610(3)
3165(3)
1376(3)
869(3)
3091(2)
3043(2)
3627(3)
4239(3)
4283(2)
3712(2)
1128(2)
787(2)
1045(3)
1618(3)
1947(2)
1718(2)
41(2)
34(2)
y581(2)
C(5)
C(6)
C(7)
C(8)
C(9)
Fig. 1. Crystal structure of Ni(dtc)(PPh3)(SeC6H5).
C(10)
C(11)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
C(31)
C(32)
C(33)
C(34)
C(35)
C(36)
C(41)
C(42)
C(43)
C(44)
C(45)
C(46)
445(4)
y993(3)
y2170(4)
y3277(4)
y3209(5)
y2024(5)
y932(4)
264(3)
Table 1
Selected bond distances and bond angles for Ni(dtc)(PPh3)(SeC6H5)
˚
Bond distances (A)
Ni(1)–P(1)
Ni(1)–S(1)
P(1)–C(21)
P(1)–C(41)
S(1)–C(1)
N(1)–C(1)
N(1)–C(4)
2.178(1)
2.208(1)
1.832(4)
1.820(4)
1.720(4)
1.307(5)
1.473(5)
Ni(1)–Se(1)
Ni(1)–S(2)
P(1)–C(31)
Se(1)–C(31)
S(2)–C(1)
N(1)–C(2)
2.292(1)
2.219(1)
1.817(4)
1.922(3)
1.712(3)
1.471(5)
y692(4)
y900(5)
y134(5)
835(5)
1048(4)
1506(3)
1154(4)
1835(4)
2857(4)
3211(5)
2542(4)
y1187(2)
y1177(2)
y565(2)
642(2)
57(2)
y96(2)
341(3)
straddle.Bond angles (8)
P(1)–Ni(1)–Se(1)
Se(1)–Ni(1)–S(1)
Se(1)–Ni(1)–S(2)
Ni(1)–P(1)–C(21)
Ni(1)–P(1)–C(41)
Ni(1)–S(1)–C(1)
S(1)–C(1)–S(2)
91.7(1)
96.3(1)
173.9(1)
118.3(1)
116.2(1)
85.4(1)
P(1)–Ni(1)–S(1)
P(1)–Ni(1)–S(2)
S(1)–Ni(1)–S(2)
Ni(1)–P(1)–C(31)
Ni(1)–Se(1)–C(6)
Ni(1)–S(2)–C(1)
171.8(1)
93.4(1)
78.7(1)
108.0(1)
108.4(1)
85.3(1)
160(3)
938(3)
1775(3)
936(2)
1077(2)
Equivalent isotropic U defined as one third of the trace of the orthogonalised
Uij tensor.
109.8(2)
ysis. Spectroscopic evidenceshowsthattheonlyligandspres-
ent in these complexes are dtc and SeR9. Based on these data,
and supported by elemental analysis, the products of reaction
(3) were formulated as [Ni(dtc)(m-SeR9)]2. Such dimeric
compounds are known for [Ni(S2CSCH2C6H5)(m-
SCH2C6H5)]2 (MsNi, Pd) [9c], withtheirstructuresestab-
lished by X-ray crystallography. It is conceivable that the
intermediate to the dimer is Ni(dtc)(PPh3)(SeR9), which
then loses PPh3 to give the coordinatively unsaturated
‘Ni(dtc)(SeR9)’, and subsequently dimerising to
[Ni(dtc)(m-SeR9)]2. Similar observations for the reactions
of Ni(dtc)(PPh3)Cl and alkyl or aryl thiols, leading to the
˚
tance of 2.178(1) A varies slightly from that observed in
5
˚
(h -C5H5)Ni(SeC6H5)(PPh3) (2.136(4) A) [14].
3.2. Dinuclear nickel(II) complexes
When the reaction in Eq. (1) was performed with dior-
ganodiselenides green dimeric products of general formula
[Ni(dtc)(m-SeR9)]2 were isolated (Eq. (3)).
2Ni(dtc)(PPh34)ClqR9SeSeR9q2NaBH
™[N2i(dtc)(m-SeR9)]2q2 2NaClqBH qH
(3)
6
formation of [Ni(dtc)(m-SR)]2 (RsCH2CH3,C H5 and
6
In these reactions PPh3 was isolated as a by-product, after
the main products had been filtered off. The benzyl analogue
was isolated as a microcrystalline solid, solvated by CH2Cl2.
