In the Classroom
Electron-Counting Rules for Transition Metal–Nitric Oxide Complexes Anthony W. Addison* Chemistry Department, Drexel University, Philadelphia, PA 19104-2875
Electron-counting schemes are used universally in chemistry, for rationalizing the constitutions of molecules and also for correlating or predicting molecular geometries. Nitric oxide (NO) binds strongly to many transition metal centers, and the resulting metal nitrosyl complexes exhibit stereochemistry that is variable with respect to the M–N– O angle, as well as exhibiting variations in reactivity and in the νN–O stretching vibration frequency. These variations arise because different types of metal centers exchange different amounts of electron occupancy with the nitric oxide’s molecular orbitals, complicated by the fact that the nitric oxide HOMO is of π* type—itself the bane of electron-counting schemes! Nitrosyl complexes usually receive only brief attention in contemporary inorganic texts (1–3); yet recent striking developments regarding the cellular biochemistry of metal nitrosyls have amplified their significance in human physiology (4). The relationship between reactivity patterns and structure makes it desirable for students to be able to perceive bases for MNO fragment stereochemistry and to predict it in the presence of other ligands of known type and number. In the simplest approach, NO is said in textbooks (1) to act as a 3-electron donor in organometallic systems, as in the conversion Fe(CO)5 + 2 NO → Fe(CO)2(NO)2 + 3 CO whereby the 18-electron rule would be obeyed At this point, this criterion would enable prediction of the constitution of the product and lead to the correct conclusion that the Fecoordination should be tetrahedral, but it does not offer any obvious insight into the stereochemistry of the coordinated NO’s themselves—that is, whether they are classifiable as “linear” or “bent”: M
N
O
M
The last reflects how an excess of electrons over the 18 accommodated by the metal will be dispersed onto the NO ligand. As an example, let us consider the nitrosyl formed by the mammalian oxygen-storage heme protein myoglobin in its functionally inactive or oxidized met-form (5). The iron(III) porphyrinate has four pyrrolic N–Fe bonds, a histidine imidazole nitrogen as fifth donor, and the sixth coordination site occupied by the NO, as in Figure 1. Focusing on the Fe-porphyrin core, we note that the Fe(III)/porphyrin dianion moiety is net monocationic, and of course remains so after NO addition. Obviously, we might first consider the possibility of the NO unit possessing its original neutral NO character. In that case, we are required to assign an oxidation state of +3 to the iron, which corresponds to a valence shell 3d5 formal configuration. These proposals are summarized in the first row for the iron(III) myoglobin–NO entry in Table 1. The iron atom possesses not only these 5 valence electrons, but also 8 from the porphyrin, 2 from the axial imidazole, and 2 from the NO. The fact that the total count of 17 violates the 18-electron rule should persuade us to seek an alternative formulation. Proposing (row 2 of Table 1) that the NO actually acquires NO + character and consequently assigning the iron as 3d 6-iron(II) ultimately yields a more tractable electron count of 18. Finally, the third possibility (NO {/Fe4+ ) is also rejected as a 16-electron system. A Lewis dot structure of NO places the odd electron on the nitrogen. In a Gillespie–Nyholm approach to molecules with XNO fragments, this electron acts as a “half lone pair”, which is stereochemically influential. Consequently, NO 2 is a bent molecule, and addition of an electron (NO 2{) builds a complete lone pair, causing a greater degree of bending, whereas removal of the electron (NO 2+) yields a linear molecule: O
N O
In our inorganic chemistry classes, we state the distinc^ O angle is large (> ~160°) tion according to whether the M N or small (< ~160°). We address this feature by constructing a decision table, which enables ready comparison of the various alternative formulations for the metal nitrosyl so that a choice may be made of the most likely structure. This approach requires application of the following criteria: 1. The NO ligand, when coordinated, may be thought of as having NO+, NO (neutral), or NO { character.
O
N O
O
O
N
O
Analogously, in metal nitrosyls, M–(NO+) moieties are expected to be linear, but M–(NO) and M–(NO{) moieties bent (the latter more so); coordinated NO +, having lost the most π*-electron density, has the highest value for νN–O. In this light, it is instructive to enquire whether the Fe(II)–NO + formulation has any basis in reality and to discover that in heme nitrosyls formulable as in the iron(III) grouping in Table 1, the Fe–N–O moiety is indeed essentially linear (6, 7). By contrast, the rather better-known NO adducts of iron(II) deoxyproteins and model iron(II) porphy-
2. This in turn governs the value of the oxidation state assignment for the metal. 3. The NO ligand always acts as a 2-electron σ-donor, regardless of its above character, so we may complete these requirements with the statement that: 4. The 18-electron (18e{) rule applies to the metal center. *Email:
[email protected].
