Kinetic studies on the reactions of HCl with trans-[MoL(CNPh)(Ph2PCH2CH2PPh2) 2] (L=N2, H2 or CO)

Article abstract of DOI:10.1016/S0020-1693(01)00819-2

The kinetics of the reactions between anhydrous HCl and trans-[MoL(CNPh)(Ph₂PCH₂CH₂PPh₂) ₂] (L=CO, N₂ or H₂) have been studied in thf at 25.0°C. When L=CO, the product is [MoH(CO)(CNPh)(Ph₂PCH₂CH₂PPh₂) ₂]⁺, and when L=H₂ or N₂ the product is trans-[MoCl(CNHPh)(Ph₂PCH₂CH₂PPh ₂)₂]. Using stopped-flow spectrophotometry reveals that the protonation chemistry of trans-[MoL(CNPh)(Ph₂PCH₂CH₂PPh₂) ₂] is complicated. It is proposed that in all cases protonation occurs initially at the nitrogen atom of the isonitrile ligand to form trans-[MoL(CNHPh)(Ph₂PCH₂CH₂PPh ₂)₂]⁺. Only when L=N₂ is this single protonation sufficient to labilise L to dissociation, and subsequent binding of Cl^- gives trans-[MoCl(CNHPh)(Ph₂PCH₂CH₂PPh ₂)₂]. At high concentrations of HCl a second protonation occurs which inhibits the substitution. It is proposed that this second proton binds to the dinitrogen ligand. When L=CO or H₂, a second protonation is also observed but in these cases the second protonation is proposed to occur at the carbon atom of the aminocarbyne ligand, generating trans-[MoL(CHNHPh)(Ph₂PCH₂CH₂PPh ₂)₂]²⁺. Addition of the second proton labilises the trans-H₂ to dissociation, and subsequent rapid binding of Cl^- and dissociation of a proton yields the product trans-[MoCl(CNHPh)(Ph₂PCH₂CH₂PPh ₂)₂]. Dissociation of L=CO does not occur from trans-[Mo(CO)(CHNHPh)(Ph₂PCH₂CH₂PPh ₂)₂]²⁺, but rather migration of the proton from carbon to molybdenum, and dissociation of the other proton produces [MoH(CO)(CNPh)(Ph₂PCH₂CH₂PPh₂) ₂]⁺.

Full text of DOI:10.1016/S0020-1693(01)00819-2

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Inorganica Chimica Acta 331 (2002) 270–278
Kinetic studies on the reactions of HCl with
trans-[MoL(CNPh)(Ph2PCH2CH2PPh2)2] (L=N2, H2 or CO)
Marie-Cecile Rosenblat, Richard A. Henderson *
Department of Chemistry, Uni6ersity of Newcastle, Bedson Building, Newcastle-upon-Tyne NE1 7RU, UK
Received 4 September 2001; accepted 15 November 2001
Dedicated to Professor A.G. Sykes
Abstract
The kinetics of the reactions between anhydrous HCl and trans-[MoL(CNPh)(Ph2PCH2CH2PPh2)2] (L=CO, N2 or H2) have
been studied in thf at 25.0 °C. When L=CO, the product is [MoH(CO)(CNPh)(Ph2PCH2CH2PPh2)2]+, and when L=H2 or N2
the product is trans-[MoCl(CNHPh)(Ph2PCH2CH2PPh2)2]. Using stopped-flow spectrophotometry reveals that the protonation
chemistry of trans-[MoL(CNPh)(Ph2PCH2CH2PPh2)2] is complicated. It is proposed that in all cases protonation occurs initially
at the nitrogen atom of the isonitrile ligand to form trans-[MoL(CNHPh)(Ph2PCH2CH2PPh2)2]+. Only when L=N2 is this single
protonation sufficient to labilise L to dissociation, and subsequent binding of Cl− gives trans-[MoCl(CNHPh)(Ph2PCH2-
CH2PPh2)2]. At high concentrations of HCl a second protonation occurs which inhibits the substitution. It is proposed that this
second proton binds to the dinitrogen ligand. When L=CO or H2, a second protonation is also observed but in these cases the
second protonation is proposed to occur at the carbon atom of the aminocarbyne ligand, generating trans-
[MoL(CHNHPh)(Ph2PCH2CH2PPh2)2]2+. Addition of the second proton labilises the trans-H2 to dissociation, and subsequent
rapid binding of Cl− and dissociation of a proton yields the product trans-[MoCl(CNHPh)(Ph2PCH2CH2PPh2)2]. Dissociation of
L=CO does not occur from trans-[Mo(CO)(CHNHPh)(Ph2PCH2CH2PPh2)2]2+, but rather migration of the proton from carbon
to molybdenum, and dissociation of the other proton produces [MoH(CO)(CNPh)(Ph2PCH2CH2PPh2)2]+. © 2002 Elsevier
Science B.V. All rights reserved.
