Synthesis, structure, and electronic properties of monomeric and dimeric trispyrazolylborate platinum(II) hydride complexes

Article abstract of DOI:10.1021/ic010099q

Tp′PtMe(H)₂ (2) [Tp′ = hydridotris(3,5-dimethylpyrazolyl)borate] has been prepared from Tp′PtMe(CO) (1) via reaction with water in a basic acetone/water mixture. Protonation of 2 at one of the pyrazole nitrogen atoms induces methane elimination, and the resulting platinum(II) monohydride solvent intermediate (3) can be trapped by added ligand. Two chiral cationic platinum(II) monohydride complexes of the type [κ²-((Hpz*)BHpz*₂)Pt-(H)(L)] [BAr′4] [L = MeCN (4), CH₂=CH₂ (5); pz* = 3,5-dimethylpyrazolyl, BAr′₄ = tetrakis(3,5-trifluoromethylphenyl)borate] have been isolated. If 2 is protonated in the absence of trapping ligand, a deep red hydride-bridged dinuclear complex, [κ²-((Hpz*)BHpz*₂)Pt(μ-H)] ₂[BAr′₄]₂ (6), forms. DFT calculations supplement intuitive expectations regarding 3-center-2-electron bridging orbital descriptions for the electronic structure of this complex. X-ray structure determinations for the monomeric acetonitrile adduct 4 and the dicationic dimer 6 are reported.

Full text of DOI:10.1021/ic010099q

4726
Inorg. Chem. 2001, 40, 4726-4732
Synthesis, Structure, and Electronic Properties of Monomeric and Dimeric
Trispyrazolylborate Platinum(II) Hydride Complexes
Stefan Reinartz, Mu-Hyun Baik, Peter S. White, Maurice Brookhart,* and
Joseph L. Templeton*
Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
ReceiVed January 24, 2001
Tp′PtMe(H)2 (2) [Tp′ ) hydridotris(3,5-dimethylpyrazolyl)borate] has been prepared from Tp′PtMe(CO) (1) via
reaction with water in a basic acetone/water mixture. Protonation of 2 at one of the pyrazole nitrogen atoms
induces methane elimination, and the resulting platinum(II) monohydride solvent intermediate (3) can be trapped
by added ligand. Two chiral cationic platinum(II) monohydride complexes of the type [κ2-((Hpz*)BHpz*2)Pt-
(H)(L)][BAr′4] [L ) MeCN (4), CH2dCH2 (5); pz* ) 3,5-dimethylpyrazolyl, BAr′4 ) tetrakis(3,5-trifluoro-
methylphenyl)borate] have been isolated. If 2 is protonated in the absence of trapping ligand, a deep red hydride-
bridged dinuclear complex, [κ2-((Hpz*)BHpz*2)Pt(µ-H)]2[BAr′4]2 (6), forms. DFT calculations supplement intuitive
expectations regarding 3-center-2-electron bridging orbital descriptions for the electronic structure of this complex.
X-ray structure determinations for the monomeric acetonitrile adduct 4 and the dicationic dimer 6 are reported.
Introduction
are doubly hydride bridged and stabilized by monophosphine3-7
or diphosphine ligands8-15 or else they are singly hydride
bridged and again stabilized by monophosphine16-20 or diphos-
phine ligands.21-23 Spencer’s dimeric complexes containing the
[Pt2H2(P∩P)2]2+ core (C in Figure 1),14,15 which are yellow or
red, are particularly relevant since they are direct analogues of
the dicationic, N-supported dihydride-bridged dimer reported
in this paper. The second large group of complexes contains
mixed bridging ligands consisting of a hydride and a carbon,11,24
sulfur,25 phosphido,26-29 or phosphine bridging ligand.30-35 “A-
frame” complexes are the principal subgroup in this set (E in
Figure 1).31-35 Platinum(I), 30-electron species of the general
Bimetallic complexes with bridging hydride ligands display
diverse structural and spectroscopic features. Dimeric hydride-
bridged platinum complexes in particular have been extensively
studied since the mid 1970s, and they exhibit a plethora of
structural types. Neutral, doubly hydride bridged compounds
[{Pt(µ-H)(SiR3)(PR′3)}]2 (A in Figure 1) were early Pt(II)
examples.1,2 A large group of dimeric platinum complexes with
bridging hydride ligands contains the [Pt2HnP4]m+ (n ) 1-3;
m ) 1,2) core (e.g., B-D in Figure 1).3-35 These compounds
(1) Green, M.; Howard, J. A. K.; Proud, J.; Spencer, J. L.; Stone, F. G.
A.; Tsipis, C. A. J. Chem. Soc., Chem. Commun. 1976, 671.
(2) Ciriano, M.; Green, M.; Howard, J. A. K.; Proud, J.; Spencer, J. L.;
Stone, F. G. A.; Tsipis, C. A. J. Chem. Soc., Dalton Trans. 1978,
801.
(18) Bachechi, F. Acta Crystallogr. 1993, C49, 460.
(19) Bachechi, F.; Mura, P.; Zambonelli, L. Acta Crystallogr. 1993, C49,
2072.
(3) Bracher, G.; Grove, D. M.; Pregosin, P. S.; Venanzi, L. M. Angew.
Chem., Int. Ed. Engl. 1979, 18, 155.
(20) Albinati, A.; Chaloupka, S.; Eckert, J.; Venanzi, L. M.; Wolfer, M.
K. Inorg. Chim. Acta 1997, 259, 305.
(21) Minghetti, G.; Banditelli, G.; Bandini, A. L. J. Organomet. Chem.
1977, 139, C80.
(22) Tulip, T. H.; Yamagata, T.; Yoshida, T.; Wilson, R. D.; Ibers, J. A.;
Otsuka, S. Inorg. Chem. 1979, 18, 2239.
(23) Rimml, H.; Venanzi, L. M. J. Organomet. Chem. 1984, 260, C52.
(24) Ara, I.; Falvello, L. R.; Fornie´s, J.; Lalinde, E.; Mart´ın, A.; Mart´ınez,
F.; Moreno, M. T. Organometallics 1997, 16, 5392.
(25) Clegg, W.; Capdevila, M.; Gonzale´z-Duarte, P.; Sola, J. Acta Crys-
tallogr. 1996, B52, 270.
(26) Goel, A. B.; Goel, S. Inorg. Chim. Acta 1984, 90, L33.
(27) Leoni, P.; Manetti, S.; Pasquali, M. Inorg. Chem. 1995, 34, 749.
(28) Leoni, P.; Pasquali, M.; Fortunelli, A.; Germano, G.; Albinati, A. J.
