_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Transition metal carbonyls
bowever, tbat bond lengtbs do not provide a sensitive measure of electron
distribution witbin tbe bond, infrared spectroscopy being mucb more informative.
Tbe M-C-0 system in terminal carbonyls is approximately linear; tbe
angles at carbon vary from about 16 5 o -180°. In tbe gas pbase intramolecular
interactions between tbe M -C-0 unit and otber ligands may be responsible for
small deviations from linearity. In tbe solid crystal packing forces may also
contribute.
Tbe C-0 bonds in bridging carbonyl are somewbat longer tban in terminal
ligands. In Fe 2 (C0) 9 (doubly bridging CO) tbe average distance is 1.176 A and in
Rb 6 (C0) 16 (triply bridging) 1.201 A. Tbe M-C-M angle in a symmetrical
bridge is ratber acute, falling in tbe range 77-90°, compared witb about 120° in
a ketone. Bridging carbonyls are less common in complexes of tbe beavier
transition elements tban in tbose oftbe First Transition Series. It bas been argued
tbat tbe bridging mode is less favoured wben tbe ligand bas to span tbe larger
metal-metal distance wbicb obtains in tbe former compounds. In any case, tbe
energy differences between bridging and terminal bonding modes are ratber
small (e.g. Co 2 (C0) 8 , p. 154). so it is difficult to predict which will occur in any
particular case.
M-C bond distances are always significantly shorter to terminal than to
bridging carbonylligands. There is a general parallel between M-CO(terminal)
and tbe atomic radius oftbe metal. Tbe radii oftbe corresponding elements at tbe
beginning oftbe Second and Tbird Transition Series are almost equal. on account
of tbe contraction wbicb occurs across tbe lantbanides. Consequently tbe Mo­
CO and W-CO bonds in tbe bexacarbonyls Mo(C0) 6 and W(C0) 6 are botb
2.06 A in lengtb.
5.2.7 Metal carbonyls-Preparation
(a) DIRECT REACTION. The first binary carbonyl, Ni(C0) 4 , was reported by
Mond, Langer and Quincke in 1890. This volatile material is formed wben carbon
monoxide is passed over metallic nickel at or just above room temperature. It
decomposes readily on warming. The Mond process for the extraction and
refining of nickel depends on these reactions. Crude nickel oxide mixed witb
oxides of iron and copper is obtained by roasting tbe sulpbide ore. Finely divided
metal is produced by beating in bydrogen, wbicb is obtained from water gas
(H 2 + CO), and tbe crude product reacted witb CO at socC/1 atm to give volatile
Ni(C0) 4 • This is then passed over pellets of pure nickel at 230 C when it
decomposes to metal of 99.97% purity, leaving CO to be recycled. A recent
modification of tbe process wbicb is operated in Canada employs sligbtly bigber
temperatures (130°C) and pressures (70atm) for the carbonylation. Under these
conditions a mixture of Ni(C0) 4 and Fe(C0) 5 is produced, which is separated by
fractional distillation or decomposed directly to give an iron/nickel alloy. Nickel
carbonyl is very toxic and strongly carcinogenic. It is present in low concentrations
in tobacco smoke.
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