Keywords: Kinetics and mechanism; Molybdenum complexes; Isocyanide complexes; Dinitrogen complexes; Protonation reaction
1. Introduction
a reaction between acid and trans-[MoL2(dppe)2] does
not necessarily reveal the true and complete protona-
tion chemistry of the complex. Most particularly, it
does not necessarily establish the initial site of protona-
tion. In order to define the protonation chemistry of
electron-rich complexes it is necessary to obtain a de-
tailed kinetic analysis of the reaction using rapid reac-
tion techniques.
Further elaboration of the above studies has involved
investigation of the protonation reactions of trans-
[MoLL%(dppe)2] (L=RCN or ArCN, L%=N2) [11]. By
studying these complexes it should be possible to estab-
lish the factors which differentiate between protonation
of L, L% or Mo. Recently, Hidai and co-workers have
reported [12–14] the preparation of a series of com-
plexes, trans-[MoL(CNPh)(dppe)2] (L=N2, H2 or CO)
and studied the products of protonation of the com-
plexes. A variety of products are obtained, which at
face value indicate protonation can occur at isonitrile
Understanding the factors controlling the sites and
rates of protonation of electron-rich complexes is fun-
damental to establishing a predictable basis for the
application of regio-, stereo- and product-selective reac-
tions, and in defining the elementary reactions of se-
lected metalloenzymes [1–3]. To this end, studies on the
reactivity of complexes of the type, trans-[MoL2(dppe)2]
((dppe=Ph2PCH2CH2PPh2); L=N2 [4], RNC [5],
C2H4 [6,7] RCCH [8,9] or 2H [10]) have been particu-
larly revealing. Protonation can occur at the metal or
L, or even both. In addition, movement of protons
between ligand and metal can occur rapidly, either by
intramolecular or acid–base-catalysed routes as shown
in Fig. 1. Consequently, the isolation of the product of
* Corresponding author. Fax: +44-191-222 6929.
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00819-2

M.-C. Rosenblat, R.A. Henderson / Inorganica Chimica Acta 331 (2002) 270–278
271
(L=N2 or H2) or molybdenum (L=CO) [14]. In this
paper, we report the results of studying the reactions of
HCl with trans-[MoL(CNPh)(dppe)2] using stopped-
flow spectrophotometry, which reveals the following
features: (i) the initial site of protonation is always the
isonitrile ligand; (ii) the protonation chemistry of trans-
[MoL(CNPh)(dppe)2] is much more complex than is
evident from isolation of the products of the reactions
and, (iii) the ultimate product is formed by a combina-
tion of reactions including protonation, labilisation of
the trans-ligands, proton migration to the metal (L=
CO) and proton dissociation.
literature (l 70 ppm) [14]. The product of the reaction
between anhydrous HCl and either trans-[Mo(N2)-
(CNPh)(dppe)2] or trans-[Mo(h2-H2)(CNPh)(dppe)2]
was isolated and shown to be trans-[MoCl(CNHPh)-
(dppe)2] by elemental analysis and spectroscopic
characterisation.
2.1. Preparation of [MoCl(CNHPh)(dppe)2]
To a stirred slurry of trans-[Mo(N2)(CNPh)(dppe)2]
(0.2 g; 0.2 mmol) in thf (approximately 20 ml) was added
MeOH (0.1 ml) followed by SiMe3Cl (0.32 ml). Immedi-
ately the solution changed colour from red to orange.
After stirring the solution for 1 h, all volatiles were
removed in vacuo to give an orange–brown solid. The
solid was dissolved in dichloromethane (approximately
20 ml) and then diethyl ether (approximately 80 ml) was
added slowly. After standing overnight orange plate-like
crystals had formed together with some amorphous
grey–green solid. The amorphous solid was removed by
decantation and the orange crystals washed onto a sinter
using diethyl ether. The crystals were washed with a
further portion of diethyl ether then dried in vacuo.
Yield=0.13 g, 65%. Elemental analysis. Found: C, 68.5;
H, 5.2; N, 1.6. Calc. for C59H54-NP4ClMo: C, 68.6; H,
5.2; N, 1.4%. 1H NMR (CDCl3): l 3.0 (br, 4H, PꢀCH2),
3.4 (br, 4H, PꢀCH2), 5.8–6.3 (m, 5H, NC6H5), 7.0–8.0
(m, 40H, PC6H5), 11.8 (br, 1H, NꢀH). 31P NMR (ppm):
l 55. IR (cm−1): wCN 1500.
2. Experimental
All manipulations in both the synthetic and kinetic
aspects of this work were performed under an atmo-
sphere of dinitrogen or argon (for trans-[Mo(h2-
H2)(CNPh)(dppe)2]) using Schlenk or syringe tech-
niques as appropriate. All solvents were dried over the
appropriate agent and distilled immediately prior to
use: diethylether (sodium); thf (sodium/benzophenone);
dichloromethane (P2O5); and benzene (sodium).
The following reagents were purchased and used as
received: Ph2PCH2CH2PPh2 (Lancaster); MoCl5 (Al-
drich); and PhCHNPh (Lancaster). The following com-
pounds were prepared by the methods described in the
literature: trans-[Mo(N2)2(dppe)2] [15]; trans-[Mo(N2)-
The same product was isolated in an analogous
manner from the reaction of trans-[Mo(h2-
H2)(CNPh)(dppe)2] with anhydrous HCl.