Am. Chem. Soc. 1998, 120, 9564.
(29) Leoni, P.; Pasquali, M.; Cittadini, V.; Fortunelli, A.; Selmi, M. Inorg.
Chem. 1999, 38, 5257.
(30) Dillinger, K.; Oberhauser, W.; Bachman, C.; Bru¨ggeller, P. Inorg.
Chim. Acta 1994, 223, 13.
(31) Brown, M. P.; Puddephatt, R. J.; Rashidi, M.; Seddon, K. R. Inorg.
Chim. Acta 1977, 23, L27.
(4) Paonessa, R. S.; Trogler, W. C. J. Am. Chem. Soc. 1982, 104, 3529.
(5) Bachechi, F.; Bracher, G.; Grove, D. M.; Kellenberger, B.; Pregosin,
P. S.; Venanzi, L. M.; Zambonelli, L. Inorg. Chem. 1983, 22, 1031.
(6) Paonessa, R. S.; Trogler, W. C. Inorg. Chem. 1983, 22, 1038.
(7) Packett, D. L.; Syed, A.; Trogler, W. C. Organometallics 1988, 7,
159.
(8) Knobler, C. B.; Kaesz, H. D.; Minghetti, G.; Bandini, A. L.; Banditelli,
G.; Bonati, F. Inorg. Chem. 1983, 22, 2324.
(9) Chiang, M. Y.; Bau, R.; Minghetti, G.; Bandini, A. L.; Banditelli, G.;
Koetzle, T. F. Inorg. Chem. 1984, 23, 122.
(10) Carmichael, D.; Hitchcock, P. B.; Nixon, J. F.; Pidcock, A. J. Chem.
Soc., Chem. Commun. 1988, 1554.
(11) Bandini, A. L.; Banditelli, G.; Cinellu, M. A.; Sanna, G.; Minghetti,
G.; Demartin, F.; Manassero, M. Inorg. Chem. 1989, 28, 404.
(12) Aime, S.; Gobetto, R.; Bandini, A. L.; Banditelli, G.; Minghetti, G.
Inorg. Chem. 1991, 30, 316.
(13) Haggerty, B. S.; Housecroft, C. E.; Rheingold, A. L.; Shaykh, B. A.
M. J. Chem. Soc., Dalton Trans. 1991, 2175.
(14) Crascall, L. E.; Spencer, J. L. J. Chem. Soc., Dalton Trans. 1995,
2391.
(32) Brown, M. P.; Puddephatt, R. J.; Rashidi, M.; Seddon, K. R. J. Chem.
Soc., Dalton Trans. 1978, 516.
(33) Puddephatt, R. J.; Azam, K. A.; Hill, R. H.; Brown, M. P.; Nelson,
C. D.; Moulding, R. P.; Seddon, K. R.; Grossel, M. C. J. Am. Chem.
Soc. 1983, 105, 5642.
(34) Xu, C.; Anderson, G. K. Organometallics 1994, 13, 3981.
(35) Xu, C.; Anderson, G. K. Organometallics 1996, 15, 1760.
(15) Mole, L.; Spencer, J. L.; Litster, S. A.; Redhouse, A. D.; Carr, N.;
Orpen, A. G. J. Chem. Soc., Dalton Trans. 1996, 2315.
(16) Bracher, G.; Grove, D. M.; Venanzi, L. M,; Bachechi, F.; Mura, P.;
Zambonelli, L. Angew. Chem., Int. Ed. Engl. 1978, 17, 778.
(17) Carmona, D.; Chaloupka, S.; Jans, J.; Thouvenot, R.; Venanzi, L. M.
J. Organomet. Chem. 1984, 275, 303.
10.1021/ic010099q CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/28/2001

Trispyrazolylborate Platinum(II) Hydride Complexes
Inorganic Chemistry, Vol. 40, No. 18, 2001 4727
mild conditions, leading to cationic platinum(II) complexes of
the type [κ2-((Hpz*)BHpz*2)Pt(Me)(L)][BAr′4] [L ) trapping
ligand; pz* ) 3,5-dimethylpyrazolyl, BAr′4 ) tetrakis(3,5-
trifluoromethylphenyl)borate] after addition of a trapping ligand.62
Haskel and Keinan recently reported synthesis of the unsub-
stituted pyrazolylborate complex TpPtMe(H)2 from TpPtMe-
(CO) via reaction with water.48 This transformation caught our
attention since an analogous Tp′PtMe(H)2 complex would
potentially provide a route into the chemistry of cationic
platinum(II) monohydride complexes analogous to the cationic
platinum(II) methyl and phenyl complexes.62 Following the
synthesis of Tp′PtMe(H)2 (2) from Tp′PtMe(CO) (1), we now
report protonation reactions of this dihydridoplatinum(IV)
reagent with the acid [H(OEt2)2][BAr′4].63 Protonation of 2
followed by addition of a trapping ligand yields Pt(II) adducts.
Protonation of 2 in methylene chloride solution without addition
of a trapping ligand leads to the formation of an unusual deep
red dimeric platinum complex, [κ2-((Hpz*)BHpz*2)Pt(µ-H)]2-
[BAr′4]2 (6). In order to understand the origin of the intense
visible absorptions of the dimeric product, we have undertaken
a DFT study of the electronic structure of this dimeric species.
The electronic structure of transition metal complexes with
bridging hydride ligands has previously been subjected to
symmetry considerations64 and EHMO,65,66 Fenske-Hall,67 and
DFT calculations.68, 69
Figure 1. Examples of dimeric platinum complexes with bridging
hydride ligands.
formula [PtHP2]2 (F in Figure 1) constitute a third group of
complexes,36-39 although hydride bridges in the ground state
structure are only observed with electron-poor phosphines as
supporting ligands.39 Note that phosphines are the prevalent
supporting ligands in these hydride-bridged dinuclear species.
