Transition metal complexes

Imido-thiolato-complexes of vanadium of the type [V(Bu N)(SR)3] have been made by the reaction of [V(Bu N)Cl3] with Li[SR] (R = Bu , SiPhj) 18). Comproportionation reactions of the trithiolate with the trihalide precursor were then used to derive the related complexes 

A systematic investigation of the reactions of [MBr2(CO)4l (M = Mo, W) with sterically hindered thiolate ligands led to the series of five-coordinate 14-electron anions [M(CO)2(SR)3] (SR = TIPT, DIPT, TMT, PFTP) (26). This contrasts with the analogous reaction with thiphenol, which gives unstable polymeric species. An X-ray crystal 

The reaction of [MoBr2(CO)4] with the DPT anion (DPT = 2,6-di-phenylthiophenolate) differs from that with other 2,6-substituted aromatic thiolates. The stoichiometry [Mo(DTP)2(CO)] of the product initially suggested three coordination however, spectroscopic studies and an X-ray crystal structure revealed that one of the 2,6-phenyl groups is 7j -bonded to the molybdenum (Fig. 6). The 7j -arene ligand is labile and can be reversibly replaced by three CO ligands. Replacement of this ligand also occurs with 2,2 -bipyridyl (bipy), 1,10-phenanthroline 

The carbyne complexes [Mo(=CBu )(SAr)3] (SAr = TMT, TIPT) have been synthesized by adding 3 eq of Li[SAr] to [Mo(=CBu )Cl3(dme)] (dme = dimethoxyethane). The analogous W derivatives were made by a slightly modified route (32). The initial aim was to probe the acetylene metathesis catalytic properties of the complexes [M ( Bu KSArlg] (M = Mo, W SAr = TMT, TIPT). However, none of the complexes were active for metathesis, which was in contrast to the high activity of the analogous alkoxide compounds for metathesis. This was attributed to the stronger electron donation power of thiolate, which reduces the electrophilic nature of the metal center (32). 

The anionic complexes [MO(SR)]4 (M = Mo, W SR = TEMPT, TIPT) were readily prepared by the reaction of M0OCI4 or WOCI4 with 

Kinetically stable complexes can be used as linkers for solid-phase synthesis. The cobalt(III) complex shown in Entry 1 in Table 3.49 was prepared in solution and then loaded onto polystyrene. Less than 5% cleavage occurred upon treatment of this support-bound complex with TFA/DCM (1 1) for 12 h or with 20% piperidine in DMF 

Entry Loaded resin Cleavage conditions Product, yield (purity) Ref. 

The cobalt(O) complex shown in Entry 2 (Table 3.49) could be prepared either by heating a mixture of an alkyne cobalt carbonyl complex with polystyrene-bound tri-phenylphosphine, or by pretreating resin-bound triphenylphosphine with dicobalt octacarbonyl and then treating the resulting support with the alkyne. 

Most transition-metal cations can adopt several different oxidation states depending on the method of preparation and the compound in which they find themselves, but which oxidation state they adopt in a particular compound is not always clear from the chemical formula or from the nature of the bonding environment. Providing that the oxidation state is not zero, the bond valence model can help because the metal ligand bond can usually be described as an 

A second problem is the widespread occurrence of compounds in which several different anionic elements bond to the metal (heteroleptic compounds). Most inorganic compounds have homoleptic coordination, i.e. the cation is bonded to only one kind of anion, making the determination of the bond valence parameters relatively simple (Appendix 1). Flomoleptic coordination is much less common among the transition-metal complexes. As described in Appendix 1, bond valence parameters for a variety of transition metals to O, N, C, Cl, S, and P have been determined using heteroleptic systems although the values obtained are often less reliable than those obtained from homoleptic systems. 

In spite of these difficulties, there have been a number of determinations of bond valence parameters for use in transition-metal complexes. In most cases the bond valence parameters determined for oxides work well with transition-metal complexes, but care is needed when the metal can be found in different spin states or the ligand allows different degrees of tt bonding. 

