Synthesis, Structure, and Bonding of d3 Molybdenum-Oxo Complexes.

Reduction of d2 metal-oxo ions of the form [MO(PP)2 Cl]+ (M=Mo, W; PP=chelating diphosphine) produces d3 MO(PP)2 Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)2 Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)2 Cl indicate the d3 compounds are low spin with a 2 [(dxy )2 (π*(MoO))1 ] configuration. X-ray crystallographic and vibrational-spectroscopic studies on d2 and d3 [MoO(dppe)2 Cl]0/+ establish that the d3 compound possesses a reduced M-O bond order and significantly longer Mo-O bond, accounting for its greater reactivity. These results indicate that the oxo-centered reactivity of d3 complexes may be controlled through ligand variation.

Light-Driven Redox Activation of CO2- and H2-Activating Complexes in a Self-Assembled Triad.

We report a self-assembled triad for artificial photosynthesis composed of a chromophore, carbon-dioxide reduction catalyst, and hydrogen-oxidation complex, which is designed to operate without conventional sacrificial redox equivalents. Excitation of the zinc-porphyrin chromophore of the triad results in ultrafast charge transfer between a tungsten-alkylidyne donor and a rhenium diimine tricarbonyl acceptor, producing a charge-separated state that persists on the time scale of tens of nanoseconds and is thermodynamically capable of the primary dihydrogen and carbon dioxide binding steps for initiating the reverse water-gas shift reaction. The charge-transfer behavior of this system was probed using transient absorption spectroscopy in the visible, near-infrared, and mid-infrared spectral regions. The behavior of the triad was compared with that of the zinc-porphyrin-rhenium-diimide dyad; the triad was found to have a significantly longer charge-separated lifetime than other previously reported porphyrin-rhenium diimine compounds.

Synthesis, structure, and reactions of a copper-sulfido cluster comprised of the parent Cu2S unit: {(NHC)Cu}2(µ-S).

The synthesis of the first CuI2(µ-S) complex, {(IPr*)Cu}2(µ-S) (IPr* = 1,3-bis(2,6-(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene; 1), has been accomplished via three synthetic routes: (1) salt metathesis between (IPr*)CuCl and Na2S; (2) silyl-deprotection reaction between (IPr*)Cu(SSiMe3) and (IPr*)CuF; and (3) acid-base reaction between (IPr*)Cu(SH) and (IPr*)Cu(O t Bu). The X-ray crystal structure of 1 exhibits two two-coordinate copper centers connected by a bent Cu-S-Cu linkage. Application of these synthetic routes to analogous precursors containing the sterically smaller ligand IPr (1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene), in place of IPr*, resulted in the formation of a transient product proposed as {(IPr)Cu}2(µ-S) (2), which decomposes quickly in solution. The instability of 2 probably results from the insufficient steric protection provided by IPr ligands to the unsaturated Cu2(µ-S) core; in contrast, 1 is stable both in solution and solid state for weeks. The nucleophilic sulfido ligand in 1 reacts with haloalkyl electrophiles (benzyl halides and dibromoalkanes) with formation of C-S bonds, affording (IPr*)Cu(SCH2Ph) and cyclic thioethers, respectively.

Axial Ligand Effects on the Structures of Self-Assembled Gallium-Porphyrin Monolayers on Highly Oriented Pyrolytic Graphite.

Monolayers of five-coordinate gallium octaethylporphyrin complexes (Ga(OEP)X; X = Cl, Br, I, O3SCF3, CCPh) on highly oriented pyrolytic graphite were studied at the solid-liquid (1-phenyloctane) interface using scanning tunneling microscopy (STM) to probe the dependence of their properties on the nature of the axial X ligand. Density functional theory calculations of the gas-phase structures of the free molecules reveal that the gallium atom is positioned above the plane of the porphyrin macrocycle, with this pyramidal distortion increasing in magnitude according to X = O3SCF3 (displacement = 0.35 Å) < Cl, Br, I (∼0.47 Å) < CCPh (0.54 Å). All compounds exhibit pseudohexagonal close-packed structures in which the porphyrin is oriented coplanar with the surface and the axial ligand is oriented perpendicular to it, and with unit-cell parameters that are within experimental error of each other (a, b = 1.34 (3)-1.39 (2) nm, Γ = 66 (2)-68 (1)°). In contrast to these close similarities, the stabilities of the monolayers are sensitive to the nature of the axial ligand: the monolayers of Ga(OEP)(O3SCF3) and Ga(OEP)(CCPh) exhibit damage during the STM experiment upon repeated scanning and upon toggling the sign of the bias potential, but monolayers of Ga(OEP)Cl, Ga(OEP)Br, and Ga(OEP)I do not. A second important ligand-influenced property is that Ga(OEP)I forms bilayer structures, whereas the other Ga(OEP)X compounds form monolayers exclusively under identical conditions. The top layer of the Ga(OEP)I bilayer is oriented with the iodo ligand directed away from the surface, like the bottom layer, but the molecules pack in a square, lower-density geometry. The comparatively large polarizability of the iodo ligand is suggested to be important in stabilizing the bilayer structure.