Few dimeric hydrido platinum complexes contain nitrogen-donor
ligands in the platinum coordination sphere.40-44
The chemistry of TpPt(R)(R′)H [Tp ) hydridotris(pyrazolyl)-
borate; Tp′ ) hydridotris(3,5-dimethylpyrazolyl)borate; R, R′
) combinations of alkyl, hydride, and aryl groups] has been
extensively studied.45-49 The stability of these complexes stems
from tridentate coordination of the tris(pyrazolyl)borate ligand,50
which anchors the octahedral coordination sphere of the
platinum(IV) center and inhibits formation of a five-coordinate
platinum(IV) intermediate which is on the pathway for reductive
elimination.51-61 On the other hand, low-temperature protonation
induces reductive elimination of methane from Tp′PtMe2H under
Experimental Section
Materials and Methods. Reactions were performed under an
atmosphere of dry nitrogen or argon using standard Schlenk and drybox
techniques. Argon and nitrogen were purified by passage through
columns of BASF R3-11 catalyst and 4 Å molecular sieves. All
glassware was oven dried before use. Methylene chloride, acetonitrile,
and pentanes were purified under an argon atmosphere by passage
through a column packed with activated alumina.70 Deuterated meth-
ylene chloride was vacuum transferred from calcium hydride and
degassed by several freeze-pump-thaw cycles. Tp′PtMe(CO)62 and
[H(OEt2)2][BAr′4]63 were synthesized according to published procedures.
1H and 13C NMR spectra were recorded on a Bruker Avance 400
spectrometer. 1H NMR and 13C NMR chemical shifts were referenced
to residual 1H and 13C signals of the deuterated solvents. UV/vis spectra
(54) Goldberg, K. I.; Yan, J. Y.; Breitung, E. M. J. Am. Chem. Soc. 1995,
117, 6889.
(36) Schwartz, D. J.; Andersen, R. A. J. Am. Chem. Soc. 1995, 117, 4014.
(37) Fryzuk, M. D.; Clentsmith, G. K. B.; Leznoff, D. B.; Rettig, S. J.;
Geib, S. J. Inorg. Chim. Acta 1997, 265, 169.
(38) Millar, S. P.; Jang, M.; Lachiotte, R. J.; Eisenberg, R. Inorg. Chim.
Acta 1998, 270, 363.
(39) Bennett, B. L.; Roddick, D. M. Inorg. Chem. 1996, 35, 4703.
(40) Grove, D. M.; van Koten, G.; Ubbels, H. J. C.; Spek, A. L. J. Am.
Chem. Soc. 1982, 104, 4285.
(55) Hill, G. S.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1999,
18, 1408.
(56) Hill, G. S.; Puddephatt, R. J. Organometallics 1998, 17, 1478.
(57) Hill, G. S.; Puddephatt, R. J. Organometallics 1997, 16, 4522.
(58) Jenkins, H. A.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1997,
16, 1946.
(59) Roy, S.; Puddephatt, R. J.; Scott, J. D. J. Chem. Soc., Dalton Trans.
1989, 2121.
(41) Terheijden, J.; van Koten, G.; Grove, D. M.; Vrieze, K.; Spek, A. L.
J. Chem. Soc., Dalton Trans. 1987, 1359.
(42) Minghetti, G.; Cinellu, M. A.; Stoccoro, S.; Chelucci, G.; Zucca, A.
Inorg. Chem. 1990, 29, 5138.
(43) Hill, G. S.; Puddephatt, R. J. J. Am. Chem. Soc. 1996, 118, 8745.
(44) Hill, G. S.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1997, 16,
1209.
(60) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E. J. Chem. Soc., Dalton
Trans. 1974, 2457.
(61) Fekl, U.; Zahl, A.; van Eldik, R. Organometallics 1999, 18, 4156.
(62) Reinartz, S.; White, P. S., Brookhart, M.; Templeton, J. L. Organo-
metallics 2000, 19, 3854.
(63) M. Brookhart, B. Grant, A. F., Volpe, Jr. Organometallics 1992, 11,
3920.
(45) O’Reilly, S. A.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc.
1996, 118, 5684.
(46) Canty, A. J.; Dedieu, A.; Jin, H.; Milet, A.; Richmond, M. K.
Organometallics 1996, 15, 2845.
(64) Cotton, F. A.; Daniels, L. M.; Jordan, G. T., IV; Lin, C.; Murillo, C.
A. J. Am. Chem. Soc. 1998, 120, 3398.
(65) Dedieu, A.; Albright, T. A.; Hoffmann, R. J. Am. Chem. Soc. 1979,
101, 3141.
(47) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235.
(48) Haskel, A.; Keinan, E. Organometallics 1999, 18, 4677.
(49) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1999, 121, 11900.
(50) Trofimenko, S. ScorpionatessThe Coordination Chemistry of Poly-
pyrazolylborate Ligands; Imperial College Press: London, 1999.
(51) Bartlett, K. L.; Goldberg, K. I.; Thatcher Bordon, W. J. Am. Chem.
Soc. 2000, 122, 1456.
(52) Crumpton, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2000, 122, 962.
(53) Williams, B. S.; Holland, A. W.; Goldberg, K. I. J. Am. Chem. Soc.
1999, 121, 252.
(66) Miller, R. L.; Lawler, K. A.; Bennett, J. L.; Wolczanski, P. T. Inorg.
Chem. 1996, 35, 3242.
(67) Jezowska-Trzebiatowska, B.; Nissen-Sobocinska, B. J. Organomet.
Chem. 1987, 322, 331.
(68) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 2000,
19, 3931.
(69) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason,
S. A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960.
(70) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.

4728 Inorganic Chemistry, Vol. 40, No. 18, 2001
Reinartz et al.
Table 1. Crystallographic Data Collection Parameters
were recorded on a Hewlett-Packard 8452A diode array spectropho-
tometer, and infrared spectra were recorded on an ASI ReactIR 1000.
Chemical analyses were performed by Atlantic Microlabs of Norcross,
GA.
