John E. Ellis Minnesota
IJnI~ersihl . ..,of -
Mmneapolis. Minnesota 55455 -
I I
The Teaching of Organometallic Chemistry to Undergraduates
The chemistry of metal carbonyls and related compounds containing d-block transition metals in low formal oxidation states is very diverse ( I ) . However, because of the strong tendency for the metals in these compounds to achieve a closed shell configuration, i t is possible to make very convincing analogies between the chemistry of many of these species and that of corresponding non-metallic main group compounds in which therk is a similar tendency for the constituent elements to achieve a closed shell configuration. Thus, the 18-electron rule, when applied with discretion (2),i s as important in predicting and rationaking the stoichiometries, structures, and reactivity patterns of transition metal compounds containing CO, PF3, and other s-acceptor lig&ds (3) as is the octet rule for compounds containing nonmetals. In order to compare transition metal and nonmetal groups which have the same electronic requirements i t is useful to introduce the concept of electronically equivalent groups (4, S).' The latter are defined as those species which require the same number of electrons to attain a closed shell configuration. Representative examples of these are given in Table 1. The close stoichiometric and structural similarities in the main group and transition metal analogs indicated in Table 1arise naturally from the equivalent electronic requirements of these species. Perhaps the most striking similarity shown is the ability of tricarhonyliridium, a 15-electron species which requires three electrons to achieve a closed shell configuration, to form a molecule containing a tetrahedral array of Ir(COh , .. units. In this manner each iridium atom thereby attains a closed shell configuration just as phosphorus, a;senic. or antimonv do in forming PA,Asd, or Sbr, respectively. ~ h u sthe , 1r(&)3 unit may be said to mimic a pnicogen atom in fulfilling its valency requirements. More significant from the standpoint of descriptive chemistry, however, is the fact that many of these stoichiometrically related main group and transition metal species also have strikingly similar reactivity patterns. For example, the parallels in the chemistry of metal carhonyl dimers, such as [Mn(CO)sJz,which contain two 17-electron metal units and halogens, and especially pseudohalogens, which contain two 7-electron units, are sufficiently far-reaching that i t is useful to classifv the former as transition metal pseudohalogens. Also, available evidence indicates that 16and 15-electron metal containing arouDs.. ex.. - . Fe(C0h and classified as tranCo(C0)3, respectively, may be sition metal pseudochalconides and pseudopnicogenides. The latter terms apparently have not been used previously and are derived from chalcogen (chalk former) and pnicogen (suffocating) which are group names for the VIa and Va family of elements, respectively. The above relationships are very useful in rationalizing manv known reactivity. patterns of electronically equiva. lent main group and transition metal species; however, it is important to recognize, at the outset, that there are two imnortant in this scheme. The first is that oreanor - - ~ -limitations - ~ ~ " transition metal species containing carbon monoxide, phos~~~~~
~~~
~
~
'
The general idea of electronically equivalent groups was first introduced by J. Halpern ( 4 ) ; however, the first to use this term was apparently L. F. Dahl(5). 2 / Journal of Chemical Education
Tabla 1. Representative Exampler of Electronically Equivalent Main Group and Transition Metal Species Elecfrom Needed
~ a iG n roup Atom and
Transition Metal Analogue I and Mn(CO1, S a n d Or(CO1, IriCO),
for Closed Shell
confiCorrarponding Anionic Forms
gurafion 1 2
b p r e r d n t ~ yknown
!-and Mn(CO1,5'-and Or(CO1,'-
&ly
[Ir(CO ,,,)
Corresponding Neutral Forms
b
12and [ M n C O l , I , SF and ror(co],l, P,and I,r(COl,l,
in t h e f o r m of derlvatwer.