(CNPh)(dppe)2];
and trans-[Mo(CO)(CNPh)(dppe)2] [13].
trans-[Mo(h2-H2)(CNPh)-(dppe)2];
The product of the reaction between trans-[Mo(CO)-
(CNPh)(dppe)2] and anhydrous HCl in thf was confi-
rmed to be [MoH(CO)(CNPh)(dppe)2]+ using 31P{1H}
NMR spectroscopy which was identical to that in the
2.2. Kinetic studies
All kinetic studies were performed using an Applied
Photophysics SX.18MV stopped-flow spectrophotome-
ter, modified to handle air-sensitive solutions. The tem-
perature was maintained at 25.090.1 °C using a Grant
LT D6G thermostat recirculating tank.
Solutions were prepared under an atmosphere of
dinitrogen and transferred by gas-tight, all-glass sy-
ringes to the stopped-flow spectrophotometer. Stock
100 mmol dm−3 solutions of anhydrous HCl were pre-
pared by mixing equimolar amounts of MeOH (0.1 ml)
and SiMe3Cl (0.32 ml) in thf (25.0 ml). Dilute solutions
of anhydrous HCl were prepared from the stock solu-
tion. All solutions were used within 1 h of preparation
to minimise complications due to acid-catalysed ring-
opening and polymerisation of thf.
All kinetics were studied under pseudo first-order
conditions with the acid in a large excess (\tenfold)
over the concentration of the complex. The ab-
sorbance–time curves were fitted using the Applied
Photophysics computer programme and the values of
the observed rate constants (kobs) were obtained from
Fig. 1. Summary of the general pathways for the protonation of
metal and ligand and the mechanisms by which protons can move
between these two sites.

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Fig. 2. Pathways observed in the reaction of trans-[MoL(CNPh)(Ph2PCH2CH2PPh2)2] with anhydrous HCl in thf at 25.0 °C.
the analysis. Derivation of the rate laws for the various
reactions were established by the normal kinetic analy-
sis of the data as indicated in Section 3.
from the beginning. A typical curve is shown in Fig. 3.
When studied at a single wavelength, it is clear that the
reactions of anhydrous HCl with trans-[MoL(C-
NPh)(dppe)2] in thf is clear that the reaction occurs in
three phases. The first phase is an initial absorbance
decrease, which is complete within the dead-time of the
stopped-flow apparatus (approximately 2 ms) and hence
too rapid to be studied. A further appreciable ab-
sorbance decrease follows, which is complete over the
course of a few seconds. The second and third phases are
associated with the absorbance–time curve. This curve
can be fitted to two exponentials of approximately equal
absorbance change. Analysis of the data reveals that the
second phase exhibits a complicated dependence on the
concentration of HCl, the specifics of which will be
discussed for each system presented below. The third
phase occurs at a rate, which is independent of the
concentration of acid for all the complexes studied, but
the associated rate constant depends on the nature of the
complex.
3. Results and discussion
The work presented in this paper describes the kinetics
of the reactions between anhydrous HCl and trans-
[MoL(CNPh)(dppe)2] (L=CO, H2 or N2) in thf [14].
The pathways, which will be outlined herein, are sum-
marised in Fig. 2. A key feature of Fig. 2 is that all
species presented, except trans-[Mo(LH)(CNHPh)-
(dppe)2]2+, have been isolated and characterised. How-
ever, trans-[Mo(LH)(CNHPh)(dppe)2]2+ has only been
invoked in the reaction of L=N2. Protonation of
coordinated dinitrogen is not without precedent, partic-
ularly at the {Mo(dppe)2} site. A variety of complexes
of the type trans-[Mo(NNH)(L%)(dppe)2]2+ (L%=N2,
EtCN, halide, etc.) [16] have been isolated and character-
ised. The discussion hereafter will focus around Fig. 2.
One feature which is common to the reactions of HCl
with all trans-[MoL(CNPh)(dppe)2] is the absorbance–
time behaviour. Consequently, it is worth outlining the
general characteristics of the absorbance–time curves
3.1. Protonation of trans-[Mo(CO)(CNPh)(dppe)2]
When studied on a stopped-flow apparatus, the reac-
tion between anhydrous HCl and trans-[Mo(CO)-

M.-C. Rosenblat, R.A. Henderson / Inorganica Chimica Acta 331 (2002) 270–278
273
Fig. 3. Stopped-flow absorbance–time curve for the reaction of trans-[Mo(N2)(CNPh)(Ph2PCH2CH2PPh2)2] with anhydrous HCl in thf at
25.0 °C, typical of that observed for all trans-[MoL(CNPh)(Ph2PCH2CH2PPh2)2]. The line at A=0.67 is the absorbance of trans-
[Mo(N2)(CNPh)(Ph2PCH2CH2PPh2)2]. The fit to the absorbance–time curve is shown superimposed on the data and is defined by the equation
At=0.294+0.07 exp(−8.0t)+0.07 exp(−0.9t).
