COMMUNICATION
1
of 3 days. Furthermore, H NMR spectroscopic studies
indicate that dissociation is facile on the NMR time scale.
For example, while a solution composed of a mixture of
Mo(PMe3)3(η2-O2CBut)(η1-O2CBut)H2 and Mo(PMe3)3(η1-
O2CBut)2(OH2)H2 exhibits distinct signals for the hydride
ligands of each complex at -30 °C, the signals coalesce at
ca. 35 °C. Assuming that the mechanism for exchange
proceeds via rate-determining dissociation of water from
Mo(PMe3)3(η1-O2CBut)2(OH2)H2, the activation parameters
for this process are ∆Hq ) 18.0(6) kcal mol-1 and ∆Sq )
11(2) eu; the corresponding activation parameters for the
tungsten complex W(PMe3)3(η1-O2CBut)2(OH2)H2 are ∆Hq
) 18.6(3) kcal mol-1 and ∆Sq ) 10(1) eu.16
The coordination mode of water to a metal center has been
extensively investigated, both in classical coordination
compounds17 and in organometallic and hydride deriva-
tives.6,14 These studies indicate that a common feature of
coordinated water is that both hydrogen atoms participate
in hydrogen-bonding interaction. In accord with this observa-
tion, the coordinated water molecule within M(PMe3)3(η1-
O2CBut)2(OH2)H2 (M ) Mo, W) is hydrogen-bonded to both
carboxylate ligands with O‚‚‚O distances of 2.559(2) and
2.655(2) Å (M ) Mo) and 2.537(3) and 2.631(3) Å (M )
W). This hydrogen-bonding motif is similar to that in Cp*Rh-
(η1-O2CR)2(OH2) (R ) Ph,18 Me6) and a variety of other
derivatives19 for which the water and carboxylate ligands
adopt a fac disposition.20,21
Figure 1. Molecular structures of Mo(PMe3)3(η2-O2CBut)(η1-O2CBut)H2
and Mo(PMe3)3(η1-O2CBut)2(OH2)H2.
aspect of M(PMe3)3(η2-O2CR)(η1-O2CR)H2 is that one of the
oxygen donors of the bidentate carboxylate ligand may be
displaced by H2O to give the corresponding aqua-dihydride
complex, M(PMe3)3(η1-O2CR)2(OH2)H2.8 Mo(PMe3)3(η1-
O2CBut)2(OH2)H2 and W(PMe3)3(η1-O2CBut)2(OH2)H2 have
been structurally characterized by X-ray diffraction, as
illustrated in Figure 1 for the molybdenum derivative; the
Mo-OH2 [2.242(1) Å] and W-OH2 [2.224(2) Å] bond
lengths in the two aqua complexes are comparable.9
The structures of M(PMe3)3(η1-O2CBut)2(OH2)H2 are
noteworthy because there are no structurally characterized
neutral metal aqua-dihydrides listed in the Cambridge
Structural Database;10-13 several cationic derivatives are,
however, known.14,15
The formation of the aqua adducts is reversible, and
treatment of a solution of M(PMe3)3(η1-O2CBut)2(OH2)H2
with either molecular sieves or KH to remove water
regenerates M(PMe3)3(η2-O2CBut)(η1-O2CBut)H2; the coor-
dinated water may also be removed by exposing a sample
of M(PMe3)3(η1-O2CBut)2(OH2)H2 to vacuum for a period
While the presence of two hydrogen bonds is common to
the majority of metal-aqua compounds, there exist a variety
of coordination geometries for the water ligand. For com-
pounds in which the water is attached to a single metal center,
(7) Green, M. L. H.; Parkin, G.; Chen, M.; Prout, K. J. Chem. Soc., Dalton
Trans. 1986, 2227-2236.
(16) The calculated energies for dissociation of water from M(PMe3)3(η1-
O2CBut)2(OH2)H2 to give M(PMe3)3(η2-O2CBut)(η1-O2CBut)H2 (∆HSCF
) 17.7 kcal mol-1, Mo; ∆HSCF ) 16.7 kcal mol-1, W) are comparable
(8) Ammonia also reacts with M(PMe3)3(η2-O2CBut)(η1-O2CBut)H2. By
analogy to the aqua complexes, we postulate that the products are
M(PMe3)3(η1-O2CBut)2(NH3)H2, in which the ammonia is involved
in a hydrogen-bonding interaction with the carboxylate ligands.
However, the ammonia is not coordinated strongly and readily
regenerates M(PMe3)3(η2-O2CBut)(η1-O2CBut)H2 upon removal of the
ammonia atmosphere.
1
to the barrier for dissociation of water as determined by dynamic H
NMR spectroscopy.
(17) (a) Ferraris, G.; Franchini-Angela, M. Acta Crystallogr. 1972, B28,
3572-3583. (b) Chiari, G.; Ferraris, G. Acta Crystallogr. 1986, B38,
2331-2341. (c) Lutz, H. D. Struct. Bond. 1988, 69, 97-125.