X-ray Crystallographic, Multifrequency Electron Paramagnetic Resonance, and Density Functional Theory Characterization of the Ni(P(Cy)2N(tBu)2)2(n+) Hydrogen Oxidation Catalyst in the Ni(I) Oxidation State.

The Ni(I) hydrogen oxidation catalyst [Ni(P(Cy)2N(tBu)2)2](+) (1(+); P(Cy)2N(tBu)2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane) has been studied using a combination of electron paramagnetic resonance (EPR) techniques (X-, Q-, and D-band, electron-nuclear double resonance, hyperfine sublevel correlation spectroscopy), X-ray crystallography, and density functional theory (DFT) calculations. Crystallographic and DFT studies indicate that the molecular structure of 1(+) is highly symmetrical. EPR spectroscopy has allowed determination of the electronic g tensor and the spin density distribution on the ligands, and revealed that the Ni(I) center does not interact strongly with the potentially coordinating solvents acetonitrile and butyronitrile. The EPR spectra and magnetic parameters of 1(+) are found to be distinctly different from those for the related compound [Ni(P(Ph)2N(Ph)2)2](+) (4(+)). One significant contributor to these differences is that the molecular structure of 4(+) is unsymmetrical, unlike that of 1(+). DFT calculations on derivatives in which the R and R' groups are systematically varied have allowed elucidation of structure/substituent relationships and their corresponding influence on the magnetic resonance parameters.

Electron-transfer sensitization of H2 oxidation and CO2 reduction catalysts using a single chromophore.

Energy-storing artificial-photosynthetic systems for CO2 reduction must derive the reducing equivalents from a renewable source rather than from sacrificial donors. To this end, a homogeneous, integrated chromophore/two-catalyst system is described that is thermodynamically capable of photochemically driving the energy-storing reverse water-gas shift reaction (CO2 + H2 → CO + H2O), where the reducing equivalents are provided by renewable H2. The system consists of the chromophore zinc tetraphenylporphyrin (ZnTPP), H2 oxidation catalysts of the form [Cp(R)Cr(CO)3](-), and CO2 reduction catalysts of the type Re(bpy-4,4'-R2)(CO)3Cl. Using time-resolved spectroscopic methods, a comprehensive mechanistic and kinetic picture of the photoinitiated reactions of mixtures of these compounds has been developed. It has been found that absorption of a single photon by broadly absorbing ZnTPP sensitizes intercatalyst electron transfer to produce the substrate-active forms of each. The initial photochemical step is the heretofore unobserved reductive quenching of the low-energy T1 state of ZnTPP. Under the experimental conditions, the catalytically competent state decays with a second-order half-life of ∼15 µs, which is of the right magnitude for substrate trapping of sensitized catalyst intermediates.

Electronic, redox, and photophysical consequences of metal-for-carbon substitution in oligo-phenylene-ethynylenes.

