Structure and chemical bonding in cis- and trans-M(NO)2(O2CR)2 (M = Cr and Mo) complexes

Article abstract of DOI:10.1016/S0277-5387(00)00556-8

New cis- and trans-nitrosyl complexes of the formula M(NO)₂(O₂CR)₂ (M = Cr and Mo) were synthesized and characterized spectroscopically. The supporting theoretical studies indicate the deviation of molecules from their ide

Full text of DOI:10.1016/S0277-5387(00)00556-8

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 2565–2572
Structure and chemical bonding in cis- and trans-M(NO) (O CR)
2
2
2
(M=Cr and Mo) complexes
a
b
a
a,
Ludmi *l a Szterenberg , Szczepan Roszak , Renata Matusiak , Antoni Keller *
a
Faculty of Chemistry, Uni6ersity of Wroc *l aw, Joliot-Curie 14, 50-383 Wroc *l aw, Poland
b
Institute of Physical and Theoretical Chemistry, Wroc *l aw Uni6ersity of Technology, Wyb. Wyspia n´ skiego 27, 50-370 Wroc *l aw, Poland
Received 28 February 2000; accepted 18 August 2000
Abstract
New cis- and trans-nitrosyl complexes of the formula M(NO) (O CR) (M=Cr and Mo) were synthesized and characterized
2
2
2
spectroscopically. The supporting theoretical studies indicate the deviation of molecules from their ideal geometries. The most
striking consequence is the deformation of the ONꢀMꢀNO fragment from its linear form, in trans complexes. This deformation
leads to the additional weak w (NO) vibration. © 2000 Elsevier Science B.V. All rights reserved.
s
Keywords: Nitrosyl complex; Chromium; Molybdenum; Carboxylate; DFT
1
. Introduction
The nitrosyl complexes with ligands coordinated via
oxygen atoms are especially important. Carbene com-
plexes of the type [M(NO) (ꢁCHR)L (AlCl ) ] (M=Cr,
The unique physical and chemical properties of metal
2
2
2 2
nitrosyl complexes [1–9], their importance related to
biological and medical aspects [10–15], and their poten-
tial application in organic synthesis [16] have led to the
constant interest in these compounds observed for more
then 100 years [17]. The molecular unit {M(NO) } (n
is the number of d electrons of the metal [2]) is formed,
inside which the electron density can move within a
wide range. The relation between electronic density
distribution in {M(NO) } and the nature of the central
atom and other coordinated ligands is still an impor-
tant issue in structural physicochemistry, biochemistry
Mo; L=alkoxy or carboxylate ligands), were obtained
in reactions of dinitrosyl complexes of the
{
M(NO) (OR) } and M(NO) (O CR) type with
2 2 n 2 2 2
RAlCl [32,47,48]. In the case of molybdenum, stable
2
n
x
alkylidene complexes are formed. Here, coordinated
alkylidene ligands have nucleophilic character in spite
of the low oxidation state of the metals. Thus, these
compounds are different carbene complexes than the
Fischer (low oxidation states, electrophilic carbene lig-
ands) or Schrock (high oxidation state, nucleophilic
carbene ligands) compounds. In the synthesis of these
n
x
(
because of the biological importance of some com-
compounds, RAlCl plays two roles — as an alkylat-
2
plexes ([10–15]) and in catalysis. Complexes of this
group are known as catalysts of oxidation [18–23],
hydrogenation [24–26], oligomerization and polymer-
ization [27–33], isomerization [34–38], and metathesis
ing agent, but first of all it is a Lewis acid, emptying the
coordination sites at the metal due to breaking the
bridging (in polymeric complexes) or chelate bonds
[32,47,48]. A similar effect is also caused by other Lewis
[
23–34,39,40] reactions. The most active catalysts of the
acids, like M%Cl4 (M%=Ti, Sn) [31–33]. Terminal
acetylenes, present in such systems, could generate the
formation of dinitrosylcarbene (vinylidene) catalysts.