In a free transition metal atom or ion (one that is not complexed with any ligands), the five different d orbitals in the 3d subshell are degenerate. 

Now consider the formation of an octahedral complex such as [N1(H20)J +. Think of six water ligands approaching the Nl + ion along the x-, y- and z-axes. The electrons in the d orbitals of the nickel ion that lie along the axes will be repelled by the electrons of the approaching ligands. 

As a result, these d orbitals now have a higher energy than the d orbitals that lie between the axes. Therefore the d orbitals are no longer degenerate. The d orbitals that lie on the axes are d,2 2 (a double dumbbell lying on both the x- and y-axes) and dy, which lies on the z-axis. The lower energy orbitals are the d, d, and d orbitals (double dumbbells that lie between the axes). 

We call this splitting of the d orbitals. The splitting is different in octahedral complexes compared with tetrahedral and other shapes of complexes. 

A short form of the spectrochemical series is CN in which the cyanide ion causes the greatest energy difference. 

Typical transition metal complexes investigated are summarized in Table 17.1 together with the polymerization conditions, the polymer properties, and the catalytic activities. 

Yang et al. examined rare earth coordination catalysts. The Nd(naph)3/ y4/(fBu)3 catalyst system was found to produce syndiotactic-rich polystyrene [8]. They proposed that the catalytically active species might be an ionic complex because the addition of CCI4 increased the catalytic activity. 

Compound Metal (mmol) MAO (mmol) Conversion (%r Stereospecificity polymerization conditions  

Recently, Wakatsuki and co-workers have shown that samarium compounds produce SPS with lower syndiotacticity than titanium compounds [9]. 

The polymerization activities of bis-cyclopentadienyl titanium compounds are lower than those of bridged bis-cyclopentadienyl titanium compounds. Miya-shita et al. reported the polymerization activities of several bridged bis-cyclopentadienyl titanium compounds [15]. They found that the catalytic activity of CH2(Cp)2TiCl2 is the highest among bis-cyclopentadienyl titanocene compounds. The data indicate that the polymerization activities and also syndiospecificity increase with a decreasing angle between the Cp centroid-Ti-Cp centroid in bis-cyclopentadienyl titanocene compounds. 

Many compounds bind to transition metals. These compounds—which can be negatively charged ions such as hydride or a hahde, or a neutral compound such as ammonia—are called ligands (Latin, ligare, to tie). A coordination complex consists of a metal atom or ion that is covalently bonded to one or more ligands. In the formation of a coordination complex, the metal ion acts as a Lewis acid and the ligands act as Lewis bases. A coordination complex in solution is in equilibrium with its component metal atom (or ion) and its hgands. 

The number of ligands that form coordinate covalent bonds in a transition metal complex is called the coordination number of the complex. The most common coordination numbers are 2, 4, and 6. The metal atom (or ion) and its bonded ligands constitute the coordination sphere of the complex. 

When a transition metal becomes covalently bonded to hgands, its charge is delocahzed across its ligands. For that reason, we do not refer to the central metal ion in a complex as having a charge instead, we refer to it as being in a certain oxidation state. 

The photochemistry of transition metal complexes is more varied than large organic molecules because of the possibility of different orbital types 

Each of the e promotional types of energy states can further split by jppin-orbital coupling interactions to give singlet and triplet states. For 

Because of the fast nonradiative deactivation of low lying energy states of transition metal complexes, the activation energy for the reactions that may occur from these states must be zero to enable them to compete effectively. For transition metal complexes both 4T2S and aEs states can be photochemically active but may follow different chemical pathways. 

Three fundamental types of photochemical reactions are known for coordination compounds (A) substitution reactions, (B) rearrangement reactions, and (Q redox reactions. 