The electronic structures, redox chemistry, and excited-state properties of tungsten-containing oligo-phenylene-ethynylenes (OPEs) of the form W[C(p-C6H4CC)n-1Ph](dppe)2Cl (n = 1-5; dppe =1,2-bis(diphenylphosphino)ethane) are reported and compared with those of organic analogues in order to elucidate the effects of metal-for-carbon substitution on OPE bonding and electronic properties. Key similarities between the metallo- and organic OPEs that bear on materials-related functions include their nearly identical effective conjugation lengths, reduction potentials, and π* orbital energies and delocalization. In addition to these conserved properties, the tungsten centers endow OPEs with reversible one-electron oxidation chemistry and long-lived emissive triplet excited states that are not accessible to organic OPEs. The electronic similarities and differences between metallo- and organic OPEs can be understood largely on the basis of π/π* orbital energy matching between tungsten and organic PE fragments and the introduction of an orthogonal mid-π/π*-gap d orbital in metallo-OPEs. These orbital energies can be tuned by varying the supporting ligands; this provides a means to rationally implement and control the emergent properties of metallo-OPE materials.

Photoinduced charge separation in zinc-porphyrin/tungsten-alkylidyne dyads: generation of reactive porphyrin and metallo radical states.

The luminescent tungsten-alkylidyne metalloligand [WCl(≡C-4,4'-C6H4CC-py)(dppe)2] (1; dppe=1,2-bis(diphenylphosphino)ethane) and the zinc-tetraarylporphyrins ZnTPP and ZnTP(Cl)P (TPP=tetraphenylporphyrin, TP(Cl)P=tetra(p-chlorophenyl)porphyrin) self-assemble in fluorobenzene solution to form the dyads ZnTPP(1) and ZnTP(Cl)P(1), in which the metalloligand is axially coordinated to the porphyrin. Excitation of the porphyrin-centered S1 excited states of these dyads initiates intramolecular energy-transfer (ZnPor→1) and electron-transfer (1→ZnPor) processes, which together efficiently quench the S1 state (~90%). Transient-absorption spectroscopy and an associated kinetic analysis reveal that the net product of the energy-transfer process is the (3)[dπ*] state of coordinated 1, which is formed by S1→(1)[dπ*] singlet-singlet (Förster) energy transfer followed by (1)[dπ*]→(3)[dπ*] intersystem crossing. The data also demonstrate that coordinated 1 reductively quenches the porphyrin S1 state to produce the [ZnPor(-)][1(+)] charge-separated state. This is a rare example of the reductive quenching of zinc porphyrin chromophores. The presence in the [ZnPor(-)][1(+)] charge-separated states of powerfully reducing zinc-porphyrin radical anions, which are capable of sensitizing a wide range of reductive electrocatalysts, and the 1(+) ion, which can initiate the oxidation of H2, produces an integrated photochemical system with the thermodynamic capability of driving photoredox processes that result in the transfer of renewable reducing equivalents instead of the consumption of conventional sacrificial donors.

Dihydrogen activation by a tungsten-alkylidyne complex: toward photoredox chromophores that deliver renewable reducing equivalents.

The d(1) tungsten-alkylidyne radical [W(CPh)(dppe)2Cl](+) reacts with H2 to give the d(0) hydride [W(CPh)(H)(dppe)2Cl](+), which on deprotonation yields the d(2) photoredox chromophore W(CPh)(dppe)2Cl. This family of reactions results in a cycle by which renewable H2 provides the reducing equivalents for photochemical reductions.

Oxidation-potential tuning of tungsten-alkylidyne complexes over a 2 V range.

The electrochemistry and electronic structures of over 30 tungsten-alkylidyne compounds of the form W(CR)L(n)L'(4-n)X (R = H, Bu(t), Ph, p-C6H4CCH, p-C6H4CCSiPr(i)3; X = F, Cl, Br, I, OTf, Bu(n), CN, OSiMe3, OPh; L/L' = PMe3, 1/2 dmpe, 1/2 depe, 1/2 dppe, 1/2 tmeda, P(OMe)3, CO, CNBu(t), py), in which the alkylidyne R group and L and X ligands are systematically varied, have been investigated using cyclic voltammetry and density functional theory calculations in order to determine the extent to which the oxidation potential may be tuned and its dependence on the nature of the metal-ligand interactions. The first oxidation potentials are found to span a range of ∼2 V. Symmetry considerations and the electronic-structure calculations indicate that the highest occupied molecular orbital (and redox orbital) is of principal d(xy) orbital parentage for most of the compounds in this series. The dependence of the oxidation potential on ligand is a strong function of the symmetry relationship between the substituent and the d(xy) orbital, being much more sensitive to the nature of the equatorial L ligands (π symmetry, with respect to d(xy), ΔE1/2 ≅ 0.5 V/L) than to the axial CR and X ligands (nonbonding with respect to d(xy), ΔE(1/2) < 0.3 V/L). The oxidation potential is linearly correlated with the calculated d(xy) orbital energy (slope ≅ 1, R(2) = 0.97), which thus provides a convenient computational descriptor for the potential. The strength of the correlation and slope of unity are proposed to be manifestations of the small inner-sphere reorganization energy associated with one-electron oxidation.