This was established from the 1H NMR and elemental anal-
C6H4CH3-4) [8], have been made. In order to investigate
that ‘Ni(dtc)(SeR9)’ are intermediates to the dimers,
[Ni(dtc)(m-SeR9)]2, we reacted Ni(dtc)(PPh3(SeR9)with
elemental sulfur. Treatment of the mononuclear complex,

S. Babikanyisa, J. Darkwa / Inorganica Chimica Acta 256 (1997) 15–20
19
Ni(dtc)(PPh3)(SeR9), with sulfur in 8:1 mole ratio gave the
dimers as in Eq. (4).
the tellurium analogues of these compounds to establish
which of these factors is the dominant one in determining the
degree of oligomerisation.
It must be noted that our complexes, which contain dtc and
PPh3 as ancillary ligands, have no biological significance in
relation to the chemistry of the hydrogenases enzyme. How-
ever, these ancillary ligands to some extent affect the electron
density at the nickel centre and hence determine the structure
and reactivity of the nickel complexes.
Ni(dtc)(PPh38)(SeR)q1/8S
™1/2[Ni(dtc)(m-SeR)]2q3 SPPh
(4)
The olive green solids showed no evidence of PPh3 in their
spectroscopic data as were observed for the SeCH3 and
SeCH2C6H5 analogues. In these two reactions PPh3 was
removed as SPPh3 and is similar to the observations made
for (h5-C5H5)Ni(PPh )(SC6H5) [16] and Ni(dtc)(PPh)Cl
3
3
[17] when reacted with sulfur. It is noteworthy that the 1H
NMR peaks observed for [Ni(dtc)(m-SeC6H5)]2 are similar
to the low intensity peaksfoundforNi(dtc)(PPh3)(SeC6H5)
(vide supra). This suggests that there is a small amount of
[Ni(dtc)(m-SeC6H5)]2 contamination in the Ni(dtc)-
(PPh3)(SeC6H5) isolated; however it is not clear if this is
due to an equilibrium between Ni(dtc)(PPh3)(SeC6H5) and
[Ni(dtc)(m-SeC6H5)]2 in solution.
4. Supplementary material
Further crystallographic details can be obtained on request
from the Director of the Cambridge Data Centre, University
Chemical Laboratory, LensfieldRoad,CambridgeCB21EW,
UK.
3.3. Thermal gravimetric analysis
Acknowledgements
Thermolysis of Ni(dtc)(PPh3)(SeC6H5) at a heating rate
of 108C miny1 in a stream of nitrogen, monitored by ther-
mogravimetry, showed no clear cut decomposition pathway.
For Ni(dtc)(PPh3)(SeC6H5) the decomposition appearedto
be an initial loss of SeC6H5. The next two weight changes
could not be specifically assigned to any unit of the complex,
but similar trends were observed for the thermogram of
Ni(dtc)(PPh3)(SeC6H4Cl-4). On the other hand, decom-
position of the dimers [Ni(dtc)(m-SeCH2C6H5)]2 and
[Ni(dtc)(m-SeCH3)]2 showed loss of dtc. In the case of
[Ni(dtc)(m-SeCH3)]2 the loss of dtc (22.03%) was fol-
lowed by a combined loss of dtc and SeCH3 (41.31%). We
also observed similar thermograms for [Ni(dtc)(m-
SeC6H5)]2 and [Ni(dtc)(m-SeC6H4Cl-4)]2, as were found
for the above dimers. This is added evidence to support the
dimeric nature of the sulfur reaction products.
It is apparent from the properties of the products isolated
that monomer or dimer formation isdependentontheelectron
density available to theseleniumatomandstericeffectaround
the nickel centre. Complexes where the selenium atom is
directly attached to aryl groups are monomeric whereasthose
with alkyl groupsaredimers. Mascharakandco-workershave
recently shown the importance of steric effect on the aggre-
gation of nickel selenolate complexes, where the less bulky
C6H5Se forms [Ni(terpy)(C6H5Se)]2 but the more bulky
2,4,6-(Me)3C6H2Se forms Ni(terpy)((2,4,6-(Me)3C6-
H2Se)2 [13]. On the other hand, decrease in electron density
on the nickel could weaken the Ni–PPh3 interaction in the
SeCH3 and SeCH2C6H5 complexes, since these ligands are
electron withdrawing [18]. When PPh3 was removed from
the monomers, increased electron density on the selenium
and decreased steric effect led to dimerisation. It is clear that
both steric and electronic factors affect the degree of oligo-
merisation in these complexes. We are currently working on
This research was supported by the Research and
Publications Committee, University of Botswana (Grant No.
10.10.00.274). WethankDrD.A. Tocher, UniversityCollege
London, who determined the crystal structure as a service
and Professor Max Herberhold, Universita¨t Bayreuth, Ger-
many for the NMR spectra. Assistance with the thermal anal-
yses measurements by Mr Bobby Naidoo, University of the
North, South Africa, is gratefully acknowledged. We are also
grateful to one of the referees for some useful suggestions.
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