Photosubstitution reactions can be aquation, anation or ligand exchange. (i) Photoaquation reactions of the type 

The potential formation of transition metal complexes with buckybowls has been of interest since a variety of transition metal complex units were reported to coordinate to buckminsterfullerene C o [54]. Interestingly, in the cases of Ceo complexes the metal is -coordinated to two carbon atoms shared between two 6-membered rings and no -coordinated haptomers were ever reported. 

In contrast, the successful formation of transition metal complexes of buckybowls turned out to be much more challenging. For some time there was only one report of an X-ray characterized buckybowl metal compound (61) the reaction of (Ph3P)2Pt(H2C=CH2) with semibuckminsterfullerene 20 [55]. In this case we 

Unfortunately, X-ray quality crystals of 64 could not be obtained and exo vs. endo (convex vs. concave) preference for the location of the transition metals could not be answered simply based on NMR data. 

Only very recently some crystal structure determinations of buckybowl metal complexes became available. Petrukhina and Scott reported successful preparation of molecular solids by gas-phase co-deposition of 1 with Rh2(02CCF3)4 which were characterized by X-ray [61]. The resulting solids consisted of ID and 2D networks of Rh2(02CCF3)4 and corannulene units with the [Rh2] fragments // -coordinated 

Similar types of molecular solids were very recently characterized by X-ray by Siegel and coworkers [62]. The co-crystallization of corannulene with various silver salts led to the formation of molecular networks in which the metal is rj -coordinated to the rim of 1. No statistically relevant deviations in the corannulene bond lengths of the complexes were found as compared to isolated 1 [62]. 

The rhenium complexes described in Section 11.2 have also been studied as electron mediators for C02 reduction at metal electrodes. Hawecker et al. used the complex Re(bpy)(CO)3Cl in DMF/water (9 1) at glassy carbon electrodes at a potential of-1.44V (versus SCE) to produce CO with 98% faradaic efficiency [15, 87]. Likewise, Sullivan et al. reported the production of CO with similar efficiency at a platinum electrode at -1.5 V (versus SCE) by using the complex fac-Re(bpy) (CO)3Cl [88]. Ruthenium complexes that have been used in photochemical 

Phosphanes possess an electron lone pair with which they can bond to a transition metal. As this bonding interaction results in the transfer of electron density from phosphorus to the metal atom, we would expect a downfleld shift in the P-NMR spectrum relative to the value for the free ligand that depends both on the r-donor strength of the phosphane, and the Lewis acidity of the transition metal fragment. 

Since most phosphane ligands also show some. -acceptor ability, coordination to a transition metal can result in r-backbonding that would increase the electron density on phosphorus, and thus result in an upheld shift of the signal in the P-NMR spectrum. Depending on the magnitude of the downheld shift due to the cr-donicity, and the upheld shift due to the. -acceptor strength of the phosphane, either a net downheld or net upheld shift is observed upon coordinahon of a phosphane to a transition metal. 

How can we estimate the sign and the magnitude of this expected chemical shift The chemical shift difference observed upon coordinahon to a transition metal is known as the coordination chemical shift, and dehned as  

We deduce from our knowledge concerning the electroihc properties of the d-block metals that the coordination chemical shift is likely to decrease going down the group. 

Phosphorus-31 NMR Spectroscopy, Springer-Verlag Berlin Heidelberg 2008 

The literature concerning the chemistry of transition-metal complexes containing 1,1-dithiolato ligands was extensively reviewed, up to 1968, by Coucouvanis (1). We attempt here to update that excellent account. 

Relatively few complexes of the early transition metals with 1,1-dithiolato ligands have been prepared and characterized. This is consistent with their classification as hard or class a acceptors. Thus, 

Modification of the acceptor properties of the metal atom may be achieved by using complexes containing rr-cyclopentadienyl ligands. 

a series of bis-cyclopentadienetitanium(III) dithiocarbamate and xanthate complexes have been prepared by Coutts et al. (49-51) by reaction of the sodium salts of the ligands with [Cp TiJCl (Cp = 7r-cyclo-pentadiene) in air-free water under an inert atmosphere the dithiocarbamate complexes are bright-green, and the xanthates are blue. 