FRET sensitization of tungsten-alkylidyne complexes by zinc porphyrins in self-assembled dyads.

The photophysical properties of self-assembled zinc-porphyrin/tungsten-alkylidyne dyads have been investigated with the aim of determining whether the porphyrin S excited state sensitizes the tungsten-alkylidyne (3)[dπ*] state. The luminescent metalloligand W(≡CC(6)H(4)CCpy)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) has been synthesized and shown by electronic and NMR spectroscopy to coordinate axially to ZnTPP and ZnTP(Cl)P (TP(Cl)P = tetra(p-chlorophenyl)porphyrin) via the terminal pyridyl group. Coordination of 1 to ZnPor results in partial quenching of porphyrin S(1) fluorescence and a decrease in the (3)[dπ*] excited-state lifetime of 1. Transient-absorption spectroscopy shows that fluorescence quenching occurs via intramolecular Förster resonance energy transfer from the porphyrin S(1) state to the (1)[dπ*] excited state of 1, which then undergoes rapid singlet-triplet intersystem crossing to produce the (3)[dπ*] excited state. Sensitization of the (3)[dπ*] state occurs with high overall efficiency (φ(EnT) ≈ 80%), thus strongly enhancing light harvesting for the tungsten-alkylidyne compound. The mechanism and rates of the net S(1)→(3)[dπ*] energy transfer are found to differ significantly from those for previously reported zinc-porphyrin/tungsten-alkylidyne dyads that are constructed from similar components but connected instead with covalent bonds at the porphyrin edge. Density functional theory calculations indicate that these differences are due in part to the degree of orbital mixing between the porphyrin and metal-alkylidyne subunits.

Ground-state and excited-state structures of tungsten-benzylidyne complexes.

The molecular structure of the tungsten-benzylidyne complex trans-W(≡CPh)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (d(xy))(2) ground state and luminescent triplet (d(xy))(1)(π*(WCPh))(1) excited state (1*) has been studied using X-ray transient absorption spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. Molecular-orbital considerations suggest that the W-C and W-P bond lengths should increase in the excited state because of the reduction of the formal W-C bond order and decrease in W→P π-backbonding, respectively, between 1 and 1*. This latter conclusion is supported by comparisons among the W-P bond lengths obtained from the X-ray crystal structures of 1, (d(xy))(1)-configured 1(+), and (d(xy))(2) [W(CPh)(dppe)(2)(NCMe)](+) (2(+)). X-ray transient absorption spectroscopic measurements of the excited-state structure of 1* reveal that the W-C bond length is the same (within experimental error) as that determined by X-ray crystallography for the ground state 1, while the average W-P/W-Cl distance increases by 0.04 Å in the excited state. The small excited-state elongation of the W-C bond relative to the M-E distortions found for M(≡E)L(n) (E = O, N) compounds with analogous (d(xy))(1)(π*(ME))(1) excited states is due to the π conjugation within the WCPh unit, which lessens the local W-C π-antibonding character of the π*(WCPh) lowest unoccupied molecular orbital (LUMO). These conclusions are supported by DFT calculations on 1 and 1*. The similar core bond distances of 1, 1(+), and 1* indicates that the inner-sphere reorganization energy associated with ground- and excited-state electron-transfer reactions is small.

1000-fold enhancement of luminescence lifetimes via energy-transfer equilibration with the T1 state of Zn(TPP).