These complexes should contain low oxidation state
metals. Their stoichiometries, M(NO) (ꢁCRR%)L (LA) ,
reaction of olefin metathesis are nitrosyl complexes of
molybdenum [34,41–43] and tungsten [43,39,44]. These
compounds also catalyze oligomerization and polymer-
ization reactions of alkenes and alkynes [30–33,45,46].
2
2
2
are similar to those of the dinitrosylcarbene complexes
obtained earlier in our laboratory [32,47,48], but the
carbene groups are electrophilic in character.
*
Corresponding author. Tel.: +48-71-320-4334; fax: +48-71-328-
348.
E-mail address: akeller@wchuwr.chem.uni.wroc.pl (A. Keller).
2
0
277-5387/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0277-5387(00)00556-8

2
566
L. Szterenberg et al. / Polyhedron 19 (2000) 2565–2572
Table 2
Synthesis of new trans- and cis-dinitrosylchromium
The relative energies (DE) and structural parameters for the cis- and
trans-Mo(NO) (O CCH ) complexes from calculations performed in
complexes, their spectroscopic characterization and the
theoretical study of the structure and chemical bonding
of these types of chromium and molybdenum com-
plexes are the subject of this paper.
2
2
3 2
LanL2DZ (L) and LanL2DZ supplemented by the single polarization
a
function (L+p) basis sets
Parameters
cis L
cis L+p
trans L
trans L+p
DE
0
0
28.6
29.5
MoꢀO2
MoꢀO3
MoꢀN1
N ꢀO
2.16
2.15
2.22
1.83
1.17
2.14
2.29
1.86
1.21
2.27
2.14
1.86
1.18
2.23
1.83
1.21
1
1
N ꢀMoꢀN
92.1
179.9
60.5
91.4
174.3
60.2
132.4
164.2
59.3
131.4
163.6
59.0
1
11
MoꢀN ꢀO
1
1
O ꢀMoꢀO
2
3
145.9
82.9
145.8
83.4
141.5
99.8
142.3
99.6
2
12
a
Energy is measured in (kcal mol−1), distance in (A
), angles in (°).
,
2
. Experimental
All experiments presented here were performed under
an argon atmosphere using standard Schlenk tech-
niques and vacuum-line procedures. Solvents were
purified and distilled under argon from appropriate
agents. The trans complexes [Cr(NO) (CO) (MeNO ) ]-
2
2
2 2
(
(
BF ) , [Cr(NO) (MeNO ) ](BF )
and {Cr(NO)2-
OR) } (R=Et, Pr) were prepared according to the
4
2
2
i
2 4
4 2
2
n
published procedure [32]. The IR spectra were mea-
sured on an Impact 400 (Nicolet) spectrophotometer.
1
The H NMR spectra were recorded using a Bruker 300
spectrometer and referenced to Me Si. Molecular
4
weights were determined using a Perkin–Elmer 115
instrument.
Fig. 1. The cis (a) and trans (b) structures of M(NO) (O CR)
2
2
2
(M=Cr or Mo) complexes. The corresponding geometrical parame-
ters are given in Tables 1 and 2.
2
.1. Synthesis of trans-Cr(NO) (O CMe) (1) and
2
2
2
trans-Cr(NO) (O CCPh ) (2)
2
2
3 2
Table 1
The relative energies (DE) and structural parameters of cis- and
trans-Cr(NO) (O CCH ) complexes from DFT calculations per-
NaO CR (8.6 mmol, R=CH , CPh ) in MeOH was
2
2
3 2
2
3
3
formed in LanL2DZ (L) and LanL2DZ supplemented by the polar-
added to [Cr(NO) (CO) (MeNO ) ](BF ) or [Cr(NO) -
2
2
2 2
4 2
2
a
ization function (L+p) basis sets
(
MeNO ) ](BF ) (4.3 mmol) in MeNO2 (20 ml) at
2 4 4 2
room temperature. The reaction mixtures were stirred
for 15 min after which the solutions were filtered and
removed under vacuum. The complexes were extracted
Parameters
cis L
cis L+p
trans L
trans L+p
DE
0
0
25.3
26.6
from the remainder with a small amount of CH Cl and
precipitated with hexane.