(CNPh)(dppe)2] occurs in the three phases typified by
the absorbance–time curve in Fig. 3. The final ab-
concentration of HCl, but at high concentrations of
acid the rate of the reaction is independent of the
sorbance corresponds to that of the product [Mo-
concentration of HCl. Analysis of the curve yields the
H(CO)(CNPh)(dppe)2]+. The absorbance–time curve
corresponds to the second and third phases. Each phase
exhibits a different dependence on the concentration of
HCl.
rate law shown in Eq. (1), with a=(1.390.2)×102
and b=330920.
−d[Mo(CO)(CNHPh)(dppe)+2 ]
dt
The second phase exhibits a complicated dependence
on the concentration of HCl as shown in Fig. 4. At low
concentration of acid the reaction is first order in the
a[HCl][Mo(CO)(CNHPh)(dppe)+2 ]
=
(1)
1+b[HCl]
Fig. 4. Kinetic data for the acid dependence in the reaction between trans-[Mo(CO)(CNPh)(Ph2PCH2CH2PPh2)2] and anhydrous HCl in thf at
25.0 °C. The data corresponds to the fast phase (ꢀ) and the slow phase (ꢁ). The curve drawn is that defined by Eq. (1) with a=(1.390.2)×102
dm3 mol−1 s−1 and b=330920 dm3 mol−1
.

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M.-C. Rosenblat, R.A. Henderson / Inorganica Chimica Acta 331 (2002) 270–278
Both the absorbance–time behaviour and the rate
crystallography of the product isolated from the reac-
tion between trans-[Mo(CO)(CNPh)(dppe)2] and 1 mol.
equiv. of [Me2OH]BF4 in thf demonstrated that the
product is cis-[Mo(CO)(CNHPh)(dppe)2]+ [14]. In the
presence of an excess of acid, it is anticipated that
cis-[Mo(CO)(CNHPh)(dppe)2]+ will be converted to
trans-[MoH(CO)(CNPh)(dppe)2]+ by a series of steps
analogous to those described above for the trans-
isomer.
law in Eq. (1) are consistent with the second phase
occurring by the pathway shown in the centre of Fig. 2.
The absorbance change which occurs within the dead-
time of the stopped-flow apparatus indicates rapid,
initial protonation, presumably at the nitrogen atom of
the isonitrile ligand to form trans-[Mo(CO)-
(CNHPh)(dppe)2]+. The rate law of Eq. (1) indicates
that this cation is further protonated. It seems likely
that the second proton binds to the carbon of the
aminocarbyne ligand to generate trans-[Mo(CO)-
(CHNHPh)(dppe)2]2+. Certainly the analogous trans-
[Mo(CO)(CHNHtBu)(dppe)2]2+, containing the elec-
tron-releasing tBu group has been isolated [14].
Spectroscopic characterisation of trans-[Mo(CO)-
(CHNHtBu)(dppe)2]2+ indicates that an agostic hydro-
gen effectively bridges between the carbon and the
molybdenum, as shown in Fig. 2.
A key difference between the earlier synthetic studies
and the kinetic studies reported herein is that the latter
are performed in the presence of a large excess of acid.
It has been noted before that the presence of an excess
of acid facilitates the conversion of cis-[Mo(CO)-
(CNHPh)(dppe)2]+
(dppe)2]+ [14] (Table 1).
to
trans-[MoH(CO)(CNPh)-
3.2. Protonation of trans-[Mo(p2-H2)(CNPh)(dppe)2]
The reaction of trans-[Mo(CO)(CHNHPh)(dppe)2]2+
is completed by the hydrogen migrating from carbon to
The reaction of anhydrous HCl with trans-[Mo(h2-
H2)(CNPh)(dppe)2] to form trans-[MoCl(CNHPh)-
(dppe)2] shows an absorbance–time curve analogous to
that shown in Fig. 3. As before, the absorbance–time
curve can be fitted to two exponentials, corresponding
to the second and third phases. The kinetics of the
second phase are shown in Fig. 6, and described by Eq.
(1) with a=(1.590.2)×103 and b=520950. The
third phase is independent of the concentration of acid
(kHH=0.1090.05 s−1).
metal to produce trans-[MoH(CO)(CNHPh)(dppe)2]2+
.
However, the combination of protonation of the metal
and the CO ligand renders the aminocarbyne ligand
sufficiently acidic that the amino-proton is released to
produce, trans-[MoH(CO)(CNPh)(dppe)2]+.
Consideration of the mechanism described above in-
dicates that initial protonation of the isonitrile nitrogen
atom is enforced since the nitrogen atom is the most
basic site. However, it is the further (presumably
slower) protonation of the carbon which facilitates the
formation of the hydride product. By the same token, it
is the combination of the electron-withdrawing CO
ligand, and the second protonation which renders the
aminocarbyne ligand sufficiently acidic to release the
amino-proton at the end of the reaction.