The new zinc porphyrin/tungsten alkylidyne dyad Zn(TPP)-C[triple bond]CC(6)H(4)C[triple bond]W(dppe)(2)Cl (1) possesses novel photophysical properties that arise from a tunable excited-state triplet-triplet equilibrium between the porphyrin and tungsten alkylidyne units. Dyad 1 exhibits (3)(d(xy) <-- pi*(WCR)) phosphorescence with a lifetime that is 20 times longer than that of the parent chromophore W(CC(6)H(4)CCPh)(dppe)(2)Cl (2). The triplet-triplet equilibrium can be tuned by the addition of ligands to the Zn center, resulting in phosphorescence lifetimes for 1(L) that are up to 1300 times longer than that of 2. The "lifetime reservoir" effect exhibited by 1(L) is approximately 1 order of magnitude larger than previously reported examples of the phenomenon.

Synthesis, structures, bonding, and redox chemistry of ditungsten butadiyne complexes with W[triple bond]C-C[triple bond]W backbones.

Complexes of the form XL(4)W[triple bond]C-C[triple bond]WL(4)X (L = 1/2 dmpe, 1/2 depe, P(OMe)(3); X = Cl, OTf) have been synthesized from (Bu(t)O)(3)WCCW(OBu(t))(3) in two steps via Cl(3)(dme)WCCW(dme)Cl(3), which undergoes facile four-electron reduction in the presence of L. The compounds possess formal d(2)-d(2) electron configurations. The molecular structures of Cl(dmpe)(2)WCCW(dmpe)(2)Cl and Cl{P(OMe)(3)}(4)WCCW{P(OMe)(3)}(4)Cl were determined by X-ray crystallography; bond distances within the backbone are consistent with a W[triple bond]C-C[triple bond]W canonical structure. Density-functional-theory calculations on Cl(dmpe)(2)WCCW(dmpe)(2)Cl and the model compound Cl(PH(3))(4)WCCW(PH(3))(4)Cl, and on their monometallic analogs W(CH)(dmpe)(2)Cl and W(CH)(PH(3))(4)Cl, indicate that the WCCW backbone is strongly pi-conjugated; this is supported by the observation of low-energy pi --> pi* transitions for the compounds. The calculations predict that delta symmetry d(xy)-derived orbitals should be (or lie near) the highest occupied molecular orbital. Consistent with this prediction, the electronic spectra of the compounds exhibit a band attributable to d(xy) --> pi* transition(s), as the lowest-energy feature and electrochemical studies demonstrate that they undergo sequential one-electron oxidations to produce (d(xy))(2)-(d(xy))(1) and (d(xy))(1)-(d(xy))(1) congeners. Due to the delta symmetry of the redox orbitals, the oxidized congeners maintain the W[triple bond]C-C[triple bond]W canonical structure of the parent d(2)-d(2) compounds. The first and second oxidation potentials of Cl(dmpe)(2)WCCW(dmpe)(2)Cl are separated by

Electronic spectra and structures of d2 molybdenum-oxo complexes. Effects of structural distortions on orbital energies, two-electron terms, and the mixing of singlet and triplet states.

Molybdenum-oxo ions of the type [Mo(IV)OL(4)Cl](+) (L = CNBu(t), PMe(3), (1)/(2)Me(2)PCH(2)CH(2)PMe(2)) have been studied by X-ray crystallography, vibrational spectroscopy, and polarized single-crystal electronic absorption spectroscopy (300 and ca. 20 K) in order to investigate the effects of the ancillary ligand geometry on the properties of the MotriplebondO bond. The idealized point symmetries of the [Mo(IV)OL(4)Cl](+) ions were established by X-ray crystallographic studies of the salts [MoO(CNBu(t)())(4)Cl][BPh(4)] (C(4)(v)), [MoO(dmpe)(2)Cl]Cl.5H(2)O (C(2)(v)), and [MoO(PMe(3))(4)Cl][PF(6)] (C(2)(v)()); the lower symmetries of the phosphine derivatives are the result of the steric properties of the phosphine ligands. The Motbd1;O stretching frequencies of these ions (948-959 cm(-)(1)) are essentially insensitive to the nature and geometry of the equatorial ligands. In contrast, the electronic absorption bands arising from the nominal d(xy)() --> d(xz), d(yz) (n --> pi(MoO)) ligand-field transition exhibit a large dependence on the geometry of the equatorial ligands. Specifically, the electronic spectrum of [MoO(CNBu(t)())(4)Cl](+) exhibits a single (1)[n --> pi(xz)(,)(yz)] band, whereas the spectra of both [MoO(dmpe)(2)Cl](+) and [MoO(PMe(3))(4)Cl](+) reveal separate (1)[n --> pi(xz)] and (1)[n --> pi(yz)] bands. A general theoretical model of the n --> pi state energies of structurally distorted d(2) M(triplebondE)L(4)X chromophores is developed in order to interpret the electronic spectra of the phosphine derivatives. Analysis of the n --> pi transition energies using this model indicates that the d(xz) and d(yz) pi(MotriplebondO) orbitals are nondegenerate for the C(2)(v)-symmetry ions and the n --> pi(xz) and n --> pi(yz) excited states are characterized by different two-electron terms. These effects lead to a significant redistribution of intensity between certain spin-allowed and spin-forbidden absorption bands. The applicability of this model to the excited states produced by delta --> pi and pi --> delta symmetry electronic transitions of other chromophores is discussed.