CrꢀO2
CrꢀO3
CrꢀN1
2.03
2.016
2.08
1.70
1.16
2.02
2.17
1.73
1.20
2.16
2.01
1.73
1.16
2
2
2.09
1.70
1.20
1
N ꢀO1
2.2. Cr(NO) (O CMe)
2
2
2
N ꢀCrꢀN
93.9
176.2
64.6
92.0
174.8
64.2
132.2
163.0
63.0
130.1
161.4
62.6
1
11
1
CrꢀN ꢀO1
IR (Nujol mulls):w(NO) 1778 sh, 1698 vs; w (CO )
as 2
O ꢀCrꢀO3
2
1
s 2 3
1
580 vs, w (CO ) 1421 s. H NMR (in CD CN at 20°C):
O ꢀCrꢀO
148.0
91.0
147.8
84.5
139.2
94.7
140.6
94.1
2
12
l 2.10 (s, 6H, CH ) ppm. Anal. Calc. for C H CrN O :
3
4
6
2
6
O ꢀCrꢀO
2
13
C, 20.88; H, 2.63; N, 12.17. Found: C, 20.96; H 2.70;
12.01%.
a
Energy is measured in (kcal mol−1), distance in (A
), angles in (°).
,

L. Szterenberg et al. / Polyhedron 19 (2000) 2565–2572
2567
2
.3. Cr(NO) (O CCPh )
2.4. Synthesis of cis-{Cr(NO) (OR) } (R=Et (3a),
Pr (4a))
2
2
3 2
2
2 n
i
IR (Nujol mulls): w(NO) 1777 sh, w, 1696 vs;
1
w (CO ) 1559 vs, w (CO ) 1447 m. H NMR (in ace-
A solution of trans-{Cr(NO) (OEt) } in toluene was
2 2 n
as
2
s
2
tone-d at 20°C): l 7.26 (m, 30H, Ph) ppm. Anal. Calc.
stirred for 30 min at 60°C. The green precipitate of the
6
for C H CrN O : C, 69.97; H, 4.40; N, 4.08. Found:
cis-isomer was filtered off, washed with CH Cl and
4
0
30
2
6
2
2
C, 70.08; H, 4.51; N, 4.00%.
dried in vacuo.
Table 3
Calculated and experimental frequencies w(NO) (cm 1) and intensities I (km mol−1) for cis- and trans-M(NO) (O CCH ) (M=Cr, Mo) a
−
2
2
3 2
Calculated
Experimental
w
I
Iw /Iw
w
Iw /Iw
wcalc/wexp
s
as
s
as
L
L+p
L
L+p
L
L+p
0.50
0.18
L
L+p
M=Cr
b
cis
was
ws
1727
1829
102
1697
1778
81
1857
1977
102
1840
1926
86
1117
498
1348
676
0.44
0.14
1699
1849
150
1698
1778
80
0.43
0.14
b
c
c
b
Dw
was
ws
trans
1431
203
1708
314
1.00
1.00
1.08
1.08
Dw
M=Mo
cis
d
was
ws
1670
1759
89
1625
1710
85
1811
1903
92
1772
1857
85
1098
472
1294
570
0.43
0.12
0.44
0.17
1650
1770
120
1665
0.42
1.01
0.99
1.09
1.07
d
c
c
d
Dw
was
ws
e
trans
1675
201
1733
293
Dw
a
Calculations were done in LanL2DZ (L) and LanL2DZ supplemented by the polarization function (L+p) basis sets.
For cis-{Cr(NO) (OEt) } .
b
c
2
2 n
Dw=w −w .
s
as
d
e
Ref. [62].
For trans-[Mo(NO) (Hsal-INH) ] [59].