4
6
formula
mol wt
C49H39B2F24N7Pt
1398.55
(C51H46B2F24N6OPt)2
2863.24
Tp′Pt(CH3)(H)2 (2). Tp′Pt(CH3)(CO) (200 mg, 0.37 mmol) was
added to 40 mL of a 1:1 acetone/water mixture to which 2 drops of
50% NaOH solution had been added. The reaction mixture was heated
to a gentle reflux for 4 h. The resulting white precipitate was filtered
through a glass frit and dried in vacuo. Product 2 (175 mg) was collected
as a white solid. Tp′Pt(CH3)(D)2 (2-d2) was synthesized in an analogous
cryst syst
space group
a, Å
b, Å
c, Å
triclinic
P1h
triclinic
P1h
12.8129(4)
13.0993(4)
18.9928(6)
78.741(1)
87.858(1)
62.287(1)
2762.52(15)
2
13.0550(6)
13.0718(6)
18.3242(8)
96.278(1)
106.327(1)
102.242(1)
2884.74(23)
1
R, deg
fashion in acetone-d6/D2O. Yield: 92%. IR (KBr): νBH ) 2517 cm-1
,
â, deg
1
4
νPtH ) 2247 cm-1. H NMR (CD2Cl2, δ): 5.86 (s, 2H, JPt-H ) 6.0
γ, deg
4
V, Å3
Hz, Tp′CH), 5.79 (s, 1H, JPt-H ) 6.8 Hz, Tp′CH), 2.39, 2.33, 2.26,
2
Z
2.08 (s, 6H, 3H, 6H, 3H, Tp′CH3), 1.02 (s, 3H, JPt-H ) 65 Hz, Pt-
density calcd, Mg/m3
F (000)
1.681
1370.75
1.648
1410.77
CH3), -19.97 (s, 2H, JPt-H ) 1260 Hz, Pt-H). 13C NMR (CD2Cl2,
1
δ): 150.0 (2C, JPt-C ) 23 Hz, Tp′CCH3), 149.3 (1C, JPt-C ) 24 Hz,
Tp′CCH3), 144.73, 144.66 (2C, 1C, Tp′CCH3), 106.3 (2C, 3JPt-C ) 10
Hz, Tp′CH), 105.3 (1C, 3JPt-C ) 10 Hz, Tp′CH), 15.8 (1C, JPt-C ) 32
Hz, Tp′CH3), 14.3 (2C, JPt-C ) 14 Hz, Tp′CH3), 12.7 (3C, Tp′CH3),
-32.5 (1JPt-C ) 570 Hz, Pt-CH3). Anal. Calcd For C16H27N6BPt: C,
37.73; H, 5.34; N, 16.50. Found: C, 37.83; H, 5.24; N, 16.50.
cryst dimens, mm
temp, °C
radiation (λ, Å)
2θ range, deg
µ, mm-1
scan mode
total no. of reflns
0.35 × 0.35 × 0.30 0.25 × 0.20 × 0.10
-100
-100
Mo KR (0.71073)
Mo KR (0.71073)
3-60
2.65
ω
3-50
2.54
ω
1
Representative [BAr′4]- NMR Data. H and 13C NMR data for
46864
27750
10104
8842
the [BAr′4]- counterion are reported separately for simplicity. 1H NMR
total no. of unique reflns 16085
(CD2Cl2, δ): 7.77 (br, 8H, o-Ar′), 7.60 (br, 4H, p-Ar′). 13C NMR (CD2-
no. of obsd data
(I >2.5σ(I))
no. of refined params
RF, %
Rw, %
RI, %
13812
1
Cl2, δ): 162.2 (1:1:1:1 pattern, JB-C ) 50 Hz, Cipso), 135.3 (Cortho),
129.4 (qq, 2JC-F ) 30 Hz, 4JC-F ) 5 Hz, Cmeta), 125.1 (q, 1JC-F ) 270
Hz, CF3), 117.9 (Cpara).
803
767
0.041
0.043
0.027
1.6604
0.039
0.045
0.039
1.7382
Protonation of Tp′Pt(CH3)(H)2 (2). Equimolar amounts of Tp′Pt-
(CH3)(H)2 (2) and [H(OEt2)2][BAr′4] were weighed into an NMR tube
in an argon-filled drybox. The NMR tube was then sealed with a septum
and secured with Teflon tape. The sample was cooled to -78 °C outside
the drybox, and about 0.6 mL of CD2Cl2 was slowly added through
GOF
(HTp′CH3). Anal. Calcd for C49H40N6B2F24Pt: C, 42.47; H, 2.91; N,
6.07. Found: C, 42.57; H, 3.05; N, 5.98.
1
the septum. 3: H NMR (CD2Cl2, 193K, δ): 11.55 (s, 1H, pz*NH),
[K2-((Hpz*)BHpz*2)Pt(µ-H)]2[BAr′4]2 (6). Tp′Pt(CH3)(H)2 (75 mg,
0.147 mmol) and [H(OEt2)2][BAr′4] (150 mg, 1 equiv) were combined
in a Schlenk flask and cooled to -78 °C. CH2Cl2 (12 mL) was slowly
added through the septum. The reaction mixture was stirred for ca. 5
min (a pink color appeared), and the flask was stored in a freezer at
-30 °C for 12 h. Dark purple needles formed, and these were collected
6.11, 5.99, 5.97 (s, 1H each, HTp′CH), 2.31, 2.29, 2.28, 2.15, 2.10,
1
1.73 (s, 3H each, HTp′CH3), -22.49 (s, 1H, JPt-H ) 1053 Hz, Pt-
H), 0.14 (s, CH4). 13C NMR (CD2Cl2, 193K, δ): 153.3, 152.5, 149.1,
148.72, 148.68, 144.4 (HTp′CCH3), 109.7, 108.1, 107.8 (HTp′CH),
17.6, 14.3, 14.1, 13.7, 12.2, 12.0 (HTp′CH3).
[K2-((Hpz*)BHpz*2)Pt(H)(NCCH3)][BAr′4] (4). Tp′Pt(CH3)(H)2 (45
mg, 0.088 mmol) and [H(OEt2)2][BAr′4] (90 mg, 1 equiv) were
combined in a Schlenk flask and cooled to -78 °C. CH2Cl2 (15 mL)
was slowly added through the septum. The reaction mixture was stirred
for ca. 5 min (a pink color appeared), and then 5 mL of CH3CN was
added. The reaction mixture was allowed to warm to ambient
temperature. The solvent was removed in vacuo, and the residue was
triturated with pentane. Colorless crystals, as well as some noncrystalline
material, were obtained from CH2Cl2/pentane at -30 °C. Yield: 72
mg (58%). IR (KBr): νPtH ) 2200 cm-1. 1H NMR (CD2Cl2, δ): 12.86
(br, 1H, pz*NH), 6.14, 6.02, 6.01 (s, 1H each, HTp′CH), 2.40, 2.36,
2.34, 2.32, 2.30, 2.22 (s, 3H, 6H, 3H, 3H, 3H, 3H, HTp′CH3 and Pt-
1
and washed with pentanes. Yield: 129 mg (65%). H NMR (CD2Cl2,
δ): 10.83 (br, 1H, pz*NH), 6.37, 5.99 (s, 1H, 2H, HTp′CH), 2.47,
2.46, 2.01, 1.99 (s, 6H, 3H, 6H, 3H, HTp′CH3), -21.57 (s, 1H, 1JPt-H
) 981 Hz, Pt-H). 13C NMR (CD2Cl2, δ): 153.6, 151.0, 150.5, 145.4
(HTp′CCH3), 110.4, 108.9 (HTp′CH), 16.1, 13.2, 11.9, 11.5 (HTp′CH3).
