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J . Phys. Chem. 1990, 94, 3541-3549 3547ar e not related. We do believe, however, that these two newtransitions are to the 4d-5p strontium a to pi c orbitals, now of bland b2 symm etry, which correlate to the A211state of the pet almonoalkoxides. The assignment of the symm etry of the B-andC state s is somewhat dtbi ous, although we prefer B2Bl and C2B2(rath er than B2B2 nd C2 BI ). From crystal field arguments, thep orbital in-plane (b,) should be higher in energy than the p orbitalout-of-plane (b ,) due to the repulsion of the negative ch_arge0;the oxygen atoms. The observed splitting between the B and Cstates is, however, so small (e200 cm-I from the strontium ca r-boxylates) that other interactions may be more important. Thecorresponding splitting between the B and C states is unresolvedfor the calcium monocarboxylates.This ordering of the in-plane and ?ut-of-plan_eexcited p orbitalsof the alkaline-earth carboxylate s (B2B I and C 2B2) s in contrastwi thj hat observed for the corresponding states of Sr NH 2 A2B2and B2Bl)where the s ymm etIy is kn_ow_nrom a high-resolutionrotational a nalysis of the A-X and B-X transition^.^ Note thatfor the carboxylates the negatively charged oxygen atom s pointdirectly at the m etal, while the partially positive hydrogens in theamides point away from the metal.A definitive high-resolution analysis was attempted to determinethe symm etry and molecular geometry of the metal carboxylatestate s. However, the molecules proved to be too relaxed for anyresonant laser-induced fluorescence to be observed, so a high-resolution analysis was impossible. This means that our assign-ments are based more on supposition than fact. Perhaps someab initio calculations would help to clarify the problem.Gas-Phase Chemistry of Alkaline-Earth Compounds

Little is known about the gas-phase chemistry of the largerpolyatomic free radicals. Several studies performed by matrixisolation techniques provide some insight into the ga s-phase re-actions of these molecules. For example, in an argon matrix thereaction of an alkaline-earth atom with a w ater atom first produces

the M-OH2 complex.36 Upon photolysis, the metal atom insertsbetween an oxygen-hydrogen bond to form H-M-OH . On U Virradiation, H-M-OH dissociates to form MO H.36The reaction between excited strontium (o r calcium) canprobably proceed directly i n a single step:Sr* + H O O C R - r O O C R + H (1 )

However, ground-state Sr (or Ca) atoms are also found to react,although reaction 1 is probably endotherm ic i n this case.Another possible mechanism for the formation of alkaline-earthmonocarboxylates is0 0I I (2)ArS r + H-0-C--R 4 H-Sr-0-C--R

m-

0 0-II \ \'0;/- + HZ (4)Sr H + H-0-C--R - r'

This mechanism accounts for ou r observation of substantialamounts of S rH in the oven. Surface and m etal cluster reactionsare also possible. It is also not clear whether t he observed S rO H(and Ca OH ) comes from H 2 0 mpurity or from a chemical re-action with the carboxylic acid. The study of the reactions ofalkaline-ea rth vapors with carboxylic acids under molecular beamconditions would be very fruitful.

Acknowledgment. This work was supported by the Natio nalScience Foundation (Grants CHE-8306504 and CHE-8608630).(36)Kauffman, J. M.; auge, R. H. ; Margrave, J. L. High Temp. Sci.1984, 8, 97.

Gas-Phase Inorganic Chemistry: Laser Spectroscopy of Calcium and StrontiumMonoformamidates

A. M . R . P. Bopegedera,+W . T. M. L. Fernando, and P. F. Bernath*,*Department of Chemistry. University of Arizona, Tucson, Arizona 85721 (Received: October IO . 1989)

The reaction products of calcium and strontium metal vapors with formam ide were studied by using laser spectroscopic techniques.Three electro nic transitions were observed for the resulting metal m onoformamidates,MN HCO H. The formamidate ligandis probably bonding to the metal i n a bidentate manner. The metal-ligand stretching vibrational frequencies were assignedfrom the low-resolution spectra.