In contrast to trans-[Mo(CO)(CNPh)(dppe)2], the re-
action of trans-[Mo(h2-H2)(CNPh)(dppe)2] with HCl
result in the dissociation of dihydrogen. Thus, although
the kinetics of the reaction of HCl with trans-[Mo(h2-
If protonation of the carbon atom in trans-
[Mo(CO)(CNHPh)(dppe)2]+ is a rapidly established
equilibrium, prior to the rate-limiting transfer of the
proton from carbon to molybdenum, the rate law asso-
ciated with the pathway described above is given by Eq.
(2). Comparison of Eqs. (1) and (2) gives KC1 O=(3.39
0.2)×102 dm3 mol−1 and kC5 O=0.3990.03 s−1
.
Fig. 5. Proposed general trans-to-cis isomerisation of trans-[MoL(C-
NHPh)(Ph2PCH2CH2PPh2)2]+ (L=CO, H2 or N2).
−d[Mo(CO)(CNHPh)(dppe)+2 ]
dt
Table 1
KC1 OkC5 O[HCl][Mo(CO)(CNHPh)(dppe)+2 ]
Kinetic data for the reaction of trans-[Mo(CO)(CNPh)(dppe)2] (0.2
mmol dm−3) with anhydrous HCl in thf at 25.0 °C (u=420 nm)
=
(2)
1+KC1 O[HCl]
fast
obs
slow
obs
[HCl] (mmol dm−3
)
k
(s−1
)
k
(s−1
)
The mechanism associated with the third phase is less
certain, in part because of the simplicity of the kinetics.
That the rate of the reaction exhibits a first order
dependence on the concentration of complex but is
independent of the concentration of HCl (kCO=(1.59
1.0
2.5
5.0
10.0
20.0
30.0
50.0
0.11
0.19
0.22
0.29
0.34
0.37
0.40
0.010
0.012
0.010
0.012
0.020
0.020
0.015
0.2)×10−2 −1) is consistent with the isomerisation of
s
trans-[Mo(CO)(CNHPh)(dppe)2]+ to cis-[Mo(CO)-
(CNHPh)(dppe)2]+ as shown in Fig. 5. Earlier, X-ray

M.-C. Rosenblat, R.A. Henderson / Inorganica Chimica Acta 331 (2002) 270–278
275
Fig. 6. Kinetic data for the acid dependence in the reaction between trans-[Mo(h2-H2)(CNPh)(Ph2PCH2CH2PPh2)2] and anhydrous HCl in thf at
25.0 °C. The data corresponds to the fast phase (ꢀ) and the slow phase (ꢁ). The curve drawn is that defined by Eq. (1) with a=(1.590.2)×103
dm3 mol−1 s−1 and b=520950 dm3 mol−1
.
p-backbonding and so dissociation does not ensue. We
will see in the next section that dinitrogen is more
sensitive to p-backbonding than the dihydrogen ligand,
since dissociation of dinitrogen occurs from trans-
[Mo(N2)(CNHPh)(dppe)2]+.
H2)(CNPh)(dppe)2] are analogous to those of trans-
[Mo(CO)(CNPh)(dppe)2], the interpretation is quite dif-
ferent. The mechanism is shown in Fig. 2 in which the
initial stages are analogous to those described for trans-
[Mo(CO)(CNPh)(dppe)2]: protonation of the isonitrile
nitrogen atom (complete within 2 ms), and further
protonation at the carbon atom produces trans-
The mechanism outlined above for trans-[Mo(h2-
H2)(CNPh)(dppe)2] is consistent with the kinetics ob-
served in Fig. 6. If protonation of the isonitrile nitrogen
atom is complete within the dead-time of the stopped-
flow apparatus, and protonation of the carbon is a
rapidly established equilibrium (KH1 H) prior to rate-lim-
iting dissociation of dihydrogen (kH4 H) the rate law
shown in Eq. (3) can be derived, with KH1 H=520950
[Mo(h2-H2)(CHNHPh)(dppe)2]2+
.
Diprotonation of
the isonitrile ligand is sufficiently electron-withdrawing
that the dihydrogen ligand dissociates from the molyb-
denum in the rate-limiting step of the reaction. Subse-
quent rapid binding of chloride to the site vacated by
dihydrogen completes the reaction.
dm3 mol−1 and k4HH=2.990.3 s−1
.