UV/vis (CH2Cl2): λ ) 526 nm (ꢀ ) 3.8 × 102 M-1 cm-1), λ ) 406
nm (ꢀ ) 5.9 × 102 M-1 cm-1). Anal. Calcd for C94H72N12B4F48Pt2: C,
41.58; H, 2.67; N, 6.19. Found: C, 41.86; H, 2.71; N, 6.12.
X-ray Crystallography. Single crystals of 4 and 6 were mounted
on a glass fiber. Diffraction data were collected on a Bruker SMART
diffractometer using the ω-scan mode. Refinement was carried out with
the full-matrix least-squares method based on F (NRCVAX) with
anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen
atoms were inserted in calculated positions and refined riding with the
corresponding atom. Complete details of X-ray data collection are given
in Table 1.
1
NCCH3), -20.02 (s, 1H, JPt-H ) 1202 Hz, Pt-H). 13C NMR (CD2-
Cl2, δ): 154.0, 152.2, 149.7, 148.6, 147.8, 144.0 (HTp′CCH3), 108.7,
108.2, 107.8 (HTp′CH), 17.2, 13.9, 13.7, 13.2, 12.6, 11.3 (HTp′CH3),
4.1 (Pt-NCCH3). Anal. Calcd For C49H39N7B2F24Pt: C, 42.08; H, 2.81;
N, 7.01. Found: C, 42.23; H, 2.78; N, 6.73.
[K2-((Hpz*)BHpz*2)Pt(H)(CH2dCH2)][BAr′4] (5). Tp′Pt(CH3)(H)2
(51 mg, 0.10 mmol) and [H(OEt2)2][BAr′4] (104 mg, 1 equiv) were
combined in a Schlenk flask and cooled to -78 °C. CH2Cl2 (15 mL)
was slowly added through the septum. The reaction mixture was stirred
for ca. 5 min (a pink color appeared), and then ethylene gas was purged
through the solution while the reaction mixture was allowed to warm
to ambient temperature. The solvent was removed in vacuo, and the
residue was triturated with pentane. A light yellow solid was obtained
after recrystallization from CH2Cl2/pentane at -30 °C. Yield: 109 mg
Computational Details. All calculations were performed using the
DFT71,72 based program DMol3 (version 4.1)73. The VWN74 local
exchange-correlation potential, augmented by exchange and correlation
functionals as suggested by Perdew and Wang (PW91),75 was employed
self-consistently for the full geometry optimization. A set of “double
numerical plus” (DNP) basis functions with a FINE mesh was used
throughout the study, and all electrons were included (no frozen core).
Relativistic calculations were performed with scalar first order correc-
tions (Pauli Hamiltonian) including all electrons.
1
(79%). IR (KBr): νPtH ) 2235 cm-1. H NMR (CD2Cl2, 203K, δ):
(71) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(72) Ziegler, T. Chem. ReV. 1991, 91, 651.
(73) Delley, B. J. Chem. Phys. 1990, 92, 508. DMol3 is available from
MSI in the Cerius2 program suite.
(74) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(75) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244.
10.59 (br, 1H, pz*NH), 6.10, 6.03, 5.96 (s, 1H each, HTp′CH), 3.86,
3.51 (m, 2H each, JPt-H ) 56 Hz, Pt-bound ethylene), 2.32, 2.30, 2.24,
2.15, 1.57 (s, 3H, 3H, 6H, 3H, 3H, HTp′CH3), -22.19 (s, 1H, 1JPt-H
)
1048 Hz, Pt-H); 13C NMR (CD2Cl2, 203K, δ): 152.6, 152.4, 148.3,
148.0, 143.2 (HTp′CCH3), 109.1, 107.8, 106.8 (HTp′CH), 60.4 (1JPt-C
) 155 Hz, Pt-bound ethylene), 14.0, 13.9, 13.0, 12.8, 10.9, 10.7

Trispyrazolylborate Platinum(II) Hydride Complexes
Inorganic Chemistry, Vol. 40, No. 18, 2001 4729
Scheme 1. Synthesis of Tp′PtMe(H)2 (2)
(Solv)][BAr′4] (3). Addition of excess MeCN or ethylene gas
to the cationic platinum hydride intermediate at low temperatures
cleanly affords the platinum(II) acetonitrile adduct [κ2-((Hpz*)-
BHpz*2)Pt(H)(NCMe)][BAr′4] (4) or the platinum(II) ethylene
adduct [κ2-((Hpz*)BHpz*2)Pt(H)(CH2dCH2)][BAr′4] (5) (Scheme
2). Compounds 4 and 5 were obtained in preparatory scale
syntheses in good yields as off-white, air-stable crystals.
If the methyl dihydride 2 is treated with deuterated acid,
[D(OEt2)2][BAr′4], no deuterium incorporation in the evolved
methane is observed by 1H NMR. Since the two hydride signals
for the isotopomers [κ2-((Dpz*)BHpz*2)Pt(H)(Solv)][BAr′4] (3-
d) and [κ2-((Hpz*)BHpz*2)Pt(H)(Solv)][BAr′4] (3) (present in
about a 4:1 ratio) integrate for a total of one proton, deuterium
incorporation into the hydride position is negligible. Hence,
protonation of the methyl dihydride 2 occurs at a pyrazole ring
rather than at either the metal or a methyl site. When Tp′PtMe-
(D)2 (2-d2) reacts with 1 equiv of [H(OEt2)2][BAr′4], both CH3D
and CH2D2 are detected by 1H NMR, and a substantial amount
of hydrogen resides in the hydride position (ca. 0.6 H) of the
product solvento complex 3. Apparently, hydrogen scrambles
from the methyl position into the hydride position, which
implicates reversible formation of a σ-methane intermediate
prior to methane loss. A plausible overall reaction sequence is
protonation of Tp′PtMe(H)2 (2) at a pyrazole ring with
concomitant generation of an open coordination site in a five-
coordinate platinum intermediate. Reversible reductive elimina-
tion of methane leads to a transient methane adduct, followed
by methane loss and coordination of a solvent molecule. The
solvent molecule is then displaced by either acetonitrile or
ethylene.
The 1H NMR spectrum of [κ2-((Hpz*)BHpz*2)Pt(H)(NCMe)]-
[BAr′4] (4) displays the N-H resonance of the protonated
pyrazole ring at 12.86 ppm and the resonance for the hydride
bound to platinum at -20.02 ppm (1JPt-H ) 1202 Hz). The
methyl carbon of the coordinated acetonitrile molecule resonates
at 4.1 ppm.62,83 The solid state IR spectrum of 3 (KBr) shows
an absorption at 2200 cm-1 for the lone Pt-H stretch.