2
2
Table 4
Frequencies of w(NO) and relative intensities Iw /Iwas of other known cis- and trans-dinitrosyl complexes of chromium and molybdenum
s
ws(NO) (cm−1)
was(NO) (cm−1)
Dw(NO) (cm−1
a
)
Iw /Iw
s
Ref.
Complex
as
b
trans-Cr(NO) (O CCPh )
3 2
1777
1696
1720
1718
1793
1798
1791
1702
1698
1766
1763
1678
1671
1678
1616
1630
1682
81
0.12
2
2
trans-{Cr(NO) (OEt) }
[32]
[32]
[32,50]
[32]
[32]
2
2 2n
i
trans-{Cr(NO) (O Pr) }
2 n
2
2
+
trans-[Cr(NO) (MeCN) ]
2
4
2
+
trans-[Cr(NO) (MeNO ) ]
2
2 4
2
+
trans-[Cr(NO) (CO) (MeNO ) ]
2
2
2 2
2+
trans-[Cr(NO) (bipy) ]
[32]
2
2
i
b
cis-{Cr(NO) (O Pr) }
1854
1887
1885
156
121
122
0.41
0.45
0.48
2
2 n
b
b
cis-[Cr(NO) (OEt) (SnCl )]
2
2
4
cis-[Cr(NO) (O CCPh ) (SnCl ) ]
2
2
3 2
4 2
trans-[Mo(NO) (Hsal-BZH) ]
[49]
[49]
[49]
[66]
[66]
[62]
2
2
trans-[Mo(NO) (Hvan-BZH) ]
2
2
trans-[Mo(NO) (Hvan-INH) ]
2
2
cis-{Mo(NO) (OEt) EtOH}
1740
1772
1790
124
142
118
0.45
0.44
0.43
2
2
n
i
cis-{Mo(NO) (O Pr) }
2 n
2
cis-Mo(NO) (O CPh)
2
2
2
a
Dw(NO)=w (NO)−w (NO).
s
as
b
This work.

2
568
L. Szterenberg et al. / Polyhedron 19 (2000) 2565–2572
Fig. 2. The HOMO for the cis-Cr(NO) (O CR) molecule plotted in the NꢀCrꢀN plane. The contour lines from 1–12 are positive (increasing
2
2
2
order), from 13–17 are negative (decreasing order). The contour 1 denotes zero. The spacing between lines is 0.0125 units. Note the p character
of the CrꢀN bond in the orbital.
2
.5. {Cr(NO) (OEt) }
brown adduct precipitates were filtered off, washed
with CH Cl and dried in vacuo.
2
2 n
2
2
IR (Nujol mulls): w (NO) 1849 s, w (NO) 1699 vs,
s
as
1
w(OR) 1093 m, 1050 m, 1025 sh. H NMR (acetone-d ):
6
2
.8. [Cr(NO) (OEt) (SnCl )]
2 2 4
l 3.62, 3.35 (s, br, 6H, CH CH ), 1.35, 1.06 (s, 9H,
3
2
CH CH ) ppm. Anal. Calc. for C H CrN O : C, 23.77;
H, 4.99; N, 13.86. Found: C, 23.89; H, 5.04; N, 13.73%.
3
2
4
10
2
4
IR (Nujol mulls): w (NO) 1887 s, w (NO) 1766 vs,
s
as
1
w(OR) 1088 m, 1008 m, 993 sh. H NMR (in CD CN at
3
2
br,
0°C): l 4.54, 3.64 (s, br, 4H, CH CH ), 1.26, 0.91 (s,
1
2 3
2
.6. {Cr(NO) (O Pr) }
2
2
n
6H,
CH CH )
ppm.
Anal.
Calc.
for
2
3
C H Cl CrN O Sn: C, 10.38; H, 2.18; Cl, 30.65; N,
4
10
4
2
4
IR (Nujol mulls): w (NO) 1854 s, w (NO) 1698 vs.
s
as
6
.05. Found: C, 10.44; H, 2.27; Cl, 35.71; N, 5.97%.