Both types of complex are extremely air-sensitive and are paramagnetic, with one unpaired electron per titanium atom. Their formulation as monomeric, symmetrical, bidentate, chelate complexes, (IV) and (V), has been established from spectral, magnetic, and molecular-weight data. 

The vast majority of homogeneous catalysts are transition metal complexes and many systems have been reported, for example, Ru(III) [129], W(VI) [130], polyoxometallates [131], Re(V) [132], Fe(III) [133], and Pt(II) [134] with hydrogen peroxide, Mn(II) [135-137] with peracetic acid, and Ti-tartrate with alkyl hydroperoxides [75]. The subject of epoxidation by H2O2 has been reviewed [138-140]. 

A summary of typical homogeneous catalysts, oxidants used, conditions of use, conversions and yields based on the olefin reactant (imless specified), and TOF is provided at the end of this section (Table 1.6). It is lamented that in general, in the homogeneous epoxidation catalysis field, TOFs are not often reported, as more emphasis is placed on selectivity than on rate. Many times, the reactions are run with hydrogen peroxide as oxidant, in excess because of its tendency to decompose, and the addition is not controlled carefully. The reported TOFs are 

Space time yield (g ot epoxide per kg catalyst per hour). 

Venturello system [PWO4O24], [(Ci8H37)o,76 (Ci6H33)0.24N(CH3)2]+/H2O2 70 °C, CH2CI2, 1 h. Yield 88% (H2O2) yield 53%  

Busch system CHsReOs, pyridine-N-oxide/ H2O2 30 °C, 20 bar N2, solvent CH3OH TOP = 5.7 X 10 3 s  

Effect of a large zero-field splitting on a quartet system (S=3/2). The separation induced between the Afs= 1/2 and Ms= il2 states is so large that only a single EPR signal, due to the Af5= —1/2— 1/2 transition, is observed near g = 2.0. The energies of the Ms = —1/2 — -3/2 and 1/2 — 3/2 transitions are outside the observable liequency range. 

Consideration of the EPR spectra of transition-metal complexes is complicated by the fact that they usually have several approximately degenerate orbitals, and several unpaired electrons. These manifest themselves first in an orbital contribution to the magnetic moment, which leads to anisotropy in g factors, and secondly in zero-field effects, like those described in the last section. Isotropic spectra of transition-metal complexes are straightforward, and are described in Section 5.2. 

Splitting of the F ground state of a ion by octahedral and tetragonal fields, showing effects of zero-field and magnetic-field splittings on the lowest resultant state. 

Splittings of the ground state of a d ion by an octahedral ligand field, with Jahn-Teller distortion, showing the effect of 

Returning to molecular orbital diagram in Fig. 1.21, a series of features of special relevance for understanding metal transition complex chemistry are apparent  

Low-lying molecular orbitals always have ligand character, and in turn practically all those metal-like in character are antibonding molecular orbitals. For a central atom with electronic configuration d HOMO as well LUMO have metal character so the complex chemistry will normally occur at least initially at the metal atom. 

Color and magnetic properties which are characteristics of the chemistry of 

The transition metals occur in many interesting and important molecular forms. Species that are assemblies of a central transition-metal ion bonded to a group of surrounding molecules or ions, such as [Ag(NH3)2] and [Fe(H20)5], are called metal complexes, or merely complexes If the complex carries a net charge, it is generally called a complex ion. (Section 17.5) Compounds that contain complexes are known as coordination compounds. 