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N
Journal of Chemical Education • Vol. 74 No. 11 November 1997
Figure 1. Schematic of the iron coordination sphere in nitrosylmyoglobins. The porphyrin is represented by the equatorial set of macrocyclic N-donors, the lower axial N representing the proximal histidine imidazole nitrogen.
In the Classroom rins (Table 1) have bent Fe–N–O moieties (6, 8). Again, this is readily deduced by constructing a decision table like the above, which leads to Fe(II)–NO as the acceptable formulation. The similar transformation of linear MNO (179°) into bent MNO (135°) during the conversion of [Co(Diars)2(NO)]2+ into [Co(NCS)(Diars)2(NO)]+ (Table 1) by addition of thiocyanate is well known (9). For our original example, this procedure indicates that in Fe(CO)2(NO)2 , the nitrosyls should be linear, and this has indeed been demonstrated experimentally (10). Table 1 also correlates the known structures of some other examples of transition metal nitrosyls with their electron counts. Some nitrosyls are renowned for their defiance of easy comprehension. These include the platinum-group species [Ir(PPh 3)2(CO)Cl]+ and [Ru(a NO)(PPh3)2(eNO)Cl]+ in Table 1. For example, the NO+BF4 { adduct of Vaska’s complex
[Ir(PPh 3)2(CO)Cl] is expected by the above criteria to possess a linear IrNO unit, but it is quite bent. However, there is another feature of transition metal complexes with high d-subshell occupancies that we must take into consideration. The factor evident in the parent compound and in classical compounds as diverse as AuCl4 { and the π-allyl complex [(η 3-C3 H5)Pd(µ-Cl)2 Pd(η3-C3 H5)] is that these systems obey a 16e{ rule. In a ligand-field model, the 16e{ factor is associated with placement of just one of the d-orbitals at high energy so that other orbitals (such as on a ligand) become occupied at its expense, yielding what have been referred to (11) as “electron-rich” nitrosyls. This arises particularly for the d x 2-y 2 metal orbital when the stereochemistry is tetragonally pentacoordinate, and especially when the ligand-field splitting is large, as it is for 4d and 5d transition metals, as shown in Figure 2. We are thus led
Table 1. Formulation Alternatives for Some Transition Metal Nitrosyls
Iron(III) myoglobin-NO a
Iron(II) myoglobin-NO b
[Co(NO)(Diars)2]2+
a,c
[Co(NO)(Diars)2(NCS)]+
b,c
[Ni(NO)(Mphos) (P{OR}3)]+
a,d
[Rh(NO)(OSO)(PPh3)2] b,e
[Ir(NO)(PPh3)2(CO)] a,f
[Ir(NO)(PPh3)2(CO)Cl]+
b,g
[Ru(aNO)(PPh3)2(eNO)Cl]+
[Tc(NO)(S ? C10H13)3Cl] a,i
b,h
NO Form
M Ox. State
M Valence Config.
NO
+3
3d 5
12
17
Reject
NO+
+2
3d 6
12
18
OK
NO{
+4
3d 4
12
16
Reject
NO+
+1
3d 7
12
19
Reject
NO
+2
3d 6
12
18
OK
NO{
+3
3d 5
12
17
Reject
Net M e{ from Ligands e{-Count
Decision
NO+
+1
3d 8
10
18
OK
NO
+2
3d 7
10
17
Reject
NO{
+3
3d 6
10
16
Reject
NO+
+1
3d 8
12
20
Reject
NO
+2
3d 7
12
19
Reject
NO{
+3
3d 6
12
18
OK
NO+
0
3d 10
8
18
OK
NO
+1
3d 9
6
17
Reject
NO{
+2
3d 8
8
16
Reject
NO+
{1
4d 10
10
20
Reject
NO
0
4d 9
10
19
Reject
NO{
+1
4d 8
10
18
OK
NO+
{1
5d 10
8
18
OK
NO
0
5d 9
8
17
Reject
NO{
+1
5d 8
8
16
Reject
NO+
+1
5d 8
10
18
5d 8 ! Reject
NO
+2
5d 7
10
17
Reject
NO{
+3
5d 6
10
16
Accept
NO+
0
4d 8
10
18
4d 8 ! Reject
NO
+1
4d 7
10
17
Reject
NO{
+2
4d 6
10
16
Accept
NO+
+3
4d 4
10
14
Accept
NO
+4
4d 3
10
13
Reject
NO{
+5
4d 2
10
12
Reject
a
Linear M–N–O. b Bent M–N–O. c Diars = o-phenylenebis(dimethylarsine) (3, p 653). ^ = 178° (12 ). Mphos = o-phenylenebis(methylphenylphosphine); R = CH 3; Ni NO ^ = 140° (13 ). f Ir NO ^ = 174° (14 ). e Ph = phenyl; SO as a 4e { donor; Rh NO 2 ^ = 124° (1, p 574). g Ph = phenyl; Ir NO ^ h The equatorial eNO + is linear; tabulation is for axial a NO, Ru NO = 138° (1, p 574). ^ = 177° (15 ). i Tc NO d
Vol. 74 No. 11 November 1997 • Journal of Chemical Education
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In the Classroom
(a)
(b)
complexes will obey a 16 or 18e{ rule may still be moot, as we have not found any structurally characterized examples of such compounds in the literature (although if the NO π* and d x2-y2 orbitals are close in energy, Jahn– Teller distortion can cause the bending [6]). Finally, one might note that the 16/18e { rules, though they limit the coordination number in nitrosyls of the later transition metals, do not disallow stable structures with fewer than 16e {/18e {, as exemplified by the 14e{ Tc(III) molecule in Table 1. Note 1. This complex also happens to be trigonally pentacoordinate, as expected from the 18-e{ rule.