IntroductionI n our laboratory, we have investigated the spectra of alkaline

earth metal containing free radicals including metal monoal-koxides ,lP2 mon~ thi o la te s ,~socyanates : cy~lopentadienides ,~monoalkylamides,6 monomethides, ' acetylides,* a ~ i d e s , ~oro-hydrides,I0 and carboxylates."J2 All these free radicals have asingle metal-ligand bond (mono dentate bonding) except for themetal borohydrides and carboxylates. The borohydride ligandbonds to the metal in a tridentate fashionlo while the carboxylateligand bonds i n a bidentate fashion.I2Although the formate anion (HCOO-) is a commonly en-countered ligand in transition-metal che mistry, the chemistry ofthe isoelectronic formamidate anion (HC ON H- ) has hardly been'Current address: NO AA, ERL, R/E/AL2 , 325 Broadway, Boulder, C O'Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar.*To whom correspondence should be addressed.

80303.

0022-3654/90/2094-3547$02.50/0

exp10red.I~ A few workers have explored the substitution offormate ligands by amidato ligands in, for example, Rh2(0NH-( 1 ) Brazier, C. R.; Bernath, P. F. ; Kinsey-Nielsen, S.; llingboe, L. C. J .Chem. Phvs. 1985. 82. 1043.(2) Brazier, C. R.; Ellingboe, L. C.; Kinsey-Nielsen,S.; Bernath, P. F. J .(3) Fernando, W. T. M. L. ; Ram, R. S. ; Bernath, P. F. Work in progress.(4)Ellingboe, L. C.; Bopegedera, A. M . R . P.; Brazier, C . R.; Bernath,P. F. Ch em . P hs y . L ef t .1986, 26 , 285. O'Brien, L. C.; Bernath, P. F. J .

Am . Chem. SOC.986, 08, 2126.

Chem. Phys . 1988, 8, 1 17.(5) 'Brien, L.C.; Bernath, P. F. J . A m . Chem. SOC.1986,108, 5017.(6)Bownedera. A. M. R. P.; Brazier, C. R.; Bernath, P. F. J . Phys. Chem.1987, 1,'2i79.(7)Brazier,C. .; Bernath, P. F. J . Chem. Phys. 1987,86,5918; J . Chem.Phys . 1989, 1, 4548.(8)Bopegedera, A . M. R. P.; Brazier, C. R.; Bernath, P. F . Chem. Phys .Lett . 1987, 36 , 91; . M o l . Spectrosc. 1988, 29 , 268.(9)Brazier, C . R.; ernath, P. F. J . Chem. Phys. 1988,88, 112.(10) Pianalto, F. S.; Bopegedera, A. M. R. P. ; Brazier, C. R.; Fernando,W T. M . L.;Hailey. R. ; Bernath, P. F. W ork i n progress.

0 990 American Chemical Society

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3548 Th e Journal of Physical Chemis try , Vol . 94 , N o . 9, I990 Bopegedera et al.

I I050 680 nmFigure 1 . Resolved fluorescence spectrum of the b2A'-i(2A' transitionof calcium monoformamidate. The asterisk marks the position of thelaser, which is tuned to the 0-0 band. The strong feature to the blue ofthe asterisk is the 'P,-'S0 tomic transition of Ca. Features at ==680 nd2645 nm are assigned to-the 0 7 1 and 1 - 0 bands, respectively. Althoughnot shown here, strong A2A'-X2A' emission was observed in th i s spec-t r um a t =706 nm (see text).CCF3)4.14 W e report here on th e gas-phase calcium and strontiummonoformamidates synthesized by the direct reaction betweenthe metal vapor and formamide.Experimental Sectio n