The molecules or ions that bond to the metal ion in a complex are known as ligands (from the Latin word ligare, to bind ). There are two NH3 ligands bonded to Ag in the complex ion [Ag(NH3)2], for instance, and six H2O ligands bonded to Fe in [Fe(H20)g]. Each ligand functions as a Lewis base and so donates a pair of electrons to form the ligand—metal bond. (Section 16.11) Thus, every ligand has 

Is the interaction between an ammonia ligand and a metal cation a Lewis acid-base interaction If so, which species acts as the Lewis acid  

TABLE 23.3 Properties of Some Ammonia Complexes of Cobalt(lll) 

Original Formulation Color Ions per Formula Unit Free Cl Ions per Formula Unit Modern Formulation 

In recent years Nakamoto (W) has pioneered in the use of Isotopes of the transition metals in order to make assignments of the vibrational bands of their complexes. ISy studying the spectra of °Cr(acac)s and Cr(acac)3, Nakamoto, Udovlch, and Takemoto ( ) were able to assign a band at h60 cm to a Cr-0 stretching mode and one at 592 cm to an out-of-plane ring mode. On the basis of 0 - isotopic substitution, the 592 

For tetrahedral XY4 species the totally symmetric Ai stretching mode involves no motion of the central atom, and hence should yield virtually no isotope shift when that atom is substituted. The trlply-degenerate F2 mode, however, should display an isotope effect. Thus Takemoto and Nakamoto (51) were able to assign bands in the Raman spectra of Zn(NI%)4 at T O cm and 1 10 cm to the Ai and F2 modes respectively. In this instance the isotopes Zn and Zn were used. The ordering of these two levels in this complex is somewhat unexpected, since in the vast majority of the XH4, tetrahalogeno, or XO4 species which have been examined the F2 band occurs at the higher frequency (52). 

As a final example of the use of isotopic substitution in the study of metal complexes, we cite the use of as a ligand by Collman, Famham, and Dolcetti ( ), who found what they termed hybridization tautomerism in several cobalt-nitrosyl complexes. From their infrared spectra they inferred a rapid equilibrium between a trigonal-bipyramidal Co(l) species having a linear Co-nitrosyl geometry, and a sqviare-pyramidal Co(lll) species, in which the Co-nitrosyl moiety is bent. 

Mich current interest in the spectroscopy of hydrogen bonded systems attaches to the question of how one might infer the shape of the potential function from the vibrational spectrum of the entity. In this connection Wood and his collaborators have recently made major contributions. They have examined the infrared and Raman spectra of a great number of cations of the form (bPB ), where B and B are nitrogen bases or perdeutero-nitrogen bases, and P is either hydrogen or deuterium. 

When B and B were both trimethylamines ( ) the NH stretching band and the ND stretching band were both singlets. The same behavior obtained also when B and B were trlmethyl-amine and pyridine (55)- When B=B =pyrldine ( ), or substituted pyrldines (5, and E=H, the HHt band was split into a 

The iodomethyl complex [CpFe(CO)2CH2I] (4) has been prepared by two routes (32). The first route involves the reaction of complex 1 with HI gas, as shown in Eq. (2). The second route involves the reaction of complex 3 with Nal [Eq. (3)]. The methoxymethyl complex (1) was isolated as an air-sensitive oil, whereas the halogenoalkyl complexes were isolated as air-and light-sensitive solids. The bromomethyl complex (3) was found to be less stable than the chloromethyl complex (2) in all respects, and the iodomethyl complex (4) was found to be even less stable. 

King and Braitsch prepared a number of halomethyl complexes of transition metals by reacting the appropriate transition metal anion with diha-lomethanes according to Eq. (4) (33). CH2C1I was used to prepare the chloromethyl derivatives of [MCH2X] directly. Since the C—I bond is weaker than the C—Cl bond, the C—I bond is preferentially cleaved in most cases. This method is a good synthetic route to the chloromethyl complexes of Mo, W, and Mn, but not for those of Fe. 