In general, protonation of the isonitrile ligand
labilises trans p-acceptor ligands. Simplistically, forma-
tion of trans-[Mo(h2-H2)(CHNHPh)(dppe)2]2+ results
in electron density being pulled towards the ꢀCHNHPh
ligand. This has three effects on the bonding of the
trans-dihydrogen ligand: (i) increases s-donation from
dihydrogen to Mo; (ii) decreases p-backbonding from
Mo to dihydrogen, and (iii) increases HꢀH bonding
[17]. It is anticipated that whilst (i) would inhibit dihy-
drogen dissociation, both (ii) and (iii) would facilitate
dissociation; (ii) by weakening the bonding between Mo
and dihydrogen and (iii) by making the HꢀH distance
closer to that of free dihydrogen (i.e. more product-
like). That diprotonation labilises dihydrogen to disso-
ciation indicates that the dihydrogen ligand is more
sensitive to changes in (ii) and (iii) than it is to changes
in (i). Similar effects to the bonding of other p-acceptor
ligands (such as CO and dinitrogen) are expected. How-
ever, the CO ligand is less sensitive to the degree of
−d[Mo(h2-H2)(CNHPh)(dppe)+2 ]
dt
KH1 HkH4 H[HCl][Mo(h2-H2)(CNHPh)(dppe)+2 ]
=
(3)
1+KH1 H[HCl]
As with the studies on trans-[Mo(CO)(CNPh)-
(dppe)2], it is not entirely clear to what the third phase
corresponds. Again, we observe that the reaction is
independent of the concentration of acid (kHH=0.109
0.05 s−1). It seems likely that, as with the CO system,
the slow phase corresponds to rate-limiting isomerisa-
tion to the cis-isomer. Such isomerisation could be a
general phenomenon for trans-[MoL(CNPh)(dppe)2],
facilitated by protonation of the isonitrile ligand (Fig.
4). Formation of the electron-withdrawing aminocar-
byne ligand trans to a p-acceptor ligand results in a
species where two trans ligands are sharing the same

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M.-C. Rosenblat, R.A. Henderson / Inorganica Chimica Acta 331 (2002) 270–278
Table 2
sented in this paper. As before, the absorbance–time
curve can be fitted to two exponentials, corresponding
to the second and third phases. The third phase is
independent of the concentration of acid, (kNN=0.59
0.1 s−1). As before, we attribute this phase to the
isomerisation of trans-[Mo(N2)(CNHPh)(dppe)2]+ to
cis-[Mo(N2)(CNHPh)(dppe)2]+ and then the relatively
rapid substitution of the cis isomer.
The second phase is markedly different to that ob-
served with L=CO or H2 complexes. As shown in Fig.
7, increasing the concentration of HCl leads to a de-
crease in the rate of the reaction. Analysis of the data
results in the rate law shown in Eq. (4). It seems likely
that the identical absorbance–time behaviour, but dif-
ferent kinetics, observed with trans-[Mo(N2)(CNPh)-
(dppe)2] is because this complex contains a ligand (dini-
trogen) which is uniquely capable of being protonated.
Kinetic data for the reaction of trans-[Mo(h2-H2)(CNPh)(dppe)2] (0.2
mmol dm−3) with anhydrous HCl in thf at 25.0 °C (u=420 nm)
[HCl] (mmol dm−3
)
k
(s−1
)
k
(s−1
)
slow
obs
fast
obs
1.0
1.5
2.0
2.5
3.0
4.0
7.5
10.0
15.0
0.55
0.95
1.55
1.8
1.9
2.0
2.2
2.4
2.5
0.07
0.07
0.10
0.09
0.12
0.10
0.12
0.14
0.15
d-orbitals for bonding, and thus competing for the
p-electron density. By isomerising to the cis configura-
tion the complex relieves the conflict of two trans
p-acceptor ligands. It is proposed that in the presence
of an excess of acid, the cis-isomer subsequently forms
−d[Mo(N2)(CNHPh)(dppe)2]
dt
trans-[MoCl(CNHPh)(dppe)2]
by
a
mechanism
10.091.0[Mo(N2)(CNHPh)(dppe)2]
analogous to that of the trans-isomer shown in Fig. 2
(Table 2).
=
(4)
1+5295[HCl]
The pathway consistent with the kinetics is shown in
3.3. Protonation of trans-[Mo(N2)(CNPh)(dppe)2]
Fig. 2. As with all the complexes discussed herein,
initial rapid protonation of the isonitrile nitrogen atom
produces trans-[Mo(N2)(CNHPh)(dppe)2]+, within the
dead-time of the stopped-flow apparatus in the first
phase of the reaction. Protonation of the isonitrile
withdraws electron-density from the metal site thus
labilising the trans-dinitrogen towards dissociation.
However, as the concentration of HCl is increased the
kinetics indicates that a further protonation must occur
The reaction of HCl with trans-[Mo(N2)(CNPh)-
(dppe)2] gives trans-[MoCl(CNHPh)(dppe)2], the same
product as with trans-[Mo(h2-H2)(CNPh)(dppe)2]. Al-
though the absorbance–time behaviour is analogous to
that shown in Fig. 3, the kinetics of the reaction
between HCl and trans-[Mo(N2)(CNPh)(dppe)2] are
different to that observed with the other systems pre-
Fig. 7. Kinetic data for the acid dependence in the reaction between trans-[Mo(N2)(CNPh)(Ph2PCH2CH2PPh2)2] and anhydrous HCl in thf at
25.0 °C. The data corresponds to the fast phase (ꢀ) and the slow phase (ꢁ). The curve drawn is that defined by Eq. (4).

M.-C. Rosenblat, R.A. Henderson / Inorganica Chimica Acta 331 (2002) 270–278
277
Table 3
Previously, an extensive series of studies on the reac-
tions of acid with trans-[MoL2(dppe)2] (L=C2H4,
MeCCH, N2, RNC or 2H) [1–10] have defined the
rates and initial sites of protonation of these complexes,
together with the subsequent steps which result in the
movement of the proton between ligand and metal. The
pathways are summarised in Fig. 1. Some general fea-
tures concerning the rates of protonation have emerged.