The gas-phase alkaline-earth monoformam idates were preparedin a Broida t ype ovenI5 by th e reaction of the metal (Ca, Sr ) vaporwith formamide (H CO NH 2). The metal was resistively heatedin an alumina crucible and the vapor entrained in 1.5 Torr of argoncarrier gas.Formam ide has a very low vapor pressure at room tempe rature( 1 Torr a t 70 "C) .I6 Therefo re, to provide a sufficient partialpressure of formamide vapor inside the oven, argon gas wasbubbled through the glass cell containing formamide. Th e totalpressure inside the oven was maintained a t approximate ly 3 Torr.Since analytical grad e formamide contains undesirable impuritiessuch as NH3 , spectrometric grade (99+%) formamide (Aldrich)was used for our experiments.Two CW broad-band (I-cm-l) dye lasers pumped by a 5.5-Wall-lines outpu t of a Coherent Innova 70 argon ion laser were usedin this experim ent. On e dye laser was tuned t o excite the 3P -'Soatomic transition of the metal (6573 A for Ca and 6892 A orSr) . The second dye laser was used to excite the moleculartransitions of the calcium and strontium monoformamidates.Several laser dyes (DCM , Pyridine 2, and Rhodamine 6G ) wererequired to cover the desired spectral region.Two types of spectra were recorded. Laser excitation spectrawere recorded by scanning the wavelength of the laser tha t wasexciting the molecular transition. Th e beam from this laser waschopped and the signal demo dulated with a lock-in amplifier. Redpass filters (Schott RG9 , RG780, and RG830), were used to blockthe scattered laser light. A photomultiplier-filter combinationwas used to detect the tota l fluorescence from the excited electronicstates.Resolved fluorescence spectra were recorded by t uning th ewavelength of the second dye laser to a molecular transition and

( 1 1 ) Brazier. C. R.; ernath, P. F.; Kinsey-Nielsen, S.; Ellingboe, L. .(12) 'Brien, L.C . ; Brazier. C. R. ; Kinsey-Nielsen, S.;Bernath. P. F. J .( I 3)Cotton. F.A.; Wilkinson, G. Advanced Inorganic Chemistry. 5t h ed.:(14)Denis, A . M.;Korp, J . D. ; Bernal, I.; Howard, R. A ,; Bear, J . L.(15 ) West, J . B .; Bradford, R. S. ;Eversole, J . D.; Jones, C . R . R e c . Sci .(16) he Merck Index. 10th ed.; Merck & Co.: Ra h w a y , N J , 1983;

J . Chem. Phys. 1985, 8 2 , 1043.Phys. Chem.. preceding paper in this issue.Wiley-Interscience: New York, 1988; p 376.Inorg. Chem. 1983, 22 , 1522.Insrrum. 1975. 46, 164.4127.

7lb i ~ o n m IFigure 2. Resolved fluorescence spectrum of the B2A'-RzA' transitionof strontium monoformamide. The asterisk ?arks t_he position of thelaser, which is tuned to the 0-0 band. Strong A2A'-X2A' emission canalso be seen to the red (see tex t). The features at ~ 7 0 5 ,7 3 5 ,nd ~ 7 4 5nm are assigned to the 1-0, 0-1, an d 0-2 vibronic bands, respectively.The feature at =780 n m is the 0-1 band of the A2A'-i?A' transition.TABLE I: Band Origins of the Calcium and StrontiumMonoformamidate Vibronic Transitions (in cm-')

band CaNHCOH SrNHCOH2-01-00-00-0-20-31-00-00- 10-22-01-00-00- 1

A2Af-i(2Al1485914 50914 I541380313457I3 IO8fi2At-R(2Af15 4401508314727

166011624815 896

1362413 35113 077I2 7891250112 222

1420113917I3 63013 389

1 5 205148951458014 296dispersing the fluorescence through a small monochromatorequipped with photon counting detection electronics.Results and Discussion

