GUARANTEE AGREEMENT

Exhibit 4.5

GUARANTEE AGREEMENT

between

KEYCORP

as Guarantor

and

DEUTSCHE BANK TRUST COMPANY AMERICAS

as Guarantee Trustee

relating to

KEYCORP CAPITAL X

Dated as of February 27, 2008

KEYCORP CAPITAL X

Certain Sections of this Guarantee Agreement relating to

Sections 310 through 318 of the

Trust Indenture Act of 1939

TABLE OF CONTENTS

GUARANTEE AGREEMENT, dated as of February 27, 2008, is executed and delivered by KEYCORP, an Ohio corporation (the “ Guarantor ”) having its principal office at 127 Public Square, Cleveland, Ohio 44114-1306, and DEUTSCHE BANK TRUST COMPANY AMERICAS, a New York banking corporation, as trustee (the “ Guarantee Trustee ”), for the benefit of the Holders (as defined herein) from time to time of the Capital Securities (as defined herein) of KEYCORP CAPITAL X, a Delaware statutory trust (the “ Issuer Trust ”).

WITNESSETH:

WHEREAS, pursuant to an Amended and Restated Trust Agreement, dated as of February 27, 2008 (the “ Trust Agreement ”), among the Guarantor, as Depositor, the Property Trustee, the Delaware Trustee and the Administrative Trustees named therein and the Holders from time to time of undivided beneficial interests in the assets of the Issuer Trust, the Issuer Trust is issuing up to $805,000,000 aggregate Liquidation Amount (as defined in the Trust Agreement) of its 8.000% Enhanced Trust Preferred Securities, Liquidation Amount $25 per Capital Security (the “ Capital Securities ”), representing preferred undivided beneficial interests in the assets of the Issuer Trust and having the terms set forth in the Trust Agreement;

WHEREAS, the Capital Securities will be issued by the Issuer Trust and the proceeds thereof, together with the proceeds from the issuance of the Issuer Trust’s Common Securities (as defined below), will be used to purchase the Junior Subordinated Debentures (as defined in the Trust Agreement) of the Guarantor which will be deposited with Deutsche Bank Trust Company Americas, as Property Trustee under the Trust Agreement, as trust assets;

WHEREAS, as an incentive for the Holders to purchase the Capital Securities, the Guarantor irrevocably and unconditionally agrees, to the extent set forth herein, to pay to the Holders of the Capital Securities the Guarantee Payments (as defined herein) and to make certain other payments on the terms and conditions set forth herein.

NOW, THEREFORE, in consideration of the purchase by each Holder of Capital Securities, which purchase the Guarantor hereby agrees shall benefit the Guarantor, the Guarantor executes and delivers this Guarantee Agreement to provide as follows for the benefit of the Holders from time to time of the Capital Securities:

ARTICLE I

INTERPRETATION AND DEFINITIONS

SECTION 1.1. Interpretation .

In this Guarantee Agreement, unless the context otherwise requires:

(a) capitalized terms used in this Guarantee Agreement but not defined in the preamble hereto have the respective meanings assigned to them in Section 1.2;

(b) a term defined anywhere in this Guarantee Agreement has the same meaning throughout;

(c) all references to “the Guarantee Agreement” or “this Guarantee Agreement” are to this Guarantee Agreement as modified, supplemented or amended from time to time;

(d) all references in this Guarantee Agreement to Articles and Sections are to Articles and Sections of this Guarantee Agreement unless otherwise specified;

(e) a term defined in the Trust Indenture Act has the same meaning when used in this Guarantee Agreement unless otherwise defined in this Guarantee Agreement or unless the context otherwise requires;

(f) a reference to the singular includes the plural and vice-versa; and

(g) the masculine, feminine or neuter genders used herein shall include the masculine, feminine and neuter genders.

SECTION 1.2. Definitions.

As used in this Guarantee Agreement, the terms set forth below shall, unless the context otherwise requires, have the following meanings:

“ Allocable Amounts ”, when used with respect to any Senior Subordinated Debt or Junior Subordinated Debt, means the amount necessary to pay all principal of (and premium, if any) and interest, if any, on such Senior Subordinated Debt or Junior Subordinated Debt, as applicable, in full less, if applicable, any portion of such amounts which would have been paid to, and retained by, the holders of such Senior Subordinated Debt or Junior Subordinated Debt, as applicable (whether as a result of the receipt of payments by the holders of such Senior Subordinated Debt or Junior Subordinated Debt, as applicable, from the Guarantor or any other obligor thereon or from any holders of, or trustee in respect of, other indebtedness that is subordinate and junior in right of payment to such Senior Subordinated Debt or Junior Subordinated Debt, as applicable, pursuant to any provision of such indebtedness for the payment over of amounts received on account of such indebtedness to the holders of such Senior Subordinated Debt or Junior Subordinated Debt, as applicable) but for the fact that such Senior Subordinated Debt or Junior Subordinated Debt, as applicable, is subordinate or junior in right of payment to trade accounts payable or accrued liabilities arising in the ordinary course of business.

