2 Recent Neutron Diffraction Studies of Metal-Hydrogen-Metal Bonds
Downloaded via YORK UNIV on December 2, 2018 at 12:28:03 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
JEFFREY L. PETERSEN—Department of Chemistry, West Virginia University, Morgantown, WV 26506 LAWRENCE F. DAHL—Department of Chemistry, University of Wisconsin, Madison, WI 53706 JACK M. WILLIAMS—Chemistry Division, Argonne National Laboratory, Argonne, IL 60439
Single-crystal neutron diffraction studies were performed for 5
+
Mo (η -C H ) (CO) (µ -H)(µ -P(CH ) ) and 2
5
5
2
4
2
2
3
the [Et N] and
2
+
4
-
[(PhP)N] salts of the [Cr (CO) (µ -H)] monoanion to obtain detailed structural information on the M-H-M bond. Results for the molybdenum complex provided the first evidence that a bridging hydrogen atom can be located symmetrically within a bent M-H-M bond even without crystallographically imposed symmetry. Neutron diffraction studies of the [Et N] salt surprisingly show that the bridging hydrogen atom is disordered between two symmetry-related sites and displaced ca. 0.3 Åfrom the crystallographic center of symmetry; the Cr-H-Cr bond angle is 158.9(6)°. The large, anisotropic thermal ellipsoid found for the bridging hydrogen atom in the [(Ph P) N] salt prevented an unambiguous interpretation of the Cr-H-Cr geometry at room temperature. 3
2
2
10
2
+
4
+
3
2
A
s a result of t h e recognized role of transition metal hydrides as reactive intermediates or catalysts i n a broad spectrum of chemical reactions such as h y d r o f o r m y l a t i o n , olefin isomerization, a n d hydrogénation, transition metal h y d r i d e chemistry has developed rapidly i n the past decade (1). Despite the increased interest i n this area, detailed structural information about the nature of hydrogen bonding to transition metals has been rather limited. This paucity of i n f o r m a t i o n p r i m a r i l y arises since, until recently, x-ray diffraction has been used m a i n l y to determine hydrogen positions either i n d i r e c t l y f r o m stereochemical considerations of the ligand disposition about the metals or directly from weak peaks of electron density i n difference F o u r i e r maps. T h e inherent l i m i 0-8412-0390-3/78/33-167-011/$05.00/0 ©
American Chemical Society
Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
12
TRANSITION M E T A L
HYDRIDES
tations of x-ray d i f f r a c t i o n methods prevent us f r o m g i v i n g a precise represen tation of the h y d r o g e n b o n d i n g i n these systems. T o extend our f u n d a m e n t a l understanding of the stereochemical aspects of m e t a l - h y d r o g e n interactions, a collaborative effort to examine the structures of a carefully selected n u m b e r of metal hydrides via single-crystal neutron d i f fraction methods has been initiated at Argonne National Laboratory.
Since the
neutron-scattering cross section for the hydrogen atom is of the same order of magnitude as those for the heavier transition metals, neutron diffraction provides the o p p o r t u n i t y to resolve not only the nuclear position but also to obtain meaningful thermal parameters for each hydrogen atom i n complexes containing m e t a l - h y d r o g e n bonds. In particular, we have been interested i n metal hydrides i n w h i c h the h y drogen atom occupies a b r i d g i n g position between at least two metal atoms.
To
date, two h y d r i d e systems c o n t a i n i n g m e t a l - h y d r o g e n - m e t a l bonds have been studied by neutron d i f f r a c t i o n at Argonne.
MO (N -C5H5) (CO)4(M2-H)(M22
5
2
P(CH )2) a n d the [Cr2(CO)io(M2-H)]~ monoanion were selected because they 3
represent two possible geometries for the b r i d g i n g h y d r o g e n atom i n a m e t a l h y d r o g e n - m e t a l bond.
F r o m earlier x-ray work, the M o - H - M o b o n d i n the
f o r m e r complex (2) was assumed to be bent whereas the C r - H - C r b o n d i n the latter (3, 4) was presumed to be the first example of a linearly protonated m e t a l - m e t a l bond.
Suitable crystals of both materials were obtained f r o m sat
urated solutions by slow solvent evaporation, and the necessary neutron diffraction data were measured w i t h the automated neutron diffractometer located at the C P - 5 reactor at A r g o n n e N a t i o n a l Laboratory.
C o m p u t e r analysis of the data
was c a r r i e d out using the I B M 3 7 0 / 1 9 5 system at Argonne.
Neutron Diffraction
Study of
Μο η