Molecular Structures, Vibrational Spectroscopy, and Normal-Mode Analysis of M(2)(C&tbd1;CR)(4)(PMe(3))(4) Dimetallatetraynes. Observation of Strongly Mixed Metal-Metal and Metal-Ligand Vibrational Modes.

The nature of the skeletal vibrational modes of complexes of the type M(2)(C&tbd1;CR)(4)(PMe(3))(4) (M = Mo, W; R = H, Me, Bu(t)(), SiMe(3)) has been deduced. Metrical data from X-ray crystallographic studies of Mo(2)(C&tbd1;CR)(4)(PMe(3))(4) (R = Me, Bu(t)(), SiMe(3)) and W(2)(C&tbd1;CMe)(4)(PMe(3))(4) reveal that the core bond distances and angles are within normal ranges and do not differ in a statistically significant way as a function of the alkynyl substituent, indicating that their associated force constants should be similarly invariant among these compounds. The crystal structures of Mo(2)(C&tbd1;CSiMe(3))(4)(PMe(3))(4) and Mo(2)(C&tbd1;CBu(t)())(4)(PMe(3))(4) are complicated by 3-fold disorder of the Mo(2) unit within apparently ordered ligand arrays. Resonance-Raman spectra ((1)(delta-->delta) excitation, THF solution) of Mo(2)(C&tbd1;CSiMe(3))(4)(PMe(3))(4) and its isotopomers (PMe(3)-d(9), C&tbd1;CSiMe(3)-d(9), (13)C&tbd1;(13)CSiMe(3)) exhibit resonance-enhanced bands due to a(1)-symmetry fundamentals (nu(a) = 362, nu(b) = 397, nu(c) = 254 cm(-)(1) for the natural-abundance complex) and their overtones and combinations. The frequencies and relative intensities of the fundamentals are highly sensitive to isotopic substitution of the C&tbd1;CSiMe(3) ligands, but are insensitive to deuteration of the PMe(3) ligands. Nonresonance-Raman spectra (FT-Raman, 1064 nm excitation, crystalline samples) for the Mo(2)(C&tbd1;CSiMe(3))(4)(PMe(3))(4) compounds and for Mo(2)(C&tbd1;CR)(4)(PMe(3))(4) (R = H, D, Me, Bu(t)(), SiMe(3)) and W(2)(C&tbd1;CMe)(4)(PMe(3))(4) exhibit nu(a), nu(b), and nu(c) and numerous bands due to alkynyl- and phosphine-localized modes, the latter of which are assigned by comparisons to FT-Raman spectra of Mo(2)X(4)L(4) (X = Cl, Br, I; L = PMe(3), PMe(3)-d(9))(4) and Mo(2)Cl(4)(AsMe(3))(4). Valence force-field normal-coordinate calculations on the model compound Mo(2)(C&tbd1;CH)(4)P(4), using core force constants transferred from a calculation on Mo(2)Cl(4)P(4), show that nu(a), nu(b), and nu(c) arise from modes of strongly mixed nu(Mo(2)), nu(MoC), and lambda(MoCC) character. The relative intensities of the resonance-Raman bands due to nu(a), nu(b), and nu(c) reflect, at least in part, their nu(M(2)) character. In contrast, the force field shows that mixing of nu(M(2)) and nu(C&tbd1;C) is negligible. The three-mode mixing is expected to be a general feature for quadruply bonded complexes with unsaturated ligands.