“ Affiliate ” of any specified Person means any other Person directly or indirectly controlling or controlled by or under direct or indirect common control with such specified Person; provided, however, that the Issuer Trust shall not be deemed to be an Affiliate of the Guarantor. For the purposes of this definition, “ control ”, when used with respect to any specified Person, means the power to direct the management and policies of such Person, directly or indirectly, whether through the ownership of voting securities, by contract or otherwise; and the terms “ controlling ” and “ controlled ” have meanings correlative to the foregoing.

“ Board of Directors ” means either the board of directors of the Guarantor or any committee of that board duly authorized to act hereunder.

“ Common Securities ” means the securities representing common undivided beneficial interests in the assets of the Issuer Trust.

“ Event of Default ” means a default by the Guarantor on any of its payment or other obligations under this Guarantee Agreement; provided, however, that, except with respect to a default in payment of any Guarantee Payments, the Guarantor shall have received notice of default and shall not have cured such default within 30 days after receipt of such notice.

“ Guarantee Payments ” means the following payments or distributions, without duplication, with respect to the Capital Securities, to the extent not paid or made by or on behalf of the Issuer Trust: (i) any accumulated and unpaid Distributions (as defined in the Trust Agreement) required to be paid on the Capital Securities, to the extent the Issuer Trust shall have funds on hand available therefor at such time,

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(ii) the redemption price, including all accumulated and unpaid Distributions to the date of redemption (the “ Redemption Price ”), with respect to any Capital Securities called for redemption by the Issuer Trust, to the extent the Issuer Trust shall have funds on hand available therefor at such time, and (iii) upon a voluntary or involuntary termination, winding up or liquidation of the Issuer Trust, unless Junior Subordinated Debentures are distributed to the Holders, the lesser of (a) the aggregate of the Liquidation Amount of $25 per Capital Security plus accumulated and unpaid Distributions on the Capital Securities to the date of payment to the extent that the Issuer Trust shall have funds available therefor at such time and (b) the amount of assets of the Issuer Trust remaining available for distribution to Holders in liquidation of the Issuer Trust (in either case, the “ Liquidation Distribution ”).

“ Guarantee Trustee ” means Deutsche Bank Trust Company Americas, until a Successor Guarantee Trustee has been appointed and has accepted such appointment pursuant to the terms of this Guarantee Agreement, and thereafter means each such Successor Guarantee Trustee.

“ Holder ” means any holder, as registered on the books and records of the Issuer Trust, of any Capital Securities; provided, however, that in determining whether the holders of the requisite percentage of Capital Securities have given any request, notice, consent or waiver hereunder, “Holder” shall not include the Guarantor, the Guarantee Trustee, or any Affiliate of the Guarantor or the Guarantee Trustee.

“ Indenture ” means the Indenture dated as of December 4, 1996, between the Guarantor and Bankers Trust Company (now known as Deutsche Bank Trust Company Americas), as trustee, as supplemented and amended from time to time.

“ Junior Subordinated Debt ” has the meaning specified in the Indenture.

“ List of Holders ” has the meaning specified in Section 2.2(a).

“ Majority in Liquidation Amount of the Capital Securities ” means, except as provided by the Trust Indenture Act, a vote by the Holder(s), voting separately as a class, of more than 50% of the Liquidation Amount of all then outstanding Capital Securities issued by the Issuer Trust.

“ Officers’ Certificate ” means, with respect to any Person, a certificate signed by the Chairman or a Vice Chairman of the Board of Directors of such Person or the President or a Vice President of such Person, and by the Treasurer, an Assistant Treasurer, the Secretary or an Assistant Secretary of such Person, and delivered to the Guarantee Trustee. Any Officers’ Certificate delivered with respect to compliance with a condition or covenant provided for in this Guarantee Agreement shall include:

(a) a statement that each officer signing the Officers’ Certificate has read the covenant or condition and the definitions relating thereto;

(b) a brief statement of the nature and scope of the examination or investigation undertaken by each officer in rendering the Officers’ Certificate;

(c) a statement that each officer has made such examination or investigation as, in such officer’s opinion, is necessary to enable such officer to express an informed opinion as to whether or not such covenant or condition has been complied with; and

(d) a statement as to whether, in the opinion of each officer, such condition or covenant has been complied with.