Thus, the rates of protonation vary with the site being
protonated and in general protonation at the metal or a
carbon site is slower than protonation of stereochemical
lone pairs of electrons on nitrogen, oxygen and halogen
atoms. The barrier to protonation of carbon sites is
usually attributed to rehybridisation whilst the barrier
to protonation of metal sites is primarily the necessary
reorganisation of the ligands. In contrast, protonation
of lone pairs of electrons in thermodynamically-fa-
vourable reactions are usually diffusion-controlled.
Currently, it is not possible to predict whether initial
protonation occurs at metal or ligand, or how transfer
of the proton occurs between the initial and final sites.
Both intramolecular and acid–base-catalysed mecha-
nisms for proton transfer have been identified. Some
specific features concerning these rearrangement reac-
tions are evident for complexes of the type trans-
[MoL2(dppe)2]. Only when L=MeCCH [8,9] is
protonation directly at the ligand and no subsequent
movement to another site (the metal) occurs. In all
other cases the kinetics indicate a more complex path-
way. When L=C2H4, or N2 the initial site can be either
the metal or the ligand depending on the nature of the
co-ligands and the concentration of acid. Thus, at low
concentrations of acid protonation of trans-[Mo(h2-
C2H4)2(dppe)2] [6,7] occurs preferentially at the coordi-
nated ethylene, to form trans-[MoEt(h2-C2H4)-
(dppe)2]+, and subsequent intramolecular b-hydrogen
transfer to the metal produces [MoH(h2-C2H4)2-
(dppe)2]+. Only at high concentrations of acid does the
rate of the direct protonation of the metal become
significant.
Kinetic data for the reaction of trans-[Mo(N2)(CNPh)(dppe)2] (0.2
mmol dm−3) with anhydrous HCl in thf at 25.0 °C (u=420 nm)
[HCl] (mmol dm−3
)
k
(s−1
)
k
(s−1
)
slow
obs
fast
obs
2.5
5.0
10.0
20.0
30.0
50.0
10.1
8.0
6.5
4.5
4.2
3.3
0.5
0.6
0.5
0.4
0.5
0.5
which suppresses this lability. Clearly, protonation at
Mo or the aminocarbyne ligand would further labilise
dinitrogen, since protonation at these sites would with-
draw electron density and diminish the p-backbonding
to dintrogen. It seems likely that the coordinated dini-
trogen is protonated to generate trans-[Mo(NNH)-
(CNHPh)(dppe)2]2+. Simplistically, protonation of the
dinitrogen ligand increases the Mo-to-nitrogen p-back-
bonding, effectively annulling the electronic effect of
the aminocarbyne ligand and making the diazenide a
less labile ligand than dinitrogen.
Comparison with the other systems discussed in this
paper indicates that the protonation of the carbon of
the aminocarbyne ligand could be in competition with
the protonation of dinitrogen. It is anticipated that
protonation of the dinitrogen would be rapid, possibly
diffusion-controlled [1,2,4]. However, earlier studies
have shown that protonation at carbon sites can be
appreciably slower than at the stereochemical lone pair
of nitrogen atoms [1,2].
If it is assumed that the initial protonation of the
isonitrile ligand is rapid and complete within the first
phase, and the protonation of the dinitrogen ligand is a
rapidly established equilibrium compared to the rate
limiting dissociation of dinitrogen from trans-
[Mo(N2)(CNHPh)(dppe)2]+ the resulting rate law is
that shown in Eq. (5), with kN3 N=10.091.0 and
KN2 N=5295 dm3 mol−1. It is worth noting that the
value of KN2 N is an order of magnitude smaller than the
values of KH1 H or K1CO consistent with KN2 N correspond-
ing to protonation at a different site (Table 3).
In contrast, protonation of dinitrogen ligands is in-
variably faster than protonation of the metal, but even
here complications can arise. In the reaction of HCl
with trans-[Mo(N2)2(depe)2] (depe=Et2PCH2CH2PEt2)
the first product isolated is [MoH(N2)2(depe)2]+ appar-
ently indicating preferential protonation at the metal
[18]. However, it seems more likely that protonation at
dinitrogen is more rapid than protonation of molybde-
num. The reason the hydride is the first species isolated
is because formation of trans-[Mo(NNH)(N2)(depe)2]+
is readily reversible whereas the protonation of the
metal is not. Thus, isolation of [MoH(N2)2(depe)2]+ is a
consequence of thermodynamics rather than kinetics.
At higher concentrations of acid, [MoH(N2)2(depe)2]+
reacts to ultimately form trans-[Mo(NNH2)Cl(depe)2]+
−d[Mo(N2)(CNHPh)(dppe)+2 ]
dt
kN3 N[Mo(N2)(CNHPh)(dppe)+2 ]
=
(5)
1+KN2 N[HCl]
3.4. Protonation of molecules bound to {Mo(dppe)2}
site
It is important to put the results presented in this
paper into context. In particular, to compare the proto-
nation chemistry proposed herein within that of other
complexes containing the robust {Mo(dppe)2} site.
by a pathway involving [MoH(NNH)(N2)(depe)2]2+
.