2
.7. Synthesis of cis-{Cr(NO) (OEt) (SnCl )} (5) and
2
2
4
n
cis-{Cr(NO) (O CCPh ) (SnCl ) } (6)
2.9. [Cr(NO) (O CCPh ) (SnCl ) ]
2
2
3 2
4 2
n
2
2
3 2
4 2
SnCl in CH Cl was added to a stirred solution of
IR (Nujol mulls): w (NO) 1885 s, w (NO) 1763 vs,
s as
w (CO) 1550 vs, w (CO) 1421 s. H NMR (in CD CN at
2
2
2
1
as s 3
the appropriate {Cr(NO) L } complex (L=OEt,
2
2
n
O CCPh ) in CH Cl in a 1:2 molar ratio. The yellow–
20°C): l 7.26 (m, 30H, Ph) ppm. Anal. Calc. for
2
3
2
2

L. Szterenberg et al. / Polyhedron 19 (2000) 2565–2572
2569
C H Cl CrN O Sn : C, 39.78; H, 2.50; Cl, 23.48; N,
4. Results and discussion
4
0
30
8
2
6
2
2.32. Found: C, 39.87; H, 2.54; Cl, 23.55; N, 2.27%.
The structure and the nature of bonding in nitrosyl
complexes were discussed in 1974 by Enemark and
3. Details of theoretical studies
Feltham [2]. In the case of dinitrosyl complexes possess-
n
ing the {M(NO) } fragment the important problem
2
The studies presented here were performed applying
concerns the NꢀMꢀN angle. The number of d electrons,
n, determines the linearity (Dꢀh) or nonlinearity (C26)
of the NꢀMꢀN angle. The six-coordinated dinitrosyl
complexes of Group 6 metals should contain the non-
density functional theory (DFT) [49]. The DFT ap-
proach utilized Becke’s three parameters hybrid func-
tional (B3LYP) [50], based on earlier works [51,52].
Calculations were performed within the LanL2DZ
linear M(NO) fragment. All the structurally character-
2
[
53,54] basis set. Additional calculations were per-
ized compounds, including Mo(NO) TTP, possess
2
formed with the LanL2DZ basis supplemented by the
single five-component d function on heavy atoms. The
exponents of polarization functions for chromium and
molybdenum are 0.058317 and 0.1270, respectively. Ex-
ponents for other atoms were adopted from Huzinaga
et al. [55]. The computations presented were made
using the program GAUSSIAN-94 [56].
nitrosyl ligands in the cis position [6,57]. However, the
dinitrosylchromium complex [Cr(NO) (MeCN) ](PF ) ,
2
4
6 2
synthesized 30 years ago by Connolly et al. [58], is trans
with regard of the nitrosyl groups. Only a single band
in the region of the w(NO) frequency was observed in
the IR spectrum of this compound. The replacement of
−
2
acetonitrile ligands by sulfur chelating ones (S CNEt
2
Fig. 3. The HOMO-1 for the cis-Cr(NO) (O CR) molecule plotted in the NꢀCrꢀN plane. The contour lines from 1–12 are positive (increasing
2
2
2
order), from 13–17 are negative (decreasing order). The contour 1 denotes zero. The spacing between lines is 0.0125 units. Note the s character
of the CrꢀN bond in the orbital.

2
570
L. Szterenberg et al. / Polyhedron 19 (2000) 2565–2572
2
2
−
1
and/or S C (CN) ) leads to the formation of respec-
in these complexes [32,63]. The H NMR spectrum of
2
2
tive cis-dinitrosylchromium complexes. Recently, the
trans-dinitrosylmolybdenum complexes with Schiff
bases were also synthesized [59]. A number of diatomic
trans-dinitrosylchromium complexes were obtained in
our laboratory [32]. The reaction of complexes contain-
the complex 3a (see Section 2) also confirms the above.