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“Pari Passu Securities” means (i) indebtedness (A) the terms of which provide that such indebtedness ranks equally with the Junior Subordinated Debentures in right of payment upon liquidation and (B) that qualifies or is issued to financing vehicles issuing securities that qualify as Tier 1 capital of the Guarantor under the capital guidelines of the Federal Reserve (as defined in the Trust Agreement); and (ii) guarantees of indebtedness described in clause (i) or securities issued by one or more financing vehicles described in clause (i)(B).

“ Person ” means a legal person, including any individual, corporation, estate, partnership, joint venture, association, joint stock company, limited liability company, trust, unincorporated association, or government or any agency or political subdivision thereof, or any other entity of whatever nature.

“ Responsible Officer ” means, with respect to the Guarantee Trustee, any Managing Director, any Director, any Vice President, any Assistant Vice President, the Secretary, any Assistant Secretary, the Treasurer, any Associate, any Assistant Treasurer, any Trust Officer or Assistant Trust Officer or any other officer of the Corporate Trust Department of the Guarantee Trustee and also means, with respect to a particular corporate trust matter, any other officer to whom such matter is referred because of that officer’s knowledge of and familiarity with the particular subject.

“ Senior Debt ” means any obligation of the Guarantor to its creditors, whether now outstanding or subsequently incurred, other than any obligation as to which, in the instrument creating or evidencing the obligation or pursuant to which the obligation is outstanding, it is provided that such obligation is not Senior Debt; provided however that Senior Debt does not include Senior Subordinated Debt, Junior Subordinated Debt or Pari Passu Securities.

“ Senior Indebtedness ” means (i) Senior Debt (but excluding trade accounts payable and accrued liabilities arising in the ordinary course of business) and (ii) the Allocable Amounts of Senior Subordinated Debt, and (iii) the Allocable Amounts of Junior Subordinated Debt.

“ Senior Subordinated Debt ” means any obligation of the Guarantor to its creditors, whether now outstanding or subsequently incurred, where the instrument creating or evidencing the obligation or pursuant to which the obligation is outstanding, provides that it is subordinate and junior in right of payment to Senior Debt. Senior Subordinated Debt includes the Guarantor’s outstanding subordinated debt securities and any subordinated debt securities issued in the future with substantially similar subordination terms and does not include the Junior Subordinated Debentures or any subordinated debt securities issued in the future or the past with substantially similar subordination terms.

“ Successor Guarantee Trustee ” means a successor Guarantee Trustee possessing the qualifications to act as Guarantee Trustee under Section 4.1.

“ Trust Indenture Act ” means the Trust Indenture Act of 1939, as amended.

Capitalized or otherwise defined terms used but not otherwise defined herein shall have the meanings assigned to such terms in the Trust Agreement as in effect on the date hereof.

ARTICLE II

TRUST INDENTURE ACT

SECTION 2.1. Trust Indenture Act; Application.

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(a) This Guarantee Agreement is subject to the provisions of the Trust Indenture Act that are required to be part of this Guarantee Agreement and shall, to the extent applicable, be governed by such provisions.

(b) If and to the extent that any provision of this Guarantee Agreement limits, qualifies or conflicts with the duties imposed by Sections 310 to 317, inclusive, of the Trust Indenture Act through the operation of Section 318(c) thereof, such imposed duties shall control. If any provision of this Guarantee Agreement modifies or excludes any provision of the Trust Indenture Act which may be so modified or excluded, the latter provision shall be deemed to apply to this Guarantee Agreement as so modified or to be excluded, as the case may be.

SECTION 2.2. List of Holders.

(a) The Guarantor shall furnish or cause to be furnished to the Guarantee Trustee (a) semiannually, on or before June 30 and December 31 of each year, commencing June 30, 2008, a list, in such form as the Guarantee Trustee may reasonably require, of the names and addresses of the Holders (the “ List of Holders ”) as of a date not more than 15 days prior to the delivery thereof, and (b) at such other times as the Guarantee Trustee may request in writing, within 30 days after the receipt by the Guarantor of any such request, a List of Holders as of a date not more than 15 days prior to the time such list is furnished, in each case to the extent such informati