Single crystals of the hydrido acetonitrile complex 4 were
subjected to X-ray structural analysis; an ORTEP diagram is
shown in Figure 2. The hydride ligand on platinum could not
be located in the difference Fourier map and was placed in a
calculated position. This structure resembles the methyl analogue
[κ2-((Hpz*)BHpz*2)PtMe(NCMe)][BAr′4] with respect to bond
lengths and the propeller disposition of the nitrogen heterocycles
of the Tp′ ligand, leaving the protonated pyrazole nitrogen atom
above the platinum square plane (see torsion angles). If the
hydrogen bound to the pyrazole nitrogen atom is placed in a
calculated position, a short Pt-H distance of 2.33 Å results,
consistent with a 3-center-4-electron N-H-Pt bond.84-86
Reflecting the strong trans influence of the hydride ligand, the
Pt1-N21 distance trans to the hydride ligand of 2.133(3) Å is
significantly longer (0.14 Å) than the Pt1-N11 distance of
1.985(2) Å trans to the acetonitrile ligand (Table 2).
Results and Discussion
Synthesis of Tp′PtMe(H)2. Haskel and Keinan recently
reported the synthesis of TpPtMe(H)2 from TpPtMe(CO) via
reaction with water.48 They suggested that nucleophilic attack
on the CO ligand of TpPtMe(CO) by water was the first step
in the overall transformation. We modified their procedure
slightly by reacting the Tp′ analogue Tp′PtMe(CO) (1), which
exists in solution in both square-planar and trigonal-bipyramidal
geometries,62 in acetone/water under basic conditions since this
should accelerate the reaction. Gentle heating of 1 for 4 h in a
1:1 acetone/water solution, to which 2 drops of 50% NaOH
had been added, results in the formation of Tp′PtMe(H)2 (2),
which is isolated after filtration as a white, air-stable powder
in 92% yield (Scheme 1). In the absence of base the reaction
was substantially slower and the yield decreased.
The solid state IR spectrum of Tp′PtMe(H)2 2 (KBr) shows
an absorption at 2517 cm-1 for the B-H stretch of the Tp′
ligand, indicating κ3 coordination,76 and a broad absorption at
2247 cm-1 is assigned to the Pt-H stretches. A single IR
absorption for the symmetric and antisymmetric modes implies
the absence of strong vibrational coupling between the two
hydride ligands, a common phenomena.77-79 The 1H NMR
spectrum of 2 displays a 2:1 pattern for the resonances of the
Tp′ ligand indicative of mirror symmetry, and the two equivalent
hydrides resonate at -19.97 ppm (1JPt-H ) 1260 Hz, 195Pt,
33.4% abundant). The spectral features of 2 are congruent with
data reported for TpPtH2Me48 and Tp′Pt(SiEt3)(H)2.79
Protonation of Tp′PtMe(H)2. When the methyl dihydride
complex 2 reacts with 1 equiv of the acid [H(OEt2)2][BAr′4] in
CD2Cl2 at -78 °C, methane formation (1H NMR: δ ) 0.14
1
ppm) is observed. No dihydrogen is detected. The H NMR
spectrum shows a single metal product with 1:1:1 patterns for
the resonances of the Tp′ ligand, indicating a chiral metal center.
An upfield resonance for a platinum-bound hydride is observed
1
(δ ) -22.49 ppm, JPt-H ) 1053 Hz), and a broad singlet at
11.55 ppm is assigned to the protonated pyrazole ring.62,80-82
These NMR data are consistent with formation of the solvent
(presumably diethyl ether) adduct, [κ2-((Hpz*)BHpz*2)Pt(H)-
The solid state IR spectrum of [κ2-((Hpz*)BHpz*2)Pt(H)-
(CH2dCH2)][BAr′4] (5) (KBr) displays an absorption at 2235
cm-1 for the Pt-H stretch. The 1H NMR spectrum shows
(76) Akita, M.; Ohta, K.; Takahashi, Y.; Hikichi, S.; Moro-oka, Y.
Organometallics 1997, 16, 4121.
(77) L’Eplattenier, F.; Calderazzo, F. Inorg. Chem. 1967, 6, 2092.
(78) Girling, R. B.; Grebenik, P.; Perutz, R. N. Inorg. Chem. 1986, 25, 31.
(79) Reinartz, S.; White, P. S., Brookhart, M.; Templeton, J. L. Organo-
metallics 2000, 19, 3748.
(80) Ball, R. G.; Ghosh, C. K.; Hoyano, J. K.; McMaster, A. D.; Graham,
W. A. G. J. Chem. Soc., Chem. Commun. 1989, 341.
(81) Rheingold, A. L.; Haggerty, B. S.; Trofimenko, S. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1983.
(83) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. Organometallics
1998, 17, 3957.
(84) Brammer, L.; Zhao, D.; Ladipo, F. L.; Braddock-Wilking, J. Acta
Crystallogr. 1995, B51, 632.
(85) Albinati, A.; Lianza, F.; Pregosin, P. S.; Mu¨ller, B. Inorg. Chem. 1994,
33, 2522.
(86) Wehman-Ooyevaar, I. C. M.; Grove, D. M.; Kooijman, H.; van der
Sluis, P.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1992, 114,
9916.
(82) Wiley: J. S.; Oldham, W. J. Jr.; Heinekey, D. M. Organometallics
2000, 19, 1670.

4730 Inorganic Chemistry, Vol. 40, No. 18, 2001
Scheme 2. Protonation of Tp′PtMe(H)2 (2)
Reinartz et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Complex 4
Table 3. Selected Bond Distances (Å) and Angles (deg) for
Complex 6
Pt1-N1
Pt1-N11
Pt1-N21
Pt1-N31
N1-C2
1.968(3)
1.985(2)
2.133(3)
3.173(3)
1.124(4)
1.458(5)
N1-Pt1-N11
177.93(11)
94.63(10)
87.42(9)
178.1(3)
-62.1 (5)
62.7(5)
Pt1-Pt1a
Pt1-N11
Pt1-N21
2.5411(4)
2.006(4)
2.005(4)
N11-Pt1-N21
85.52(16)
136.88(11)
137.58(11)
169.9(8)
N1-Pt1-N21
Pt1a-Pt1-N11
N11-Pt1-N21
Pt1-N1-C2
N12-B10-N32-N31
N22-B10-N32-N31
Pt1a-Pt1-N21
N12-B10-N32-N31
N22-B10-N32-N31
51.1(5)
C2-C3
distance of 2.54 Å (Table 3), which is among the shortest
distances reported for similar complexes. The two platinum
square planes are almost coplanar, consistent with the assign-
ment of a double hydride bridge to this dimeric species.