Complex 9 and [CpFe(o-BPXCO)CH 2C1 ] [o-BP = tri(o-biphenyl)-phosphite] were also prepared by the reaction of HC1 with [CpFe(L)-(CO)CH2OEt] (L = PPh3, o-BP) (47). Alkylation of the two complexes with sodium /crt-butyl acetoacetate and pyrroline cyclohexanone enamine yielded six of the eight possible alkylation products. The two products where L was o-BP and the nucleophile was tort-butyl acetoacetate did not form, presumably because of excessive steric hindrance. The excess of one diastereomer over the other ranged from 10 to 64%. 

Roper and co-workers reported that [0s(PPh3)2(C0)2(i/2-CH20)] proved to be a useful synthetic precursor for stable hydroxymethyl, methoxy-methyl, and halomethyl complexes (51,52). Thus [Os( PPh3)2(CO)2( /2-CH20)] reacts with excess HX (X = Cl, Br, or I) to yield the halomethyl 

Cr—C single bond. Thus, as observed for the iodomethyl iron complex [Fe P(OCHMe2)3 2(CO)2(I)CH2I] (see Section II,A), the CH2I carbon atom may have some sjp character (57). 

Some of the most important and commonly encountered compounds which involve the d orbitals in bonding are the transition metal complexes. The term complex in this context means that the molecule is composed of two or more kinds of species, each of which can have an independent existence. 

For example, the ions Pt2+ and CF can form the ion [PtCl4]2. To understand the hybridization scheme, it helps to start with the neutral Pt atom, then imagine it losing two electrons to become an ion, followed by grouping of the two unpaired 5d electrons into a single d orbital, leaving one vacant.This vacant orbital, along with the 6s and two of the 6p orbitals, can then accept an electron pair from four chlorines. 

All four Pt-CI bonds are equivalent and point to the corners of a square centered on the Pt 

All of the four-coordinated molecules we have discussed so far have tetrahedral geometry around the central atom. Methane, CH4, is the most well known example. It may come as something as a surprise, then, to discover that the tetrachlorplatinum (II) ion [PtCl4]2- has an essentially two-dimensional square-planar configuration. This 

Many of the most commonly encountered transition metal ions accept electron pairs from donors such as CN and NH3 (or lacking these, even from H20) to form octahedral coordination complexes. The hexamminezinc(II) cation depicted below is typical. 

Although the general features of photochemistry and photochemical reactions are applicable to metal complexes, there are some particular features that need to be considered when choosing a system to be studied, or when understanding why particular metal complexes are chosen for study. The majority of studies have been carried out with complexes of the transition metal series, which is a group of metal ions that have particular properties that need to be borne in mind when considering their photoreactivity. 

Transition metal compounds can be divided into two broad categories classical complexes and organometallics. The former have the metal in a high oxidation state, and the metal center can be considered to have Lewis acid character. Organometallic complexes of the transition metals, however, usually have the metal in a low oxidation state, and the metal center can be considered to have Lewis base character. 

Tris chelates (L-L) of type M(L-L)3, where M is a kinetically inert metal ion can give optically active complexes where the chirality is at the metal center (Fig. 1.11). This is a particularly attractive feature of transition metal complexes if it is planned to use the complexes for selective binding to an optically active target molecule, or to a bipolymer such as DNA. 

Exercise 6.3-1 Write down, from eqs. (18) and (19), three separate expressions for the bond orbitals 7/, i 2, and i )3. 

The characters of the MRs for the basis dj can now be written down using the transformation of the second subset of d orbitals given in Table 6.4 and eq. (3). Note that the characters for I), simply change sign in the second half of the table (for the classes I T ) this tells us that it is either a u IR, or a direct sum of u IRs. The characters for both ds and d f simply repeat in the second half of the table, so they are either g IRs, or direct sums of g IRs. This is because the p functions have odd parity and the d functions have even parity. 