Th ree ele ctro nic tran siti ons A2A'-W2A', B2A'-R2A' andC2A"-R2Af were observed in the excitation spectra of the metalmonoformamidates. Figures 1 an 2 show parts of the resolvedfluorescence spec tra of the B2A'-X2A' transi tion of calcium an dstrontium monoformamidate molecules, respectively. To obtainthese spectra, the dye laser exciticg the-molecular transition wastuned to the 0-0 ban d of the B2A'-X2A' transit ion, and thefluorescence dispersed with the monochromator. Emission fromthe excited elec tronic sta te to higher vibrational levels of the groundelectronic stat e was observed in all of the resolved fluorescencespectra. The band origins of these vibronic transitions are givenin Table I .When t he B2A'-R2A' transition of the m_etal mono formam idateswas excited by th e laser, rela_xation_to he A2A' stat e was observed(Figure 2). In fac t, stro ng A2A'-X2A' emissio_nwas _observed in:he spectra of both molecules when either the B2A'-X2A' or theC2A"-X2A' transition was excited by the laser.The formam idate anion (H CO NH -) is isoelectronic with theformate (HC OO- ) anion. The low-resolution spectra of calciumand strontium monoformates have been reported previously.12Similar to the formate anion, the formam idate anion could bondto the metal ion in a mo nodenta te (to give molecule I or 11) orbidenta te (t o give molecule 111) manner. Although both types

H111

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Gas-Phase inorganic ChemistryCorrelation Diagram

2A -/?f - _._..._Al - _ _ _ _ _ _2a -

01- -.----A'-np-:: __._0,-

2 .-.?n - .-. -..?The Journal of Physical Chemistry, Vol. 94, N o . 9, 1990 3549

ns________!,+-E _ _ _ _ _ _ _A - ...... __._._A -M+-NH2 M+-NHR M+ 0 . c ~HN'+ M+-CCHc-, C2" c, c,

Figure 3. Correlation diagram for Ca' and Sr' with CCH- (C-"), NH C(Cb),H R - (Cs,monodentate) and HCONH- (C8,bidentate) ligands.Note tha t i n our previous paper on calcium and strontium monoalkyl-amides (ref 6) th e corresponding figure (Figure 4) is i n error. Theout-of-plane B, states correlate with A" states while the in-plane B, statescorrelate with A'.of bonding result i n a molecule i n th e C, oint group, the natureof bonding will have a strong effect on the electronic spectra ofthe metal monoformamidates. I f the bonding is monodentate,then the spectra of the M-NH-COH or M - O - C H N H ( M = Ca,Sr) molecules will resemble those of th e m onoalkylamides6 or themonoalkoxides.2 I f the bonding is bidentate, the spectra ofmolecule 111 will resemble those of the isoelectronic metal m ono-formates.I2 A close look at the spectra of the metal m onoform-amidates reveals considerable similarity with those of the metalmon oform ates, suggesting tha t the form amid ate anion is a bi-dentate ligand.The metal monoformamidates are ionic molecules representedby the st ructure M+H NC OH -, where the HC ON H- l igand isclosed shell. Therefore, the m olecular orbitals of the metal mo-nofor mam ides can be described as the orbitals of the M + ionperturbed by the HCONH- ligand i n th e C, point group. Thecorrelation diagram in Figure 3 is helpful to describe these mo-lecular orbitals and the electronic structure of Ca N H C O H andS r N H C O H .The valence ns (n = 4 or Ca, n = 5 for Sr) orbital of the M +ion contains one unpaired electron . This results i n a 2Z + groundstate for the linear (C,,) MC CH molecule. In the C, point group,this transforms to a 2A' state (Figure 3) . The 5-fold degeneracyof the ( n - I)d orbitals is lifted i n th e C,, point grou p, giving riseto 6 (dxLyyl,dxy),a (d,,, d y r ) , an d u (d22) orbitals. Similarly, the3-fold degenerate np orbitals split into a (px, py) and u (p,) orbitalsi n th e C,, point group. I n addition, the presence of the linearligand mixes d a with pa and d u with pa so that these orbitals arenow da-p?r and do-pu mixtures. Transition to the 2A state (ds-9,d, orbitals) from the 2Z+ ground state is forbidden. Consequently,in the C,,point gro up the first allowed electronic transition isfrom the X2Z + ground state to the A2 H state.When the symmetry is reduced to C2, (a s i n metal mono-amides), the degeneracy of th e 6 nd a orbitals is lifted, givingrise to a,, a 2 (dXLyz. ,) an d b2, b, (m ixtur e of d,, and p,, d, andp,) orbitals, respectively. Th e corresponding electronic states aregiven i n Figure 3. -Note that th e x-axis is out of plane. The relativeordering of the A2B2and B2B, states was determined experi-mentally" for SrN H2 . Although transitions are allowed from theground 2A , state to all the other states in the C2, point group,except to the 2A z state, a transition w as not observed to thelow-lying 2A , state in any of the Ca - and Sr-containing molecules