(18) Kisenyi, J. M.; Sunley, G. J.; Cabeza, J. A.; Smith, A. J.; Adams, H.;
Salt, N. J.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1987, 2459-
2466.
(9) The average Mo-OH2 and W-OH2 bond lengths for complexes listed
in the Cambridge Structural Database are 2.18 and 2.14 Å, respectively.
(10) Cambridge Structural Database (Version 5.26). 3D Search and
Research Using the Cambridge Structural Database. Allen, F. H.;
Kennard, O. Chem. Des. Autom. News 1993, 8 (1), 1 and 31-37.
(11) The molecular structure of the octahedral aqua-dihydride Ru(Imes)2-
(CO)(OH2)H2 has been recently reported (see ref 11a), but it is now
recognized that the structure actually corresponds to a disordered
square-pyramidal hydroxy-hydride complex, Ru(Imes)2(CO)(OH)H
(see ref 11b). (a) Jazzar, R. F. R.; Bhatia, P. H.; Mahon, M. F.;
Whittlesey, M. K. Organometallics 2003, 22, 670-683. (b) Chatwin,
S. L.; Davidson, M. G.; Doherty, C.; Donald, S. M.; Jazzar, R. F. R.;
Macgregor, S. A.; McIntyre, G.; Mahon, M. F.; Whittlesey, M. K., to
be submitted for publication.
(19) (a) Albers, M. O.; Liles, D. C.; Singleton, E. Acta Crystallogr. 1987,
C43, 860-863. (b) Chaudhuri, P.; Stockheim, C.; Wieghardt, K.;
Deck, W.; Gregorzik, R.; Vahrenkamp, H.; Nuber, B.; Weiss, J. Inorg.
Chem. 1992, 31, 1451-1457. (c) Mieczynska, E.; Trzeciak, A.
M.; Zio´lkowski, J. J.; Lis, T. J. Chem. Soc., Dalton Trans. 1995,
105-109. (d) Chuan H.; Lippard, S. J. J. Am. Chem. Soc. 1998, 120,
105-113. (e) Ye, B. H.; Chen, X.-M.; Xue, F.; Ji, L. N.; Mak, T. C.
W. Inorg. Chim. Acta 2000, 299, 1-8. (f) Malik, K. Z.; Robinson, S.
D.; Steed, J. W. Polyhedron 2000, 19, 1589-1592. (g) Speier, G.;
Tyekla´r, Z.; To´th, P.; Speier, E.; Tisza, S.; Rockenbauer, A.; Whalen,
A. M.; Alkire, N.; Pierpont, C. G. Inorg. Chem. 2001, 40, 5653-
5659. (h) Warzeska, S. T.; Micciche`, F.; Mimmi, M. C.; Bouwman,
E.; Kooijman, H.; Spek, A. L.; Reedijk, J. J. Chem. Soc., Dalton Trans.
2001, 3507-3512. (i) Liu, G.-F.; Ye, B.-H.; Ling, G.-H.; Chen,
X.-M. Chem. Commun. 2002, 1442-1443.
(20) Complexes that exhibit a similar motif but with a trans arrangement
of carboxylate ligands and a mer arrangement with the aqua ligand
are also known. See: (a) Wu, X.-L.; Tong, Y.-X.; Chen, X.-M.; Mak,
T. C. W. Acta Crystallogr. 1998, C54, 606-608. (b) Kuznetsov, V.
P.; Yap, G. P. A.; Alper, H. Organometallics 2001, 20, 1300-1309.
(c) Jensen, D. R.; Schultz, M. J.; Mueller, J. A.; Sigman, M. S. Angew.
Chem., Int. Ed. 2003, 42, 3810-3813.
(21) For an example in which the aqua ligand also bridges two metals,
see: Coucouvanis, D.; Reynolds, R. A., III; Dunham, W. R. J. Am.
Chem. Soc. 1995, 117, 7570-7571.
(12) The complex [W(PMe3)4H2(OH2)F][F] has been reported, but the
complex is now recognized to be [W(PMe3)4H2F(FHF)]. See: Murphy,
V. J.; Rabinovich, D.; Hascall, T.; Klooster, W. T.; Koetzle, T. F.;
Parkin, G. J. Am. Chem. Soc. 1998, 120, 4372-4387.
(13) The complex [Os(PPri3)2(OTf)2(H2O)H2] has been reported, but the
X-ray data were not of sufficient quality to determine either the
presence or number of hydrides or to verify the identity of the
coordinated water on two of the four crystallographically independent
molecules. See: Kuhlman, R.; Streib, W. E.; Huffman, J. C.; Caulton,
K. G. J. Am. Chem. Soc. 1996, 118, 6934-6945.
(14) Luo, X.-L.; Schulte, G. K.; Crabtree, R. H. Inorg. Chem. 1990, 29,
682-686.
(15) Eremenko, I. L.; Rosenberger, S.; Nefedov, S. E.; Berke, H. Inorg.
Chem. 1995, 34, 830-840.
9638 Inorganic Chemistry, Vol. 44, No. 26, 2005