It contains two resonance signals for the CH and CH
2
3
protons corresponding to the terminal and bridging
ethoxy ligands.
Reactions of complexes 2 and 3 with Lewis acids (e.g.
SnCl ) induce similar trans-[Cr(NO) ]“cis-[Cr(NO) ]
2
+
I
I
ing the trans-[Cr(NO)]
core with M OR (M =Na,
4
2
2
Li) leads to the formation of respective trans com-
plexes. A similar reaction Eq. (1) yields dicarboxyla-
tochromium complexes, i.e. Cr(NO) (O CR) (where
transformations, in 1:1 as well as in 1:2 molar ratios of
reagents. The adducts cis-[Cr(NO) (OEt) (SnCl )] (5)
2
2
4
and cis-[Cr(NO) (O CCPh ) (SnCl ) ] (6) are produced
2
2
2
2
2
3 2
4 2
R=CH or CPh ).
in such reactions. Their IR spectra contain two charac-
teristic w(NO) frequencies. The character of the spec-
trum of complex 5, in the range of w(OEt) frequencies,
indicates the bridging and terminal coordination of
ethoxy ligands [32,63]. Examination of the H NMR
spectrum (see Section 2) also leads to the same
conclusion.
3
3
[
Cr(NO) (CO) (MeNO ) ](BF )
2 2 2 2 4 2
or
Cr(NO) L ](BF )
1
[
2
L=MeCN or MeNO
4
4 2
)
(
2
+
2NaO CR“Cr(NO) (O CR) +2NaBF
(1)
The brown complex 1, soluble in donor solvents
2
2
2
2
4
(
3
R=Me (1) or CPh (2))
4.1. Theoretical results
(
MeCN, MeNO , MeOH) is polymeric ({Cr(NO) -
2
2
(
1
O CMe) } ). The Dw(CO )=w −w splitting equals
Theoretical studies were performed for the M(NO)2-
2
2
−
n
1
2
as
s
51 cm
confirms the above [60,61]. Very low
(O CMe) (M=Cr and Mo) complexes. The valence
2
2
Dw(CO ) values (B105) generally indicate chelation
orbitals of chromium and molybdenum undergo the
2
5
1
[
61]. For example, in [Mo(NO) (O CMe) ]·MeOH,
d s hybridization which leads to a trigonal bipyramid
structure. In the molecules studied, however, the
metal–acetate rings destroy the perfect D4h symmetry.
Two equilibrium structures were located (Fig. 1) with
2
2
2
−
1
1
Dw(CO ) is 97 cm
[62]. The H NMR spectrum of 1
2
in CD NO contains one methyl proton signal at 2.10
3
2
ppm.
Complex 2 (soluble in CH Cl , benzene, toluene,
the cis isomers (C symmetry) being more stable then its
2
2
2
CCl ) exists in the monomeric form as indicated by its
molecular weight (Found: 683.91. Calc. for
trans partners (C26 symmetry) (Tables 1 and 2). The
X-ray structures for the trans-dinitrosyl complexes of
Group 6 metals are unknown. The main structural
difference between the ideal structures of the cis and
trans isomers concerns the NꢀMꢀN angle. In the ideal
trans structure the ONꢀMꢀNO fragment is linear. The
studies performed, however, indicated a significant de-
viation from linearity with the NꢀMꢀN angle being
130.1 and 163.6° for Cr and Mo compounds, respec-
tively. In the case of the cis isomers NꢀMꢀN angles of
92.0° (Cr) and 91.0° (Mo) are close to the values found
for the perfect geometry (90.0°). The MꢀNꢀO branch
was also found to be nonlinear. It was noted several
years ago [64] that slightly bent MꢀNꢀO fragments are
a common feature of polynitrosyl complexes. The elec-
tronic rationalization of this phenomenon was given by
Kettle [65]. In an every molecule studied, the OꢀMꢀO
angles of metal–acetate rings amount to about 60°,
again far from the 90° expected for the ideal D2h
symmetry. This distortion leads to two different MꢀO
bonds within the same ring.