The Tp′ resonances of complex 6 display a 2:1 pattern in
both 1H and 13C NMR, indicating C2h symmetry in solution. A
resonances at 10.1 ppm for the protonated pyrazole ring and at
-22.19 ppm for the hydride ligand (1JPt-H ) 1048 Hz). Two
multiplets with broad platinum satellites (JPt-H ) 69 Hz)
integrating for two hydrogens each are observed between 3.5
and 4.0 ppm for the four protons of the coordinated ethylene
molecule, indicating rapid rotation of the ethylene ligand about
the midpoint of the ethylene-platinum bond on the NMR time
scale at room temperature. Accordingly, both ethylene carbons
resonate at 60.6 ppm (1JPt-C ) 150 Hz) in the 13C NMR
spectrum.
Formation of a Hydride-Bridged Dimer. When Tp′PtMe-
(H)2 (2) is protonated with [H(OEt2)2][BAr′4] in CH2Cl2 without
addition of trapping ligand, dark red-purple needles of X-ray
quality form after standing overnight at -30 °C.87 Single crystals
of [κ2-((Hpz*)BHpz*2)Pt(µ-H)]2[BAr′4]2 (6) were subjected to
X-ray structural analysis; an ORTEP diagram is shown in Figure
3. The bridging hydride ligands could not be located in the
difference Fourier map and were placed in calculated positions.
The most important feature of this structure is the short Pt-Pt
1
hydride resonance at -21.57 ppm is observed in the H NMR
with very broad satellites (1JPt-H ) 981 Hz), and the triplet for
the bridging hydrides coupling to two spin active platinum
centers can only be observed after extended acquisition times.43
Presumably relaxation due to chemical shift anisotropy accounts
for the difficulty in observing these satellites, each of which
accounts for roughly 3% of the signal.88 No IR absorption for
the bridging hydride atoms was located, a common occur-
rence.8,21,30,32 Although dimer 6 is stable for a period of months
in the solid state at room temperature, it decomposes over a
period of several hours in CH2Cl2 at room temperature.
The electronic spectrum (CH2Cl2) of the colorful dimeric
complex 6 displays absorption maxima in the visible region at
Figure 2. ORTEP diagram of [κ2-((Hpz*)BHpz*2)Pt(H)(NCMe)]-
[BAr′4] (4); ellipsoids are drawn at the 50% probability level, and the
BAr′4 counterion is omitted for clarity.
Figure 3. ORTEP diagram of [κ2-((Hpz*)BHpz*2)Pt(µ-H)]2[BAr′4]2
(6); ellipsoids are drawn at the 50% probability level, and the BAr′4
counterions are omitted for clarity.

Trispyrazolylborate Platinum(II) Hydride Complexes
Inorganic Chemistry, Vol. 40, No. 18, 2001 4731
Table 4. Distances and HOMO/LUMO Energies for the
DFT-Optimized Structure of [κ2-((Hpz)BHpz2)PtH(NCMe)]+
(model-4)
Pt-H Pt-N11 Pt-N21 Pt-N1 N1-C2 ELUMO EHOMO
(Å)
(Å)
(Å)
(Å)
(Å)
(eV)
(eV)
exptl
(Table 2)
DFT calcd,
gas phaserel
1.985
2.133
1.968
1.124
1.554 1.985
2.113
1.919
1.161 -5.38 -8.60
Table 5. Distances and HOMO/LUMO Energies for the
DFT-Optimized Structure of [κ2-((Hpz)BHpz2)Pt(µ-H)]2 (model-6)
Pt-Pt Pt-H Pt-N21 ELUMO EHOMO
2+
(Å)
(Å)
(Å)
(eV)
(eV)
exptl (Table 3)
DFT calcd, gas phaserel 2.542 1.748
2.541
2.005
1.998
-9.09 -10.93
406 nm (ꢀ ) 5.9 × 102 M-1 cm-1) and 526 nm (ꢀ ) 3.8 × 102
M-1 cm-1). Given the absence of comparable absorptions in
related Pt(II) monomers, these absorptions prompted us to
investigate the electronic structure of the dinuclear complex 6.
How does the electronic structure change upon going from a
monomer (e.g., complex 4) to a dimer (e.g., complex 6)?
Comparison of 6 with diborane, B2H6, provides a qualitative
explanation for its deep red color. 6 is similar to B2H6 in that
the coordinatively unsaturated metal centers complete their
valences with bridging hydride ligands: the electronic structure
thus approximates a simple 3-center-2-electron bond. The lowest
energy transitions in such an orbital scheme will surely excite
an electron to the central nonbonding MO of the 3-center-2-
electron orbital. In a monomeric complex like 4, such non-
bonding levels are absent.
Figure 4. (a) Experimental absorption spectrum of 4 and the calculated
spectrum of its computational model [κ2-((Hpz)BHpz2)Pt(H)(NCMe)]+.
(b) Experimental absorption spectrum of 6 and its computational model
2+
[κ2-((Hpz)BHpz2)Pt(µ-H)]2
.
Geometry Optimization. To investigate the electronic
description of dimer 6, density functional theory (DFT) calcula-
tions were conducted. Geometrical parameters obtained after
geometry optimization are listed in Tables 4 and 5. Model
systems containing the protonated Tp ligand rather than Tp′ were
employed in order to reduce the computational effort. The
structural features of the calculated geometries for the mono-
meric acetonitrile adduct [κ2-((Hpz)BHpz2)PtH(NCMe)]+ (model-
Table 6. Calculated Excitation Wavelengths and Relative
Intensities (Relativistic).
excitation
(wavelength, nm)
rel intensity
Monomer (model-4)
321
310
308
304
0.41
3.99
0.34
0.65
4) and the hydride-bridged dimer [κ2-((Hpz)BHpz2)Pt(µ-H)]2
2+
(model-6) are in good agreement with the X-ray structures of
4 and 6 only when relativistic corrections are included. The
DFT calculations illustrate the trans influence of the hydride
ligand in the monomeric system model-4 evident from the X-ray
structure of complex 4: a Pt-N distance of 2.113 Å trans to
the hydride is computed, which is significantly longer than the
calculated Pt-N bond length of 1.985 Å trans to the acetonitrile
ligand.