Literature Cited
Figure 2. A simplistic orbital layout for 18e{ metal nitrosyls. The split shell represents the metal 4d or 5d subshell in (a) octahedral (t 2g < eg in O h) and (b) square-pyramidal stereochemistries. At lowest energy lie ligand- and metal-derived σ-orbitals appropriate in number to our naive electron-counting rules. In (b), the C4v ligand field raises the dx 2-y 2 orbital above the NO π* level, so that the MNO unit becomes bent. In most stereochemistries, the dx 2-y 2 energy is lower, as in (a).
to a fifth criterion: 5. When a 4d or 5d transition metal nitrosyl is square pyramidal, it obeys a 16-electron rule.
We act on this by rejecting 18e{ counts if they are associated with a 4 d 8 or 5d 8 formal configuration ([Ir(PPh3 )2(CO)Cl]+ and [Ru(a NO)(PPh3)2 (eNO)Cl]+ entries in Table 1) and proceeding to the corresponding 16e { entry when it is feasible. Contrast, though, the entry for [Ir(NO)(PPh 3 ) 2 (CO)], a four-coordinate (pseudotetrahedral) Ir-nitrosyl, and the preceding entry for [Rh(NO)(OSO)(PPH3)2 ], for which no 16e{ count is accessible.1 The question whether square-pyramidal 3d 8-NO
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1. Douglas, B. E.; McDaniel, D. H.; Alexander, J. J. Concepts and Models of Inorganic Chemistry, 3rd ed.; Wiley: New York: 1994. 2. Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry, 2nd ed.; Freeman: New York, 1994. 3. Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry—Principles of Structure & Reactivity; Harper-Collins: New York, 1993. 4. Butler, A. R.; Williams, D. L. H. Chem. Soc. Rev. 1993, 22, 233–241; Lowenstein, C. J.; Dinerman, J. L.; Snyder, S. H. Ann. Intern. Med. 1994, 120, 227; Bredt, D. S.; Snyder, S. H. Annu. Rev. Biochem. 1994, 63, 175. 5. Ehrenberg, A.; Sczcepkowski, T. W. Acta Chem. Scand. 1960, 14, 1684. 6. Addison, A. W.; Stephanos, J. J. Biochemistry 1986, 25, 4104. 7. Wayland, B. B.; Olson, L. W. J. Am. Chem. Soc. 1974, 96, 6037. 8. Scheidt, W. R.; Piciulo, P. L. J. Am. Chem. Soc. 1976, 98, 1913. 9. Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry—Principles of Structure & Reactivity; Harper-Collins: New York, 1993; p 653. 10. Hedberg, L.; Hedberg, K.; Satija, S. K.; Swanson, B. I. Inorg. Chem. 1985, 24, 2766. 11. Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry—Principles of Structure & Reactivity; Harper-Collins: New York, 1993; p 651. 12. Rahman, A. F. M.; Salem, G.; Stephens, F. S.; Wild, B. S. Inorg. Chem. 1990, 29, 5225. 13. Moody, D. C; Ryan, R. R. Inorg. Chem. 1977, 10, 2473. 14. Brock, C. P.; Ibers, J. A. Inorg. Chem. 1972, 11, 2812. 15. De Vries, N.; Cook, J.; Davison, A.; Nicholson, T.; Jones, A. G. Inorg. Chem. 1990, 29, 1062.
Journal of Chemical Education • Vol. 74 No. 11 November 1997