phines, phosphites, olefins, and similar *-acceptor ligands can lose such ligands readily to form coordinatively unsaturated molecules which have different electronic requirements than the original species. This and related reactions are extremely i m p & n t in the ability of these substances. especially metal carbonyls, to function as effective reagents in-organic synthesis ( 6 ) ;As a result of the lability of carbon monoxide groups in these compounds, whenever two or more metal carbonyl groups are hound to an atom or ion there is a strong tendency for a condensation process t o occur which effectively results from intramolecular displacement of carhonyl groups. The formation of metal carhonyl cluster compounds, which have no precedence in main group chemistry, is the net result of such an interaction. For example, although PhSn(Co(CO)& analogous to PhSnX3 (X = halide or pseudohalide group), is a well defined molecule, attempts to prepare the unknown carbon analog, PhC(Co(CO)a)s, result in the formation of a cluster species PhC(Co(C0)3)3. Effectively, in this reaction, the Co(C0)a group, of pseudohalogen character, by losing CO has changed to a Co(C0)3 group, of pseudopnicogen character. In the case of germanium both PhGe(Co(C0)d~ (III), PhGe(Co(C0)s)s (I), and an intermediate PhGeCos(CO)11 (11) are known and may he interconverted as follows (7)
The second important limitation is that organotransition metal species, unlike their main group counterparts, do not exist as stable entities when the transition metal has more electrons than i t requiresto attain a closed shell configuration (3). Cobaltocene and nickelocene are two notable exceptions to this rule. They are both highly reactive and paramagnetic molecules which tend to formally lose their "excess" electrons in reactions. I n contrast, numerous second and higher row main group nonmetals and a few metals often undergo valence shell expansions in reactions, e.g. Pa,
+ CI,
+
Seventeen-Electron Organometalllc Groups Seventeen-Electron metal groups like their seven-electron main erouo. counteroarts are normallv found as dimeric species, are often volatile, unless they are ionic such as C O ~ ( C N ) ~ &and , have chemical reactivity patterns which are strikingly similar to those of the higher halogens, particularlv Iz and main -group Some pseudo. ~seudohalogens. . haloge& are known only in the form of neutral iadicals, e.g., bis(trifluoromethyl)nitroxide, (CF&NO (8)or monoanions, e.g., azide, N3-. and fulminate, CNO-. Similarly, there exist a few isolable but highly reactive metal carbonyl free radicals, e.g., V(C0)e and ICr(C0)5 as well as other species which are only known in the anionic form, e.g., N b ( c 0 ) ~ -and Ta(CO)6- (I). The principal characteristic reactions of the metal carbonyls which are also characteristic of main group pseudohalogens and halogens are as follows. Each is illustrated with representative examples. 1) Reduction by electropositive metals and other reducing agents to metallic salts and other metal carhonyl derivatives; i.e., like halogens, these metal dimers can behave as oxidizing agents.
-
2KCo(CO), C O ~ ( C O+) ~2K + 2Mg + 1,2-C,H,Br2 2BrMgFe(CO),C,H, + C,H, Zn + Mn,(CO),, Zn(Mn(CO),),
+ Mn2(CO),,
[MnfC0),I2
+ (SCN),
(9) (10)
(11) (12)
2[fCjH,),Cot1I-
(13)
2CJ3s(C0)2FeMn(CO)5 (15) ZNCSMn(CO), (16)
-
+
(17) (13) (13)
-
+
3) Disproportionation by Lewis bases.
main group analogs Br,
+ HOH
H+ +
HrO
+
+ Br-
{[H,OBr]Brl =+ HOBr H,O+ Me,N + [Me,NCI]CI CI,
+
(21) (22)
The Lewis base induced disproportionation of carbonyl dimers emphasizes the close similarity in the chemistry of these species and halogens. Although Lewis base attack of neutral carhonyl dimers can also result in substitution reactions, with hard bases such as those containing oxygen or nitrogen donor functions, disproportionation is favored over substitution. eaceot with ReK!Oim. .'There appear to be nb reports of 1.ewis base mdueed disproportionnrion of the latter substance, presumably because of the dXfificulty oicleaving the Re-Re bond. The disproportionation of halogens in neutral or basic aqueous solution to give hypohalous acid (or hypohalites) and halide ion is often cited as an important criterion for a substance to he considered a pseudohalogen (23). If the initial attack of water (or hydroxide ion) on hromine is analogous to that of trimethylamine on bromine, it can be represented by the following scheme
Such an initial step seems reasonable in that the existence of HzOBrf, B hmm~xoniumion, has been confirmed in acidified aqueous solutions of HOBr (21). Water does attack C O ~ C O ) ~ , beit slowly, because the earbonyl is practically insoluble in this medium, but only salts of the type [Co(HnO).][Co(CO)dz have been isolated (24). However, the presumed initial intermediate in this reaction is CO(CO)IOH~+or CO(CO)IOH (20). The latter species can he compared with HOI, also an extremely unstable substance which readily disproportionates to iodide and iodate (13). The detection of thermally unstable salts of the type [Co(CO)bROH][Co(CO)dgenerated from the interaction of alcohols (20) with C d C 0 ) 8 provides additional support that water as well as other hard Lewis bases interact with Coz(CO)a to initially form salts of the type [Co(CO)~B][Co(CO)~] (18). Although other neutral csrbonyl dimers are less reactive than C O ~ ( C Oexcept ) ~ , far RedCOh the" are also thoueht -. " to interact with hard Lewis bases to give similar salts. 4) Hydrogenation
+-
Co,(CO),
+ H,
[C&,Cr(COhL
3 UOCC, CO, H,
+ Ha
+
Just as neutral carhonyl dimers may be considered to be transition metal pseudohalogens, the corresponding carhonyl monoanions
+
[h(W),I, + I, ~m&o), main group analogs CIS Ix + 2IC1 I, (SCN), 2ISCN
+
+ [CjHjCr(W)J2 2[iCSH,),Co+][C,H,Cr(C0).,-] main group analog 2(C-,H,),Co + I* XC,H,So
[C,H,Fe(COXL
+
PCI,
Thus, there are many main group compounds such as SbFc-. - , (.C H-.~-I S O-.I .SO?. -. IF?. -. and SCL which have n o counterpart in organotransition metal chemistry. This observation serves to emohasize the i m ~ o r t a n c eof the 18-electron rule in rationalizkg the stoichiimetry and the reactions of orrranotransition metal species (2, 3). Indeed, there Dresen