Electronic Spectroscopy. The computation of accurate
excitation energies is a difficult task, often requiring multicon-
figurational wave functions as available in the CASSCF level
of theory.89 Recently, time-dependent DFT has emerged as an
alternative to expensive ab initio methods.90,91 To achieve our
limited goal of qualitatively understanding changes in the
electronic structure upon dimerization of two Pt monomers, a
more simple protocol was used. We approximate the excitation
energies as the energy differences between the occupied and
Dimer (model-6)
562
355
328
0.48
0.82
2.89
unoccupied Kohn-Sham orbitals responsible for the excitation.
The intensity of the excitation is computed by numerically
evaluating the transition dipole moments ψ(occ)|x,y,z|ψ(unocc) ,
where ψ are the occupied and unoccupied Kohn-Sham orbitals.
This is a severe approximation, and only qualitative agreement
with experiment is expected.
Figure 4 shows the experimental and computed absorption
spectra, where Lorentzian functions have been scaled according
to the computed relative intensities. Table 6 lists the calculated
absorption wavelengths and their relative intensities. As antici-
pated, the quantitative agreement of the computed and observed
spectra is poor. However, the calculation reveals an important
qualitative difference between monomer 4 and dimer 6: no
absorption above 320 nm is predicted for model-4, whereas two
low-energy absorptions (562 and 355 nm) are calculated for
model-6.
(87) Protonation of 2 with HBF4 leads to the immediate formation of the
deep purple hydride-bridged dimer. An intermediate solvent complex
could not be observed by NMR.
Figure 5 shows 3D plots of the two electron-donating orbitals
and the one accepting orbital of the two low-energy excitations
in model-6. The transition at 562 nm is perhaps best character-
ized as a ligand-to-metal charge transfer (LMCT) excitation:
the electron is elevated from an orbital with significant ligand
(88) Ghosh, P.; Desrosiers, P. J.; Parkin, G. J. Am. Chem. Soc. 1998, 120,
10416.
(89) For an example of a recent study, see: Crespo, R.; Merchan, M.; Michl,
J. J. Phys. Chem. A 2000, 104, 8593.
(90) Heinze, H. H.; Go¨rling, A.; Ro¨sch, N. J. Chem. Phys. 2000, 113, 2088.
(91) Casida, M. E.; Salahub, D. R. J. Chem. Phys. 2000, 113, 8918.

4732 Inorganic Chemistry, Vol. 40, No. 18, 2001
Reinartz et al.
Figure 6. Lowest unoccupied molecular orbital (LUMO) of model-4
showing significant Pt d atomic orbital character.
2
(dz , dxz, dyz) to the LUMO+1 orbital, a ligand-based π orbital
(see Supporting Information). Note the contrasting origins of
the low-energy absorptions of model-4 and model-6: none of
the low-energy absorptions in model-4 involves a transition to
the metal center. In fact, the Pt dxy based MO of the monomer
which correlates with the dimer LUMO (Figure 6) is so high in
energy that a transition from a Pt dyz based MO (MO-104) to
the Pt dxy based MO (MO-113 ) LUMO+6) would require a
transition energy of 5.52 eV (corresponding to a 225 nm
absorption band), if the transition were symmetry-allowed.92
Conclusion
The dihydridomethyl complex Tp′PtMe(H)2 (2) was prepared
from Tp′PtMe(CO) (1) in a basic acetone/water mixture by a
slight modification of Keinan’s procedure for TpPtMe(H)2.
Protonation of Tp′PtMe(H)2 at a pyrazolyl ring promotes
reductive elimination of methane, and the resulting platinum-
(II) monohydride intermediate can be trapped with acetonitrile
or ethylene to yield chiral cationic complexes. The platinum-
(II) intermediate dimerizes in the absence of trapping ligand to
form a dark-red dihydrido-bridged dimer 6 of C2h symmetry,
which was structurally characterized.
DFT calculations reproduce the structure of dimer 6 with two
bridging hydride ligands. The electronic spectrum of the
monomeric acetonitrile adduct model-4 shows no absorption
in the visible region below 321 nm, while low-energy absorp-
tions at 562 and 355 nm are computed for dimer model-6. The
two low-energy absorptions in dimer model-6 involve transitions
to a low-lying metal-metal nonbonding MO which is a central
part of the 3-center-2-electron orbital scheme tying the Pt(µ-
H)2Pt entity together.
Figure 5. 3D plots of selected molecular orbitals of model-6. The
orbital energies are given in parentheses. Two views of the same orbital
are shown, respectively: The illustration on the right (top view) is
related to the plot on the left (side view) by a rotation of 90° around
the metal-metal vector.
character (MO-188) to a metal-dominated MO. MO-188 (HOMO-
2) is metal-metal δ-antibonding in the Pt dyz orbitals with a
large out-of-phase contribution from the ligand π orbitals. The
accepting orbital (MO-191) is the lowest unoccupied molecular
orbital (LUMO) of the dimer, formally an out-of-phase com-
bination of the Pt dxy orbitals, which approximates the non-
bonding combination of a 3-center-2-electron bond reminiscent
of B2H6. A similar orbital is not available in the monomer model.
The plots in Figure 5 show that the LUMO is mainly metal-
based with significant out-of-phase contributions from the p
orbitals of the directly coordinated nitrogen atoms. The second
lowest excitation at 355 nm, for which the LUMO acts again
as the charge-accepting orbital, has a charge-donating orbital,
MO-177, somewhat similar to MO-188. MO-177 is also metal-
metal dyz-antibonding, but the metal-ligand interaction is in-
phase, hence the lower energy of MO-177.
Acknowledgment. This work was supported by the National
Science Foundation (Grant CHE-9727500) and the National
Institutes of Health (Grant GM 28938). We thank Prof. H.
Holden Thorp (UNCsChapel Hill) for access to his UV/vis
spectrometer. The North Carolina Supercomputing Center
(NCSC) is gratefully acknowledged for providing computational
time and technical assistance.
The orbitals involved in the lowest energy absorptions of the
monomer [κ2-((Hpz)BHpz2)Pt(H)(NCMe)]+ (model-4) are all
MLCT bands involving transitions from platinum-based orbitals
Supporting Information Available: Complete crystallographic
data, in CIF format, for 4 and 6. 3D plots of selected molecular orbitals
of model-4. Electronic structure of the model systems [PtH4]2- and
[Pt2H6]2-. This material is available free of charge via the Internet at
http://pubs.acs.org.
(92) The orbital energy differences between the dimer and monomer make
a quantitative analysis of the orbital energy shifts upon dimerization
desirable in order to distinguish effects arising from the Tp ligand
from those due to dimer formation. A discussion of the simple model
systems [Pt(H)4]2- and [Pt2(H)6]2- is given as Supporting Information.
IC010099Q