278
M.-C. Rosenblat, R.A. Henderson / Inorganica Chimica Acta 331 (2002) 270–278
With trans-[Mo(CNR)2(dppe)2] initial protonation oc-
limit for kH3 H50.1 s−1, and the data in Fig. 7 shows that
kN3 N=10.0 s−1. Thus, dinitrogen is at least 100 times
more labile than dihydrogen in these complexes. At first
sight this appears to be surprising (and counter-intuitive)
however, comparison with other systems indicates that
it may be not so unusual. Studies on the labilities of
L=N2 and L=H2 in [WL(CO)3-{P(C6H11)3}2] show
curs at the isonitrile to produce trans-[Mo(C-
NHR)(CNR)(dppe)2]+. However, the proton ultimately
moves to the metal by a mixture of intramolecular
rearrangement pathway and an acid-catalysed route,
presumably involving [MoH(CNHR)(CNR)(dppe)2]2+
[5].
NN/kHH=0.16 [20]. In only one other study has the
Studies on the protonation of [MoH4(dppe)2] show
that diprotonation occurs to produce a species [10] which
must contain at least one dihydrogen ligand, such as
[MoH4(h2-H2)(dppe)2]2+. The mechanism of formation
of this species could either involve direct protonation of
a hydride ligand (as observed in [WH4(PMePh2)4] [19])
or initial protonation of the metal followed by in-
tramolecular coupling of the hydride ligands [17].
More recently, complexes of the type trans-
[MoLL%(dppe)2] have been reported and allow the study
of the preferential protonation site between two basic
ligands, giving further insight into the factors controlling
protonation at different sites bound to the same metal.
Recent studies on trans-[Mo(N2)(NCC6H4R-4)(dppe)2]
[11] showed that although protonation at the carbon of
the nitrile ligand was the initial site of protonation, the
rate of protonation reached a maximum with k=1×104
dm3 mol−1 s−1, irrespective of the electron-releasing
capability of R. The electron releasing capability of R
acts in opposition to the p-backbonding from Mo to
nitrile leading to a compensatory effect. However, the
electron-releasing effect of R reinforces the p-backbond-
ing from Mo to dinitrogen. Thus, with very electron-re-
leasing nitriles protonation at the dinitrogen is observed.
The important point is that the product of the reaction
is controlled by a combination of kinetic and thermody-
namic factors.
In this paper, we have explored the protonation of
trans-[MoL(CNPh)(dppe)2] (L=N2, CO or H2). Several
general features have emerged from this work. (i) The
initial site of protonation is always the isonitrile ligand.
(ii) When L=CO, formation of [MoH(CO)(CNPh)-
(dppe)2]+ occurs via the diprotonated species trans-
[Mo(CO)(CHNHPh)(dppe)2]2+ and the analogous
trans-[Mo(h2-H2)(CHNHPh)(dppe)2]2+ is necessary to
labilise the dihydrogen ligand to dissociate. (iii) Protona-
tion of trans-[Mo(N2)(CNPh)(dppe)2] shows that proto-
nation of the isonitrile ligand occurs preferentially, and
only at higher concentrations of acid does protonation
of dinitrogen become detectable. (iv) Surprisingly, al-
though diprotonation of trans-[Mo(h2-H2)(CNPh)-
(dppe)2] is necessary to labilise dihydrogen, only
monoprotonation of trans-[Mo(N2)(CNPh)(dppe)2] is
necessary to labilise dinitrogen. This last point deserves
further comment.
k
effect of protonation on the relative labilities of dinitro-
gen and dihydrogen ligands been quantified. In the
reaction of acid with [MoH4(dppe)2] [10] although dipro-
tonation is necessary to labilise the system towards
dihydrogen dissociation, a limit for the rate of dissocia-
tion of dihydrogen from [MoH5(dppe)2]+ can be esti-
mated, kHHB1×10−2
dinitrogen from the analogous [MoH(N2)2(dppe)2]+ has
been measured [4] kNN=2×10−2 −1, giving kNN
s
−1. The rate of dissociation of
s
/
kHH\2. It appears that protonation of a metal site
labilises dinitrogen ligands more than dihydrogen. The
origin of this effect is still obscure but it seems likely that
it is a consequence of the subtle effects protonation has
on the s- and p-bonding between ligand and metal.
References
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references therein).
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references therein).
[4] R.A. Henderson, J. Chem. Soc., Dalton Trans. (1982) 917.
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(1992) 441.
[20] K.Z. Lang, A. Gonzalez, C.D. Hoff, J. Am. Chem. Soc. 111 (1989)
3627.
The results presented in this paper indicate that
dinitrogen is more labile to dissociation at the trans-
{Mo(CNHPh)(dppe)2}+ site than is dihydrogen. From
the data presented in Fig. 6 we can calculate an upper