The classes for the non-zero characters of 1 m its character system, and reduction, are 

The characters for E and 3 C2 have opposite signs, and so to reach a sum of 48 in the reduction test will be unlikely except for IRs with a negative character for the class of 3 C2. Therefore we try first those IRs for which (3C2) is negative. T2g, Tiu, and T 2 all have 

Since T2g, T u, and T2m have the same characters as T g for these classes, they must also occur once in the direct sum, which therefore is 

The X-ray crystallographic structure of the base stabilized terminal stannylene complex f-Bu2Sn(py)Cr(CO)5 is shown in Fig. 12 (95). The slightly longer Sn—Cr bond length in this complex, and the slight eleva- 

Organostannylene and plumbylene complexes of transition metals can be synthesized from both divalent and tetravalent starting materials. 

Crystal structure of (CpjSnFe(C0)4)j (96). Reprinted with permission from J. Chem. Soc., Dalton Trans, p. 2097 (1975). Copyright by The Chemical Society. 

When the divalent compound is stable, the complexes are synthesized by ligand displacement reactions such as those shown in Eqs. (47) (52), (48) (78, 100), and 49 (78). 

In addition to serving as ligands, organostannylenes can also insert into metal-ligand bonds (cf. Fig. 10). In the reaction shown in Eq. (50) (78), two moles of the divalent tin compound react, one forming a terminal stannylene complex, and the other inserting into the Pt—Cl bond. 

Inukai, and H. Muramatsu, Bull. Chem. Soc. Japan, 1969, 42, 1684. 

CFg-PFa and iron pentacarbonyl. Metallic nickel (formed by decarboxylation of nickel oxalate) has been shown to react directly at 60 °C with several fluorophosphines, including CFs-PF and (CF8)2PF, to give the corresponding zerovalent nickel complexes, e.g. Ni(CFj PF2)4 analogous complexes of zerovalent platinum, e.g. Pt[(CF3)jPF]4 (which can also be obtained from potassium tetrachloroplatinite and fluorobistrifluoromethylphosphine ), have been prepared from platinum(n) chloride and PFj, CFj-PFj, and (CF3)2PF at 60°C. =  

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Wight C A and Armentrout P B 1993 Laser photoionization probes of ligand-binding effects in multiphoton dissociation of gas-phase transition-metal complexes ACS Symposium Series 530 61-74... 

Benedetti M, Biscarini P and Brillante A, The effect of pressure on circular dichroism spectra of chiral transition metal complexes Physica B 265 1... 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

The detailed theory of bonding in transition metal complexes is beyond the scope of this book, but further references will be made to the effects of the energy splitting in the d orbitals in Chapter 13. 

DFT calculations offer a good compromise between speed and accuracy. They are well suited for problem molecules such as transition metal complexes. This feature has revolutionized computational inorganic chemistry. DFT often underestimates activation energies and many functionals reproduce hydrogen bonds poorly. Weak van der Waals interactions (dispersion) are not reproduced by DFT a weakness that is shared with current semi-empirical MO techniques. 

V S, C M Kelly and C R Landis 1991. SHAPES Empirical Force-Field - New Treatment of igular Potentials and Its Application to Square-Planar Transition-Metal Complexes. Journal of American Chemical Society 113 1-12. 

For transition metal complexes, techniques derived from a crystal-field theory or ligand-field theory description of the molecules have been created. These tend to be more often qualitative than quantitative. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

The preparation of a series of transition metal complexes (Co. Ni. Pd. Pt, Ir. Au. Cu. Ag) with ambident anion (70) and phosphines as ligands has been reported recently (885). According to the infrared and NMR spectra the thiazoline-2-thione anion is bounded through the exocyclic sulfur atom to the metal. The copper and silver complexes have been found to be dimeric. 

A large number of organometallic compounds are based on transition metals Examples include organic derivatives of iron nickel chromium platinum and rhodium Many important industrial processes are catalyzed by transition metals or their complexes Before we look at these processes a few words about the structures of transition metal complexes are m order... 

Many transition metal complexes including Ni(CO)4 obey the 18 electron rule, which IS to transition metal complexes as the octet rule is to mam group elements like carbon and oxygen It states that... 