4
Cr(NO) (O CCPh ) : 686.68). The Dw(CO ) value for
2
2
3 2
2
−
1
1
chelate triphenylacetate ligands is 112 cm . The H
NMR spectrum (in CD Cl at 20°C) exhibits a multi-
2
2
plet at 7.26 ppm due to phenyl protons. The IR spectra
of 1 and 2 are similar to all the trans-dinitrosyl-
chromium complexes [32,58] and are characterized by a
−
1
single strong band at 1700 cm
vibration. However weak shoulders at 1778 and 1777
related to the w(NO)
−
1
cm
may also be observed.
Dinitrosylchromium complexes with alkoxy ligands
i
(
i.e. trans-{Cr(NO) (OR) } , R=Et (3), Pr (4)), when
2
2 n
heated (60°C) in noncoordinating solvents, undergo
isomerization to cis complexes (3a, 4a). However, only
the cis-{Cr(NO) (OEt) } (3a) complex was obtained in
2
2 n
pure form, as a brown powder soluble only in polar
solvents. Complex 4 partially disintegrates during such
treatment, while complex 2 does not isomerize at all
and at higher temperatures decomposes. Complex 1 is
not soluble in noncoordinating solvents.
The IR spectra of complexes 3a and 4a (see Section
The vibrational spectra of studied compounds reflects
the geometry of the molecules. Two different frequen-
cies for NO vibrations (asymmetrical and symmetrical)
were expected and both are observed. The calculated
and experimental frequencies are presented in Table 3.
The most striking effect comparing this structure to the
ideal is that there are two, instead of just one, vibra-
2
) contain two w(NO) frequencies characteristic for the
6
cis-{Cr(NO) } unit (C symmetry), corresponding to
2
26
symmetric and asymmetric stretching motions of the
−
NꢀO bond. The w(CO) frequencies of RO ligands (e.g.
−
1
for R=Et 1093, 1050 and 1025 cm ) may indicate the
presence of bridging as well as terminal alkoxy ligands

L. Szterenberg et al. / Polyhedron 19 (2000) 2565–2572
2571
tional motions for NO in the trans case. The frequen-
cies calculated applying the LanL2DZ basis set closely
reproduces the available experimental spectra. The ex-
tension of the basis set by the polarization functions,
however, leads to the systematically higher values of
frequencies which have to scaled by 0.9 to reproduce
experimental data. This scaling factor reflects the qual-
ity of the method and the basis set applied. The excel-
lent agreement of the smaller basis results with
experimental values has to be treated with caution. The
differences between asymmetric and symmetric modes
of NꢀO vibrations are reproduced satisfactorily. The
calculated ratio of Iw /Iw is also close to that observed
6. Based on the previous IR studies [32,50,51], D2h
symmetry was expected for trans structures with a
linear ONꢀMꢀNO fragment.
Theoretical studies determined two isomers (Fig. 1):
cis (C symmetry) and trans (C symmetry), with the
2
26
cis forms being energetically more stable (Tables 1 and
2). The trans NO groups are divided by a plane defined
by the acetate rings. The latter structures are signifi-
cantly distorted from ideal geometries. The expected
D2h symmetry of trans complexes, due to the nonlinear-
ity of the ONꢀMꢀNO fragment, is lowered to C . This
2
6
distortion is observed as an additional symmetric NO
vibration. The weak band corresponding to w (NO) may
s
as
s
experimentally. This agreement gives us confidence in
the calculations performed for trans isomers where
experimental results are scarce. The inflection observed
in the wide band of the trans isomer in the Cr based
molecule fits the theoretically found symmetric vibra-
tion. The much lower ratio of Iw /Iw (0.14) for the
be easily hidden in the strong one associated with the
w (NO) vibrational motion. In the case of compounds 1
as
−
1
and 2, the single bands at about 1700 cm is related to
was(NO), but weak shoulders at 1778 and 1777 cm
−
1
may correspond to the symmetrical NO vibrations. The
reasonable reproduction of the properties of the cis
isomers gives confidence for the overall theoretical stud-
ies presented. The calculated intensities of observed
bands are comparable in the cis case, while the second
(symmetrical) vibration in the trans molecule is barely
seen. The calculations led to the distortion of
ONꢀMꢀNO structures in every isomer studied. The
study of structures of other complexes would help to
answer if this is a property of Cr and Mo complexes
only or if this is a more general feature of nitrosyl
complexes. The MꢀNꢀO branches were also distorted
from linearity in every structure studied. The geometry
optimization also indicated a significant deformation of
the acetate rings. Different lengths of MꢀO bonds
within the same rings were observed.