(17) Brazier, C. R.; Bernath, P. F. Manuscript i n preparation

TABLE II : Vibrational Frequencies of Calcium and StrontiumMonoformamidates (in cm-I)s ta te CaNHCOH SrNHCOHP A ' 35 1 28 8A2A' 355 278F A ' 35 7 284CZA!I 353 315

that we studied. When a H atom of a metal monoamide isreplaced by an alkyl group to obtain the metal monoalkylamides,the sy Fm et ry is reduce! from C2, to C,. The relative orderingof the A2A f,B2Arf, nd C2A' states of the metal monoalkylamidesis obtained by correlating to the C2, point group.Since the formamidate ligand bonds to the metal in a bidentatefashion, the nitrogen and the oxygen atoms on the anion*-/-"H-C.+.IH

are partially negatively charged. W e believe tha t these off-axisnegative charges destabilize the metal orbitals that are perpen-dicular to the z axis ( a orbitals) relative to the metal orbitals thatare parallel to the z axis (u orbitals). This has pronounced effectson the relative ordering of the electronic states in_ the metalmonoformamidate. As a result of this, the A and B el_ectroni_cstates of the metal monoalkylamides correlate with_ he B an d Cstates of the metal monoformamidates while the C state of themonoalkylamides correlates with the A state of the monoform-amidates (Figure 3) . Therefore, the three observed electronictransitions of the metal monoformgm idates ar e assigned asA2A'-X2A ', fi2Ar-j(12A r, nd C2A"-X2A'. W e have observed asimilar switching of state s in two other f amilies of molecules wehave studied previously: the metal borohydridesI0 and the metalmonoformates.I2 The borohydride (B HJ anion bonds in a tri-denta te fashion. In both these cases off-axis negative charges arepresent on the ligands.The low-resolution electronic spectra enabled us to obtain vi -brational frequencies for the metal monoformamidates which ar ereported in Table 11 . Since only metal-centered orbitals areinvolved in the electronic transitions, any vibratio nal activity tha tis observed is also associated with the metal atom . We there foreassign the single observed F ranck-Condon-active mode as ametal-ligand stretching vibration. T he observation of progressionsin the metal-ligand stretching mode suggests that the re ar e sig-nificant changes in the metal-ligand bond lengths in the excitedelectronic states.The electronic assignments of the metal monoformamidatespectra were based on qualitative arguments made by comparingthe spectra of metal mono form amid ates, monoformates, andmonoboroh ydrides. This was necessary because definitive high-resolution spectra ar e not available.Boldyrev a nd co-workers18 have carried out ab initio calculationsof the molecular properties of LiBH,. They find tha t the C,,tridendate structure has the lowest energy on th e potential surface.Similar ab initio calculations on metal mon oformates and mo-nofor mam idates would provide some additional insight into thestructures of these molecules.Conclusion

The gas-phase reaction between alkaline-earth metals andformamide produces the alkaline-earth-monoformapidate_re eradicals. Thr ee electronic transitions (A2A'-X2Af, B2A'-X2A',and c2A"-X2A') were observed at low resolution. Comparisonof the spectra with those of the alkaline-earth monoborohydridesand monoformates enables us to conclude that the formamideligand bonds to the metal in a bidentate manner.(18) Boldyrev, A. 1.; Charkin, 0. .; Rambidi, N. G.; vdeev, V. . Chem.Phys . Letr. 1976, 44 , 20.