Eor transition metal complexes the number of ligands that can be attached to a metal will be such that the sum of the electrons brought by the ligands plus the valence electrons of the metal equals 18... 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

The 18 electron rule is a general but not universal guide for assessing whether a certain transition metal complex is stable or not Both of the following are stable compounds but only one obeys the 18 electron rule Which one" ... 

Metallocene (Section 14 14) A transition metal complex that bears a cyclopentadienyl ligand Metalloenzyme (Section 27 20) An enzyme in which a metal ion at the active site contributes in a chemically significant way to the catalytic activity... 

For transition metal complexes with several possible spin arrangements, a separate calculation within each spin multiplicity may be required to find the ground state of the complex. 

Polyatomic molecules cover such a wide range of different types that it is not possible here to discuss the MOs and electron configurations of more than a very few. The molecules that we shall discuss are those of the general type AFI2, where A is a first-row element, formaldehyde (FI2CO), benzene and some regular octahedral transition metal complexes. 

Flowever, transition metal complexes do absorb in the visible region, giving them a characteristic colour. Flow can this happen if the transitions are forbidden The answer is that interaction may occur between the motion of the electrons and vibrational motions so that some vibronic transitions are allowed (see Section 7.3.4.2b). 

All the forbidden electronic transitions of regular octahedral transition metal complexes, mentioned in Section 7.3.1.4, are induced by non-totally symmetric vibrations. 

Acetonitrile also is used as a catalyst and as an ingredient in transition-metal complex catalysts (35,36). There are many uses for it in the photographic industry and for the extraction and refining of copper and by-product ammonium sulfate (37—39). It also is used for dyeing textiles and in coating compositions (40,41). It is an effective stabilizer for chlorinated solvents, particularly in the presence of aluminum, and it has some appflcation in... 

Numerous explosives are based on hydrazine and its derivatives, including the simple azide, nitrate, perchlorate, and diperchlorate salts. These are sometimes dissolved in anhydrous hydrazine for propeUant appUcations or in mixtures with other explosives (207). Hydrazine transition-metal complexes of nitrates, azides, and perchlorates are primary explosives (208). 

S. F. Ashcroft and C. T. Mortimer, Thermochemisty of Transition Metal Complexes Academic Press, Inc., New York, 1970. 

R. G. Wilkins, The Study of Kinetics andMechanisms of Reactions of Transition Metal Complexes JSRyn2iadR2LConH < -yRos. on lsl. 2LSs. 1974. 

Condensation of vinyl chloride with formaldehyde and HCl (Prins reaction) yields 3,3-dichloro-l-propanol [83682-72-8] and 2,3-dichloro-l-propanol [616-23-9]. The 1,1-addition of chloroform [67-66-3] as well as the addition of other polyhalogen compounds to vinyl chloride are cataly2ed by transition-metal complexes (58). In the presence of iron pentacarbonyl [13463-40-6] both bromoform [75-25-2] CHBr, and iodoform [75-47-8] CHl, add to vinyl chloride (59,60). Other useful products of vinyl chloride addition reactions include 2,2-di luoro-4-chloro-l,3-dioxolane [162970-83-4] (61), 2-chloro-l-propanol [78-89-7] (62), 2-chloropropionaldehyde [683-50-1] (63), 4-nitrophenyl-p,p-dichloroethyl ketone [31689-13-1] (64), and p,p-dichloroethyl phenyl sulfone [3123-10-2] (65). 

Tertiary stibines have been widely employed as ligands in a variety of transition metal complexes (99), and they appear to have numerous uses in synthetic organic chemistry (66), eg, for the olefination of carbonyl compounds (100). They have also been used for the formation of semiconductors by the metal—organic chemical vapor deposition process (101), as catalysts or cocatalysts for a number of polymerization reactions (102), as ingredients of light-sensitive substances (103), and for many other industrial purposes. 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

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