s
as
trans isomer compared to that of the cis form (0.43) is
consistent with molecular structures. The weak symmet-
ric band would vanish in the case of the perfect D4h
structure. Table 4 collects the w(NO) frequencies and
relative intensities Iw /Iw of other known cis- and
s
as
trans-dinitrosyl
complexes
of
chromium
and
molybdenum.
The nature of bonds formed by the metal are similar
in Cr and Mo compounds. The analysis of bonding
performed here is based on the Mulliken population
analysis. The two highest molecular orbitals (Figs. 2
and 3) describe the MeꢀNO bonds. These bonds are
covalent, and may be described as two three-centered
NꢀMeꢀN s and p bonds. The central metal atom
donates 0.5 of an electron to the acetate rings, although
the total loss of electrons in d orbitals amounts to 1.2
electrons. Due to the rearrangement of the electronic
density of metal, its s orbitals gain 0.4 of an electron,
while 0.3 of an electron is shifted to the metal p
orbitals. The involvement of p orbitals in the formation
The electronic density rearrangement on the central
atom is almost the same in both cis and trans isomers.
The change in the electronic density is due to the
transfer of electrons from d orbitals to the acetate rings
(0.5 electron) and s and p orbitals (0.7 electron).
The reported results, including the charge distribu-
tion picture on the central and ligand atoms confirm
the suggested mechanism of the formation of carbene
and vinylidene catalysts in cis-M(NO) (O CR) –Lewis
5
of bonds indicates the deviation from the d s hybridiza-
tion and in effect the distortion of the perfect D2h
symmetry. The electronic structure of the center of
coordination is similar in both the cis and trans iso-
mers, and is visible as an almost equal occupation of
the orbitals on the central atom.
2
2
2
acid and {M(NO) (OR) } –Lewis acid systems [31–
2
2 n
33,47,48,54,66]. The favorable conditions for the elec-
trophilic attack of Lewis acid molecules on the
carboxylate ligand oxygen atoms are in the equatorial
plane (M1O3O14N9N11 plane in Fig. 1). In the same
plane, on the side opposite to the NO groups, carbene
and vinylidene ligands will coordinate.
5. Conclusions
n
The number of d electrons in the {M(NO) } frag-
2
ment determines its structural properties. The six-coor-
dinated dinitrosyl complexes of Group 6 metals should
have the cis form. However, trans isomers containing
Cr or Mo are also known (Tables 3 and 4) and some of
them were synthesized in this work. X-ray structures
are not known for trans-dinitrosyl complexes of Group
Acknowledgements
We thank the State Committee for Scientific Re-
search (grant no 3T09A 059 17) and Wroc *l aw Univer-
sity of Technology (grant no 341-831) for supporting

2
572
L. Szterenberg et al. / Polyhedron 19 (2000) 2565–2572
[
[
[
32] A. Keller, R. Matusiak, J. Mol. Catal. 145 (1999) 127.
this work. We would like to thank Pozna n´ and
Wroc *l aw Supercomputing and Networking Centers
and Interdisciplinary Center for Mathematical and
Computational Modeling of Warsaw University for the
generous allotment of computer time.
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