(3".Rev.
41
1905. 85. 41-49
The Transition-Metal-Hydrogen Bond RALPH 0. PEARSON D s p a m n f of Chmnlsby. I A h I f y of CaXfm!a. Sanfa Barbara. CaXromla 93108
Recshred MardB 1, 1984 (Revpped Manuscript R e c e h g d October 22. 1984)
Contents I. Introduction 11. Diatomic Hydrides 111. The Hydridic-Protmic Preference IV. Bond Energies In HML, V. HydrMic-Protonic Preference for HML. VI. Brwnsted Acuity VII. Influence of the Ligands on Acidity VIII. Homolytic Bond Energies and Brwnsted Acidity IX. Concluding Remahs
41 41 43 44 45 45 47 48 48
I . Introductbn There is great current interest in the transition-metal hydrides, both because of their unusual reactivity and because of their potential as homogeneous catalysts for hydrogenation and other reactions of organic substrates. Though several thousand such hydrides have been synthesized and their properties studied, there is still a dearth of basic information on metal-hydrogen bond energies.' This paper is an attempt at drawing together the available data and at making extrapolations useful in future work on hydrides. Actually there are three bond energies of interest. Writing HML. for a general transition-metal hydride, where L represents other ligands on the metal HML, H* + *MLn (1)
HML. HML,
--
+ ML,+ H+ + ML, H-
(2)
(3)
We have the homolytic bond energy, and the two possible heterolytic bond energies. The first of these is important in determining the thermal stability of the hydride toward hydrogen gas evolution. It also determines, in a reverse sense, the ability of the fragment ML, to cleave a hydrogen molecule to form the hydride. Also of considerable importance, the ease of reaction 1 determines the stability of the hydride to free-radical attack, with hydrogen atom abstraction. Such a freeradical attack can lead to very rapid substitution reactions of L for L', as Brown has ably demonstrated? HML, + L' HML,,L' + L (4)
-
free radical chain path Reaction 2 is important in determining the power of the metal hydride on a reducing agent, by way of hydride ion transfer. This leads to reactions such as the reduction of suitable ketones. It also determines the stability of the compound towards hydrogen gas evolution in the presence of protons, or proton donors. 0009-2885/85/07854041$08.50/0
L
Ralph G. Pearson was born in Chicago and received the Ph.D. degree tram NMlhwestem University' in 1943. Afler service In the Air Face during World War 11. he returned to Northwestem and began hk wa* on agank and iwganic reaction mechanisms. He has written several well-known books, including "Kinetics and Mechanism". with A. A. Frost and (later) J. W. Moore. "Mechanisms of Inorganic Reactions", with F. Basolo. "Some Aspects of Crystal Field Theory". with D. S. McClure and T. M. Dunn. "Hard and Soft Acids and Bases". and "Symmetry Rules lor Chemical Reactions". In 1976 he moved to the University of Califomia. Santa Barbara. where he is Professor 01 Chemistry. His current research interest is in the properties and reactions 01 transition-metal hydrues. Reaction 3, in which the metal hydride acts on a Brwnsted acid, is a fascinating and originally unexpected reaction. The reaction is often critical in determining the species which are actually present in solution. The anion ML,- is usually a good reducing agent, but by way of electron-transfer mechanisms. The anion is usually a good nucleophile, especially when strongly basic. Also reaction 3 is very important, in the reverse direction, hy making it possible to easily synthesize a large number of new hydrides. Many neutral organometallic compounds can be protonated to form cationic hydrides, as originally shown by Wilkinson.
ML,
+ H+
-
HML,+
(5)
11. Diatomic Hydrides The simplest systems with metal-hydrogen bonds would be the diatomic MH molecules. These are transient species, usually formed in the gas phase and studied spectroscopically. Some homolytic bond energies, Do, are known, hut it is instructive to first examine bonds to hydrogen formed by the representative elements. It is found that average E H bond energies, such as those tabulated by Pauling? are large when E is an electronegativeelement and small when E is of low electronegativity. For example Dois 135 kcal/mol for 0 1985 Amwican Chemical Soclehl
Chemical Reviews, 1985, Vol. 85, No. 1
42
Pearson
TABLE I. Some Properties of the Transition Metals
'ri V Cr Mn Fe Co Ni Cu Zr Nb Mo Tc €tu Rh Pd Ag Hf Ta W Re
Os Ir Pt Au
dZs2 d3sz dks! d5sz d6s' d7s2 d8s2
d%' d's? d'sl d5s'
d5s2 d's' des1 d'Oso dL0s1 d2s2 d3s2 d4s2 d5sz d's' d7sL d9s' d'*ql
3.51 3.64 3.76 3.52 4.03 4.26 4.44 4.48 3.69 3.88 3.92 4.00 4.24 4.30 4.44 4.44 3.50 4.25 4.19 4.01 4.90 5.35 5.70 5.80
18.7
37
6.0 0
42d
48.8 19.8
36' 39f
45
10.0
4Y
71
0.6
68 66
72 3 5 . 102
ii
I
